FITTING - MB Engine Packers/Gaskets

In this section we will cover con rod lengths, crank strokes, piston crown heights, cylinder packing plates and head gaskets and how to create a better engine to set up, to suit the customer.

There's many misconceptions with regards Lambrettas and their engines and setting them up. There's plenty of experts offering advice on what they have read, but a lot of it is incorrect or not explained correctly and leads to engines which under perform when they should perform much better. Part of this comes from the factory and how the engine was designed. For years we have been stuck with what Innocenti invented, but Innocenti couldn't have imagined what us, the Brits and other people around the world were going to do with these simple cheaply made shopping trollies many years later.

One major problem I have faced over the years as a tuner and engine builder is the limitation in con rod lengths designed by Innocenti.

One theory is a 'con rod' that's a 'connecting rod' ok the bit between the crankshaft webs where the bearings run that 'connects the crankshaft to the piston' ............ should be twice the length of it's stroke. The stroke is the length of the crankshafts movement from top dead center to bottom dead center. So a standard crank at 58mm stroke should have a 116mm long con rod. And a con rod on a very popular after market 60mm crank should be 120mm.

Why? Well someone at some point must have made it up based on a theory and tried it out in practice and it works and it can make for a smoother engine. But Innocenti in their wisdom went and used a 107mm rod in a 58mm crank which in theory is wrong! Apart from a Tv175 engine which did use a 116mm con rod. So which is best? 

Lets look at the stroke of the crank – as said the stroke is the movement from top dead center to bottom dead center. All Lambretta Series 1, 2 and 3's have a 58mm stroke and uses a 107mm con rod apart from a Tv175 which was 58mm stroke and used a 116mm con rod. (not including the Tv175 Series 1 which is a totally different engine) To compensate on rod lengths the piston crown height changes to suit the con rod.


The piston 'compression height' or 'crown height' also gets confused with different people. The crown height of a piston is from the center of the gudgeon pin to the top of the piston where it starts the curve of the dome.

  • All Lambretta engines use a 107mm con rod and uses a 39mm crown height piston
  • A Lambretta Tv175 uses a 116mm con rod and uses a 30mm crown height piston

Lets look at a simple sum which we will work through to find why I like using cylinder packing plates.

  • 107mm con rod + 39mm compression height piston = 146mm
  • 116mm con rod + 30mm compression height piston = 146mm 

You will see where I'm going in a bit.

This figure of 146mm you won't see in any book, you will rarely see it as advice on a forum. But it's important in Lambretta engines where we have mismatched cranks, con rods and pistons like no other engine in the world................ you don't know how lucky you are!


Over the years to improve the Lambretta engine, tuners have found different pistons and made them fit to improve reliability. But there isn't a direct piston which has been a straight fit. There's been plenty of suitable pistons available from motorbikes. All have different crown and dome heights, the skirt lengths alter as does the gudgeon pin size and ring peg positions.

All Lambrettas use a 16mm gudgeon pin and small end bearing. Most bike pistons used 18mm gudgeon pins so if your using a Lambretta standard type con rod you needed piston bushes fitting, which go wrong. I hunted for different con rods for years and perfected the Lambretta con rod conversion which worked with whatever piston was used! This allowed us to use better pistons with better stronger con rods. Con rods stop snapping, big and small end bearings stop melting and pistons stopped cracking and we lifted power out puts with the increase in port sizes that created no problems. 

But Innocenti used funny con rod lengths at 107 and 116mm! Bikes used 105, 110, 115 and 120mm con rods so nothing was a direct replacement. (there was oddball rods appearing over time at 106mm and 108mm and even 117mm but these were not normal and were expensive and hard to get)

If we look at some common well know piston conversions used over the years you will see why I'm using the above sum.


  • Honda 205; 107mm con rod + 38mm crown height = 145mm, which means 1mm needs removing from the cylinder
  • Suzuki 205; 107mm con rod + 37mm crown height = 144mm which means 2mm needs removing from the cylinder
  • Yamaha 250; 107mm con rod + 32mm crown height = 139mm which means 10mm needs removing from the cylinder BUT 10mm is hard to remove so tuners rightly used the longer con rod at 116mm. So 116mm con rod + 32mm compression height = 148mm! Now you can see where I'm going, 2mm needs adding to the cylinder to make it all fit, so we require a 2mm packing plate


And the list goes on with lots of pistons. Today with my con rod conversions have been used by other tuners and also manufacturers are now making cranks with different length con rods, but we have made a problem that most can not get to grasps with so I hope this article helps to explain some engine internal basics.

Moving to today and modern times of improved crankshafts and piston conversions.

Since the 80's I've been perfecting and standardising engines to use the Yamaha 110mm rod (which comes from many different engines some with different dimensions). Why? It's not double the stroke! Well it's like this............ they are quite easy to fit in a crank and easy to set up the engine and it's a totally bullet proof con rod, easy to locate, quiet cheap to buy and fit.

Back to the sum; take a crank with a 110mm con rod and a standard style Lambretta piston with a compression height of 39mm – 149mm which is 3mm longer (the extra length of the con rod), so a 3mm cylinder packer will be needed to get things set up as if they were in a standard set up.

Go one step further to the extreme which some are doing these days. Of course I've also done in this in the past and that would be a crank fitted with a 115/116mm con rod and using a standard 39mm crown height piston = 154/155mm which is 8-9mm difference and needs a big packing plate to compensate! Sounds ok in theory, but it will push the cylinder towards the frame, it will create a gap from the head cowling to the mag housing so something extra and odd ball needs doing there. And the plug becomes close to the frame. Why? Well I've done it in the past to use a crank already fitted when there hasn't been a perfect low compression height piston available. Today people are doing this conversion on purpose when I think it's not the way to go, especially now we make our full range of our Race-Tour pistons in any crown height required.

Now throw into the mix of a 60, 61, 62, 63 or 64mm crankshafts which are available today and go back to the theory of double the length of con rod and we should be using rods up to 128mm! With a standard Lambretta piston you would need a 21mm packer! Now we are getting silly, so to compensate you really have to reduce the crown height of the piston to bring the sum down and put the cylinder closer to the casing. This is where the 116mm/30mm set up comes into it's own, no spacers, packers or packing plates. BUT this limits where you can set the very important port timings because there is no movement in sliding the cylinder up and down the studs using packers and head gaskets. Ok you can machine the cylinder base and add the difference in the head gasket to get the dimensions back to where you want to go or you could also add a base packer and machine the top of the cylinder and use no head gasket.

Lets go back to a standard engine, but one you want to improve it's power. Take the 58 x 107mm crankshaft and a standard piston with a 39mm crown height in a TS1 engine. In standard form you would fit the cylinder with a standard base gasket and a standard 0.5mm head gasket – this gives the standard TS1 set up which is in my opinion a high revving road race set up and lots of Lambretta owners find them hard to ride especially with a race pipe and over gearing as originally suggested by the manufacturers. The limiting factor which controls the cylinder set up is the con rod and crown height factors and also the height of the face on the casing where the gasket goes. A perfect Italian casing uses the above set up, but casing heights vary. Spanish casings are known to be up to around 1mm higher and some Indian casings can be 2mm higher. So ports are lifted up making the standard set up even worse! And it will also increase the all important squish clearance making the engine even worse to ride and set up. You can remove the base gasket and head gasket and if your lucky the squish is correct but the port timings are still too big because of the casing face height! So what do we need to do? Well, I've been one to offer a conversion to remove either 0.5 or 1.00mm off the base of the cylinder, which lowers the port timings helping it to ride better and gives you the choice of swapping head gasket thicknesses to tweak the all important squish clearance! But this is a workshop job and a total guess when youre sending out a kit or working on a kit over the phone in the future.

Now take this last paragraph and remove the standard limiting factor the 107mm con rod and add in the longer 110mm con rod. The difference is 3mm, so instead of having to remove metal off the base of the cylinder to get port timings you have to add a packer between the casing and the cylinder and you can play with base packing plates (packers) to get the important port timings.

Lets say the difference in con rods is 3mm, there is no difference in compression height of the piston, so basically you need a 3mm packer for it to assemble as a standard designed TS1 kit. This should need a 0.5mm head gasket as described previous. Because a casing height can vary, let say it's 0.5mm higher than normal then either the head gasket or the base packer needs reducing to get the important 1mm squish clearance. If you remove the head gasket then port timings increase, if you reduce the base packer then port timings should near standard.

In the old days when I used to machine the cylinder base and add a fat head gasket (or as some did, machined the difference of the head gasket into the head) it lowers the cylinder down on the con rod towards the casing and lowers the port timings to make the engine more ride-able and drive-able making a better engine. Ok it may lower the peak power output but the low down riding ability compensates and allows you to fit higher gearing and you get a better engine!

Lets look at it in this way using the same set up.

TS1 engine, 58 x 110mm with a standard crown height piston of 39mm, you can set it up using different cylinder packing plates and different head gaskets to make the engine work the way you want it. But remember the important 1mm squish, which should be the same with all scenarios listed below.



  • 3.50mm base packer - no head gasket = top end race power
  • 3.00mm base packer – 0.5mm head gasket = mid to top end road race power
  • 2.50mm base packer – 1.00mm head gasket = mid range power
  • 2.00mm base packer – 1.50mm head gasket = low to mid end touring power


As you can see by simply swapping packers and head gaskets you can very easily set up your engine without been a master mechanic or engine tuner. You can fit want you want, take the bike for a ride, see how it feels for you, go home, strip it down, do a swap and try again until it suits you.

By doing it this way I'm helping to simplify and standardise how Lambretta engines can be set up and I'm helping to avoid the complicated set up that people are promoting in public forums with cheap ebay pistons then having to find a special con rod and make it fit.


Now add into the equation a very common 60mm crankshaft. Take what I've said about the standard 58mm crank. Using a 60mm with a 107mm con rod the piston will go to bottom dead center a further 1mm. Some cylinder kits like the TS1/RB/Mugello and others with a slightly longer piston skirt will hit the casing! It only has to just clip it or hit the mag housing and within half a mile the piston will explode. Now we don't want this so your only cure is to trim the skirt by 1mm (this is the same as a squish clearance setting, allowing for con rod stretch at BDC) ...... BUT this increases the inlet port timing on a piston ported cylinder (makes no difference on a Reed cylinder) and this can increase the inlet timing by too much which gives the knock on effect of lack of low down power and even more spit back which we want to avoid by improving our engines! Use a 62mm crank and a 107mm con rod and it's even worse! Use a 110mm con rod and you can see instantly there is another advantage, no need to trim a piston! No cracked pistons! Port timings can be kept standard or you can improve them!

The misconceptions of the longer 60mm crank is it has a 2mm longer stroke so the cylinder needs lifting an extra 2mm! This will lift port timings beyond a joke, this does not work! In practice the piston goes 1mm towards the crankshaft and will go 1mm towards the head! This alone creates more problems! Without extra head gaskets it would be difficult to set up a cylinder correctly. So what do we do with a 60 x 107mm crank? Really if you know what you are doing you need to lower the cylinder by machining the base gasket area, this will get transfer positionings to where they are needed, you may need to cut the piston, so you have two problems machining and Inlet timing problems. Then on the head side, the piston will stick out of the cylinder so you need to, either add a fat head gasket or machine the head different with a recess or both. None of this is easy to do in your back garden. Easy for me, it's my job and I know what I'm doing.

But change the rod to my standardised 110mm Yamaha as before and you don't need to trim the piston, so inlet timings can be controlled better, you can even fit a longer skirted piston reducing the inlet timings, again to improve the engine. You don't need to machine the cylinder base! Instantly you have advantages over the 60 x 107mm crankshaft. Now you can see to set up the cylinder is easy by using different packing plate thickness's. Setting up the head and getting clearance is much easier with thicker head gaskets and or recessing the head.

One to watch out for is a normal 60mm crank with standard or uprated 107mm con rod, these will have the disadvantage in that the con rod will likely hit the casings and some grinding to the casing will be required. Some say grind the con rod......... no, do not do this especially whilst it is still in the crank assembly! It will go wrong! Now with the Yamaha rod that I have been using since the 80's, 95% of the time they fit with no grinding, another plus for the right 110mm rod!


There are so many different scenarios of stroke, rod and crown height configurations. One favorite of mine in the past is the 115mm Yamaha con rod conversion, all the dimensions are the same as the 110mm Yamaha con rod apart from the extra 5mm in length. Now if your clever and can find a piston with a smaller crown height so no or a small packer is needed then you can use the double length theory to a bit of advantage, ok 115 isn't double 58 or 60mm but it's getting better. I've used the old Kawasaki 1970's H2 750 piston at 71mm diameter but has a compression height of 31mm. Lets go back to the sum - 115mm + 31mm = 146mm, bang on the magic standard of 146mm! When I found the Wiseco Jet Skis big bore kit piston it made sense to do this conversion in a few TS1 engines. The Jet skis piston is basically the same as the H2 Kawasaki, a bit to big for a cast cylinder but if you do it right it works perfect in an alloy cylinder conversion.


There are rods at 120mm which has the same dimensions as the Yamaha rod but if you used, say the short crown height H2 piston you need to add a 5mm packer, again pushing the cylinder towards the frame. We can can make one off MB pistons with smaller crown heights and we've made these pistons with 29mm crown heights to avoid using these bigger packers.


I think I've covered why cylinder packers are becoming more popular. I once spoke with Terry Frankland about packers when he asked 'what do you do for packers'? I replied I make them by hand, he couldn't believe I made one off packers for every engine tune I did. He was having them CNC machined at the time. As time went on and I got busier with these conversions I had them laser cut to suit my requirements. If hand making them is hard in production, laser cutting makes it easier but you get burn slag so you have to clean each side and de-burr holes, you can water jet cut these packers but again it's not perfect cutting alloy, we have a new process where packers are cleaned with no burrs, but his cost money!

Packer materials

Whats a packer made of? Well you can use all sorts of materials. Modern base gaskets in fibre has become a problem in the last few years, these can fail and I see it on many occasions. It doesn't matter who recommends a material or supplies them, we have fallen foul of the non asbestos materials in the past. We have tested many materials since asbestos was band and now manufacturer's use rubber as a binding agent, which I don't think is a good as the older gaskets. So we only use these gaskets on bog standard style engines. Today we prefer to supply alloy gaskets in varying thicknesses to suit the application used including the standard 0.5mm base gasket.

We use alloy for our packers, why Alloy? Well...... it's perfect....... light, reasonably cheap, soft and easy to work when trimming port shapes. Other have used plastic but I found them too soft. Alloy is soft and will crush, but that's the idea as gaskets they need some crush, but alloy doesn't over crush. BUT use them too many times and you will see they crush and you will see an indent of the cylinder or casing shape, if they go like this then change them for a new one.

Packer preparation

An alloy packer is perfect, it will seal if fitted correctly and prepared. Some of our packers are pre prepared to remove any slag from the cutting process. By prepared I mean each side of the packer needs roughing up, this will allow the sealing process to work with whatever sealer you will use. To prepare each surface I suggest you use either; emery cloth, Scotch Brite or wire wool or anything to just rough the alloy up to give a dry matt look. Our new packers come water grit cleaned and give this matt effect. Fit cylinder packers dry with no grease and always use a silicone sealer both sides. If done right they will not leak.

What packers can we offer?

Well whatever your engine set up is we should have it. You can use the perfect packer size or you can, if you want add a base gasket on one or both sides but I don't suggest it, I personally don't recommend standard paper style gaskets. These days I prefer to use an alloy gasket for the strength over the paper materials. Especially with some of the over tuned casings we see these days with some of the super over tuned cylinders. We offer sizes without having to have special sizes made or have packers machined down as this would be near impossible. We have these sizes for both small and large block engines.

Small block


Large block



You can see 3.50mm is not available as its not made, 3.2mm is, which is an imperial size but not needed, you can use a mixture of any of the above to get the perfect size. What is the perfect size? Well it comes down to how anal you are setting up port timings. I'm anal in that way but then no one complains how my engines run so it's worth it if you know what you are doing. Always silicone seal each side of the packer to guarantee a seal.


I've looked at all the kits on the market, both small and large block, standard, tuned and with the potential to over tune them. The limiting factor is always the engine casing transfer ports and the cylinder transfer ports. I've looked at each cylinder casting and tweaked the packers to suit. You can get cylinders with massive transfer feeds and small sealing areas. You can get welded casings with massive transfer feeds. Of course you can get perfect standard transfer feeds and slightly tuned transfer feeds. You can get some cylinders where the transfer feeds are a bit bigger than the casing and if used without a gasket they would be open to thin air! Not only is there more advantages using a 110mm con rod but there are advantages in using a cylinder packing plate. Our gaskets are diner plate size, these match our Race-Tour cylinder where I've increased the base area size to stop distortion. Our packers are cut to suit most cylinders as an average so a packer fits with a standard cylinder, TS1/RB/Rapido/Mugello etc. If you have tuned casings and transfers like a old skool stage 6 then it's a case of matching the packer to the casing and packer to the cylinder. This is done with tuning tools, dremils, drills and cutters or hand filed. This is easy to do, fit the packer to a casing using studs, scribe from underneath, do the same on the cylinder by dropping bolts into the holes and scribe again, then its a case of matching the two scribe lines either side.

If we are looking at the advantage of using the 110mm con rod and the improvement the engine can gain by the versatility of using different cylinder packers at some point you will need different cylinder head gaskets. Ok depending on who you talk to, people love or hate head gaskets. Some prefer not to use a head gasket, why? Well there are advantages of no head gasket. When the head is torqued down there is no crush on a gasket! You can if you know what you are doing dowel or pin the head to the cylinder to centralise the head to the bore. I went one step further on the idea perfected the spigotted cylinder and head in the 80's/90's, by this the cylinder is machined with a top hat spigot shape, do the same to the head and you make sure you have the all important squish clearance and you get perfect centralisation without a possible weak link........... the head gasket. But again this is only any good if you have a workshop and machines to do it. Some cylinders are made in this way now, but you have no movement, especially if you are using the fixed standard con rod! So this is old hat. So back to adjustable packers and head gaskets – you have adjustment this is the beauty of the set up!

Ok there are some very poor head gaskets on the market and surprisingly these are made in the UK, they melt and blow and then distort the head and cylinder and we all know the end result. We haven't sold these since the early 90's but others never learn and still sell them. Lets not talk about others we are talking the way we do it. We have sourced our head gaskets from all over the world, in the end we came back to the UK and use a local company who we've worked with on special shims. The material is a mixture of soft and tough and does the job in hand.

So what head gaskets can we offer? We can offer the same thickness head gaskets for small and large block cylinders and we can offer gaskets to suit all cylinders ever made including the MB Race-Tour cylinder with the extra 4 bolts holes, which addresses the issues where Innocenti got it wrong and made the large block engine with a wide gap between the stud holes holding the head on and it's at these points gaskets can blow.

With good quality head gaskets you don't get many head gasket problems so the argument of head gasket or no gasket starts to equal out, which is now backed up with stronger and thicker cylinder heads available in the last few years. Get rid of weak Spanish and Innocenti 200 cylinder heads and swap them for thicker stronger Indian cylinder heads (yes I did say improved better Indian cylinder heads) and of course MB have made thicker cylinder heads for 20 years to improve the head gasket problem! If we are talking about standardising an engine set up which suits the masses then you have no choice, you need a head gasket of varying thicknesses. Now with a bigger variety of packers and head gaskets you can work at that all important squish clearance by simply replacing a thicker or thinner gasket. Ok there's still an argument a head gasket can become offset, it could be said the gasket gets in the way of the piston at TDC or it could be oversized creating a problem. In reality non of these arguments come into how a Lambretta runs so it works with little or no problem.

We now offer these head gaskets


125 (52mm bore)
150 (57mm bore)
175 (63mm bore)
190 (64mm bore)
195 (65mm bore for Race-Tour cylinders, 8 stud)
0.5mm RT
0,7mm RT
1.0mm RT
1.2mm RT
1.5mm RT
2.0mm RT


200 (66mm bore)
210 (69mm bore)
225/230 (70mm bore)
225/230 (70mm bore for Race-Tour cylinders 8 stud)
0.5mm RT
0.7mm RT
1.0mm RT
1.2mm RT
1.5mm RT
2.0mm RT
230/240 (71mm bore)

As with cylinder base packers always rough up the surface, we suggest using any of the loctite range to seal the head gasket. We don't recommend any silicone sealer as we have seen problems but other have it work with no problems. Head gaskets come in different internal diameters to suit different piston diameters. Usually we make a head gasket 1mm over size of the piston diameter. This is done to allow for mismatching of cylinders, gaskets and heads been just slightly offset in the factories machining processes. On assembly if you do it with the bike on it's stand and rear shocker removed as you assemble the top end, the gasket and head can drop towards the floor. You can mess about and slightly nip up the head, check the squish clearance and tap the head in the direction required BUT this does not stop the gasket sagging which may hit the piston. If you want to do it right then assemble the top end with the engine vertical (out of the frame of course) this bench mounted way means you can position the gasket so it's not a problem, then do the squish test set up.



From fat to thin we have them all

Trim the packer to the casing transfers so there is no overlap and restrictions

Showing our packers are as large as we can get them to help seal all casings and cylinder sizes

Where possible when we do engines and cylinder kits we mark each cylinder with the packer supplied in our tuning information

Packers can be roughed up to a matt finish to help sealers bond by using any of these cleaning items

Piles of packers in different sizes

Different shapes and sizes suit small and large block engines

Race-Tour head gaskets with extra stud holes, these are cleaned by a new special process which gives us a matt finish required

Mark Broadhurst 4.2013 if you have any questions please email