Piston spacer washers

[stonecat]

Hi Guy's

This is my first resto project it's a LIs 150 1964 vintage bought in from Italy - it had had a real tough life and I suspect ti was used by kids for riding around the fields etc, the upshot is that it needed among lots of other items a new barrel and piston and crankshaft.

So I'm putting this thing back together and see there are no piston spacer washers either side of the little end and with about a 1mm of space either side figured I should put spacer washers either side.

The problem is after extensive looking I cant find any anywhere !

My question is do I actually need them ? and why cant I buy them ?

Any Ideas ?

Richard

[dickie]

You could use a 22mm wide small end bearing.

I suspect that's what most do now and is why you can't buy the 1mm shims anymore.

I had the same problem last year.

[Adam_Winstone]

No, a wider bearing gets rid of bearing movement but does not affect the side to side movement of the conrod, which is what the shims were designed to eliminate.

Have a read up on this matter and you'll find almost as many opinions but Sticky's manual gives as good advice as you'll find anywhere.

Adam

[dickie]

Why do you need bearing movement side to side? I appreciate that a small amount is needed for effective lubrication, but would have thought 0.5mm would suffice?

I see what you're saying about conrod movement, but if the big end is shimmed then surely it's a bad thing to shim the small end too?

Of course I have no idea what crank you have so this is pure conjecture. What crank do you have?

[Adam_Winstone]

^... yep, whether the crank is shimmed/aligned at the bottom end is the key issue. As Sticky's states, providing your conrod is aligned at the big end then you don't need small end shims.

The bearing alignment is normally not an issue, if the rod's aligned, but the wrong combination of floating rod moving to one side and narrow small end (or wide gap between piston bosses) moving to the other side can mean that the ends of the needle rollers run in the small end eye. This can quickly result in the bearing failing or the ends of the rollers wearing into the small end eye and/or the gudgeon pin.

Wide small end bearings simply ensure that the small end eye is always running on the bearing face, rather than the edges. Because the bearing fits through the rod's small end eye, it plays no part in rod alignment, which was the whole purpose of the shims. Address the alignment by fitting a shimmed big-end type crank and 'job done'. However, use a crank/rod/piston assembly (let's not forget that the original pistons for small end shims are machined accordingly and have a small square on the crown for identification), just leaving the shims out means that the alignment is not controlled small end or big end.

Everyone has an opinion as to whether you can just leave the shims out but successive editions of The Official Home Workshop Manual state that this must not be done, spending considerable text space on this specific topic. This is a debate that comes up regularly, then goes around and around until people get bored and ignore it.

Best of luck with it.

Adam