Small end play

[Nbowe]

Have fitted a mec Eur crank to a gp150 casing.i have a standard Beedspeed 190cc iron barrel and piston top end. I understand that the big end is shimmed on these cranks but do I need to shim the small end. There does seem to be quite a bit of play at small end when piston is fitted.
Cheers

[NorthernJordan]

If the big end is shimmed, do NOT shim the small end!
I assume the play is side to side, rather than forward and backwards. If so that is fine as it will allow the piston to center itself on the bore without there being and side ward stress.

[Nbowe]

yes its side to side thanks for your help

[dickie]

Can you post a picture please?

By side to side, you could mean one of two things.

If the small end can float on the bearing, then that's fine. However, I've seen small end bearings with around 2mm float either side within the piston. This means the bearing can float and the small end is only running on part of the bearing. Not good.

[HxPaul]

If there is room for a longer small end bearing MB and Cambridge Lambretta sell a Yamaha 16x20x22 small end bearing to fit a Lambretta and it stops the movement that you get with the original bearing on the gudgeon pin.The standard original one is 16x20x20.

[Warkton Tornado No.1]

Post by Nbowe » Wed Dec 28, 2016 12:28 am

Have fitted a mec Eur crank to a gp150 casing.i have a standard Beedspeed 190cc iron barrel and piston top end. I understand that the big end is shimmed on these cranks but do I need to shim the small end. There does seem to be quite a bit of play at small end when piston is fitted.
Cheers

You could do worse than research the (very) old Lambretta recommendations. However, a bit more recently, A. Graham Bell advocates critical clearances:

Big end 0.5 to 0.6 mm (0.020 to 0.024”)

Little end (shake) 0.8 to 1.3 mm (0.031 to 0.051”)

I hope that helps.

[Nbowe]

The small end bearing floating around was exactly what I was worried about.I'll get some pictures up as soon as I can cheers fellas

[dickie]

If there is room for a longer small end bearing MB and Cambridge Lambretta sell a Yamaha 16x20x22 small end bearing to fit a Lambretta and it stops the movement that you get with the original bearing on the gudgeon pin.The standard original one is 16x20x20.

The ones I'm talking about still had about 4mm (total) float with a 22mm bearing.

[johnnyXS]

from what i remember reading about this , the earlier conrods had sufficient float on the big end to allow the piston to find a natural centre in the bore to avoid asymetrical wear.

Later cranks the conrod had shims fitted either side to reduce the float of the conrod so to compensate ,the float was switched to the piston small end.
You must have the float one end or the other .
If the big end is shimmed then the small end must have the correct tolerance to allow the piston to centre itself in the bore

Ahh i've just found the reference I read originally its MB Dev.
This link goes to the correct page and the explanation and instructions are half way down the page under the heading 'Checking a Small End bearing'

http://www.mbscooters.co.uk/info/setting-up-*-engine-carbs-3/setting-up-*-cylinders+163.html

[a-teamlambretta]

hope you dont mind me jumping in , if you have no option but to use shims at the little end and shim choise is limited , would it matter if the shim used was bigger in diameter than the little end of the rod ??. the reason i ask is that i have a piston with a 12mm pin and will need shiming up. the best solution i can come up with for my piston is to use a pair of thrust washers which are hardend and ground/polished spring steel with a 12mm inner diamer x 26mm outer x 1 mm thick (they come in standard sizes and this is the nearest) , its the outer at 26mm which is going to be bigger than the little end of the rod which is around 22mm on the ground polished area. cheers simon. ps there are no shims in the big end of the crank.

[Warkton Tornado No.1]

This comment is purely to communicate & hopefully clarify my understanding of the clearances in generic Lambretta designed/based crankshafts, not to dispute anything that anybody has contributed. Please see it only as my opinion through experience & it may even overlap something already said, possibly, for which we should consider ourselves fortunate in ‘singing from the same hymn sheet’

It is only, like all other opinions, merely an attempt to help.....


Over the years, I have had a lot of crankshafts that have been rebuilt through necessity.

Often, it will be to utilise a con-rod that will suit the piston & porting layout.

As the OP has asked, he is quite correct to be concerned about the importance in the control of the side float, whether that is achieved @ the big end or little end.

When Innocenti increased the side clearance @ the big-end to allow a leaner petroil mix, they controlled the con-rod float via shims fitted @ the small end.

As things have progressed, the realisation that this resulted in less material for the webs to fit the pin has meant that crankshafts now tend to control the float @ the big end. Ultimately, a con-rod may have its sides shaved to help the scenario & no shims whatever utilised. Even that may still require the pin to be welded into place for extreme circumstances, much as I'd like to place faith in an interference fit plus Loctite....

Whatever figures are stated for side float from whatever source, it is wise to adhere to the limits selected.

If a brand new crankshaft does not control side float to the figures that the owner put’s their faith in, it should be rejected, or, @ worst, shims fitted @ the little end.

Excess side float will wreck an engine.

[HxPaul]

If there is room for a longer small end bearing MB and Cambridge Lambretta sell a Yamaha 16x20x22 small end bearing to fit a Lambretta and it stops the movement that you get with the original bearing on the gudgeon pin.The standard original one is 16x20x20.

The ones I'm talking about still had about 4mm (total) float with a 22mm bearing.

the conrod will still have 4mm side float with the 16x20x22 or the standard one.

[HxPaul]

I think you're missing the point paul

It depends on the piston that you are fitting. You need to measure the piston casting then decide if piston shims are required depending on the crank being used

No it doesn't.If you have shims on the crank you dont need them on the piston,irrespective of the type of piston.

[Nbowe]

Sorry fellas didn't mean to start an argument. This is only the second engine I have built. The last one was a gp200 which was a standard rebuild of original barrel and piston but with a racing crank which was not shimmed at either end which someone has rightly said on here.
The original 175 piston which came off this engine was a slightly different design to the one I am putting back. This one is tapered on the inside where the gudgeon pin sits whereas the original wasn't. There was some play in the original but there seems to be excessive play on the new one . When you rock the small end from side to side you can see about 2mm of the bearing at each end as it floats about independently of the small end.Thanks everybody for their interest in my dilemma.
By the way I tried to load a pic but the file was too big and won't load.

[Steve J]

Correct me if I'm wrong, but I'd always thought that the piston shims were introduced around 1967 for the SX150 & SX200 engine, using a conrod with a much narrower big end. The big end side play on those was enormous, and relied totally on the piston shims to keep it aligned. I've just checked the big end sidefloat on an old SX150 crank, and it's about 2.25mm. Unless you have that sort of clearance, you don't need piston shims.

[Steve J]

Wrong on that - there seem to have been two types of MecEur cranks, one with the big end shims and a cheaper one without, which I'm pretty sure is the type I've got in my GP200. The point is this - both MecEur types are similar to the Indian GP types and the original Innocenti types (i.e. the ones before 1967), where the sideways alignment of the conrod on the crankpin is controlled solely by the big end, and the relatively small sideways clearance at the big end. None of these types (with a standard type of piston) requires any shims at the small end to centre the conrod.

As the OP has an iron 190 barrel, I'm assuming it's based on an overbored Tv175 or maybe Indian GP150 cylinder, fitted with an aftermarket 64mm piston. This configuration is probably very similar to an Indian Stage 4 200 setup, so is unlikely to put out a huge amount of power. The introduction of shims on the aftermarket cranks seems to have required by the higher power produced by kits such as the TS1, where it became necessary to address the friction between the sides of the big end on the crank and the webs themselves. Having the cast webs and conrod in direct contact at high rpm and high power is guaranteed to wear the surfaces rapidly. OK, so it happens on lower power engines to some degree - I've seen that sort of wear on old Li cranks, but it takes many thousands of miles to get to that state. On an iron 190, whilst it might be better from a purist's point of view to change to a better crank with big end shims and a Japanese conrod (for example), the MecEur crank with or without shims will be fine as it stands.

[Nbowe]

The big end is shimmed so no movement there. The small end bearing and Conrod measure 20mm and the gap in the piston casting is 24.5mm so about 2.25mm movement either way. The Conrod doesn't travel that far but the bearing does. Mb site doesn't really convince me either way on wether I should shim it or not

[HxPaul]

As I've said before,if the crank is shimmed the piston shouldn't be,get a 16x20x22 small end bearing to take up the space.

[dickie]

The big end is shimmed so no movement there. The small end bearing and Conrod measure 20mm and the gap in the piston casting is 24.5mm so about 2.25mm movement either way. The Conrod doesn't travel that far but the bearing does. Mb site doesn't really convince me either way on wether I should shim it or not

I think you're thinking more clearly than people are giving you credit for. You're dead right, the bearing should not be rattling around with such little axial restraint.

If you can find shims that will stop your small end bearing from floating so much then use them. Otherwise I'd get a new piston with a gap that's more appropriate for either a 20mm or 22mm wide bearing.

I went through this on my first engine build and couldn't get a straight answer as everyone was referring to a different issue to the one I was actually trying to address which (I believe) is the same one that I have.

There's are two VERY distinct issues here. Firstly that the small end bearing needs enough axial float to stop it overheating but not enough that it runs through the small end of the con rod. The second very different issue is that the Conrod needs to be restrained at one end only (for the same reason) but not both ends or it will be twisted.

To be honest though the op needs to post a photo so that we all stop blathering on about different issues and getting on each other's nerves.

[Warkton Tornado No.1]

I think Dickie is correct in realising the nub of the problem. This has become more evident as the thread has progressed. My own initial response was taking the OP too literally.

Post by Nbowe » Wed Dec 28, 2016 12:28 am

Have fitted a mec Eur crank to a gp150 casing.i have a standard Beedspeed 190cc iron barrel and piston top end. I understand that the big end is shimmed on these cranks but do I need to shim the small end. There does seem to be quite a bit of play at small end when piston is fitted.
Cheers


You could do worse than research the (very) old Lambretta recommendations. However, a bit more recently, A. Graham Bell advocates critical clearances:

Big end 0.5 to 0.6 mm (0.020 to 0.024”)

Little end (shake) 0.8 to 1.3 mm (0.031 to 0.051”)

I hope that helps.

But I can't read minds or see around corners either!

Trying to help further, though, as I'm sure we all are, a small end bearing that was 24 mm long would appear to solve what the actual issue is. I'm guessing! Unfortunately, such a bearing does not exist.

The bearing of 22 mm long as suggested by HxPaul would be better than 20 mm but even that will float to one side. Actually, not better - Essential.

However, if it is centred with the use of piston shims, it will only be centring the bearing, not the con-rod, which is fine (HxPaul) especially as the big end is already doing that job.

Sufficient little end lubrication will prevail as there will be 1 mm each side to catch the mist.

Simple! Who needs Christmas quizzes in newspaper supplements?