LCGB Forums

Hi all,
I'm looking to replace the rings on my piston. I've got a pair of casa ones 57mm x 2.5mm, however the gap allowance is 0.5mm on one and the other 0.3mm. If I were to use a set of 57.2mm oversize there would be a lot of filing to get the to fit in the piston. I have included an image of the piston. I can't make out the markings but one could be 0.30. Can anyone shed any light and advice what's the best pistols to go for.
I have some old NOS ones from the 60s would it be worth using these or better to get newer ones, doe it matter?
The old rings I took out had a gap of 1.1mm,

Before anything else, you should assess the cylinder & bore, prior to spending any more money & time. After all, you may need a rebore & fresh piston. Hopefully not…..

You should measure the piston across the widest point @ the base with something accurate like a Vernier calliper. That will give you a proper indication of the size, which will likely have started off @ a size such as 57.0 mm, 57.2 mm etc although increments of 0.25 mm are not unknown.

Then check for the running clearance within the bore by fitting the piston & slipping feeler gauges between the base of the piston & around the top of the bore.

That clearance is critical so without leading you as to what it should be(!) if you report back here, or PM, & I’m sure that one of us will respond as quickly as possible

Hi WT1,
I've just had a measure up and results are as follows:
Cylinder bore is 57mm, (56.891 - 56.997mm)
Piston 56.642mm at crown, 56.896mm at skirt.
How do these add up.. anything still serviceable?

Sorry, forgot to add I can get a 0.10mm feeler gauge snug between the two at the top bore and skirt.

0.10mm is fine. Can you get 0.15 in there. If not, then you should be OK.

Hi Chris, no I can't get anything more than 0.1mm in there and that's at a very snug fitting.

Dan
Your piston measures up ok but looking at the picture you posted I would check the integrity of the piston ring pegs as they look a bit dodgy. This is typical of what happens when rings are allowed to wear as far as yours have. If the pegs are the old brass type then I wouldn't re-use this piston. Call it a day and fit a new one.
New rings should be gapped to 0.23mm for this bore.

As Chris said, that clearance is good, but by no means tight. For the record, with a 66 mm bore, I aim for only 0.0035" (0.089 mm) so don't go mad if you do run a three leg honer down the bore to deglaze it. If you do so, remember to deburr all port edges.

Your piston should clean up nicely with some light abrasive pads ("Scotchbrite") Check that the windows in the skirt are not cracking & remove them if they are. All piston edges around the skirt can benefit from chamfering & polishing in general, particuarly so that oil on the bore face is not scraped away, as well as the inside face having more of a knife edge to function better closing the inlet port.

To get the optimum ring gap, personally I would buy oversize & regap them. It really doesn't take long with a good file, in fact it's all too easy to go too far, though I would advise good lighting & even a magnifying glass to get things right

Fast n Furious wrote:Dan
Your piston measures up ok but looking at the picture you posted I would check the integrity of the piston ring pegs as they look a bit dodgy. This is typical of what happens when rings are allowed to wear as far as yours have. If the pegs are the old brass type then I wouldn't re-use this piston. Call it a day and fit a new one.
New rings should be gapped to 0.23mm for this bore.

Hi FNF,
Thanks I will have a look at the condition of the pegs late, they could well be the brass type ones too. are these generally no good or thae fact that they could be worn now? Any advice on a decent brand to go for, also the markings on top are very feint but it appears to be the remains of a 030 marking on the cron, would this make the piston a 1st oversize? if so should I be replacing it with the same or stepping up to the 2nd oversize?

Warkton Tornado No.1 wrote:As Chris said, that clearance is good, but by no means tight. For the record, with a 66 mm bore, I aim for only 0.0035" (0.089 mm) so don't go mad if you do run a three leg honer down the bore to deglaze it. If you do so, remember to deburr all port edges.

Your piston should clean up nicely with some light abrasive pads ("Scotchbrite") Check that the windows in the skirt are not cracking & remove them if they are. All piston edges around the skirt can benefit from chamfering & polishing in general, particuarly so that oil on the bore face is not scraped away, as well as the inside face having more of a knife edge to function better closing the inlet port.

To get the optimum ring gap, personally I would buy oversize & regap them. It really doesn't take long with a good file, in fact it's all too easy to go too far, though I would advise good lighting & even a magnifying glass to get things right

Thanks for the info, I'll give it a light hone and re measure. Then consider replacing the piston to achieve smaller clearance, any recommendations on brand to go for?

Personally I believe that your piston is standard size; any other markings probably represent the grade/tolerance.
I would source a reputable dealer and take the cylinder with you to ensure you get the correct piston if condidering replacing it.
A rebore and quality 1st oversize piston will cost you about £80. Peace of mind when you are hurtling along at 50mph wondering if you should have replaced the piston when you had the opportunity.

I don't advocate spending other people's money so if you are happy with the existing piston, just buy new rings, oversized as recommended.
I wish you well.

Thanks Chris, It makes sense to me to consider a new piston. So to reduce the clearance would be going 1st oversize and a rebore.

A new standard piston would give me the same (or there abouts) clearance as I currently have. (unless the new piston comes in slightly over my original) 0.1mm is still ok.

Dan,
Your piston is 57mm standard bore. It looks to be an original AE piston made in England by Hepolite or Borgo in Italy.
The markings on the top of this piston are actually the letters SC. This is an abbreviation for the italian word "Scarico" which means Exhaust. So, this is the side of the piston that faces the exhaust port.
Original pistons are hard to come by, but are available from discrete sources.
PM me if you want any further info on where to acquire.

I've got a new piston and ring kit, bore to piston clearance is now 0.04mm. I'll check the clearance again after the bore is honed. Is 0.089mm (3.5 thou) the clearance I should have?

Danbretta wrote:I've got a new piston and ring kit, bore to piston clearance is now 0.04mm. I'll check the clearance again after the bore is honed. Is 0.089mm (3.5 thou) the clearance I should have?

Personally, in the majority of instances with a non-alloy cylinder with nikasil bore, which should be tighter, I use the standard Lambretta workshop minimum figures. I know the minimum for a 200 as 0.0034" (hence 3-1/2 thou") but not the 150 without checking. For your 150, it should be less than 0.0034" & I'd say 0.003" if I was a betting man....

The workshop manual states 0.034 to 0.046 if I'm reading it right. Although some recommendation I've just read say 0.051 to 0.064mm.

Danbretta wrote:The workshop manual states 0.034 to 0.046 if I'm reading it right. Although some recommendation I've just read say 0.051 to 0.064mm.

Yes, it would be useful especially to stick to either metric or imperial in some of these discussions.

The workshop manual figure assumes that the piston is an AE piston and that the air cooling system is working optimally.
The larger figures quoted by others are more relevant to the slap happy crappy pattern pistons out there.