What's the best method to downsize a piston?

I think it is more sensible to re-hone or bore the cylinder..this is pretty cheap to get done.

Doing it by hand with wet&dry paper should only be a last resort, although, with care and some engineers blue it is doubtless feasible but somewhat masochistic.

On problem with turning down a piston is that the grooves will do longer be the same depth....

I'm doing a performance comparison between a coated piston and an uncoated piston on a single cylinder two stroke and it's a lot easier and cheaper if I can use the same cylinder to test both pistons.

Yeh...but your test will be meaningless if the pistons aren't 'as standard', e.g.A hand finished or turned down piston won't perform the same as the 'off the shelf' one.

So 'penny pinching' will actually be counter productive.

I eagerly await the comments of the guys who do this stuff professionally.

Del

Thanks Del but our problem is that both these pistons have been ceramic coated on their domes and friction free material coated on their skirts. One is a Wiseco performance piston and the other is a stock Yamaha piston. The Yamaha one was 0.5mm oversize and the Wiseco one was 0.020in oversize when they were both sent to the coaters for coating. We didn't actually measure them but the Yamaha one came out of the cylinder that we're using and we figured the Wiseco one would fit the same cylinder since those two measurements are theoretically only 0.0003in difference so we figured they would come back with the same minute difference in their dimensions. Now however, we find that the Wiseco one, which is theoretically the bigger of the two, goes in the cylinder just fine but the Yamaha one doesn't go in there anymore. So we want to remove the skirt coating back to a point where we get the right clearance again. The coating doesn't go on the ring area so the ring groove depth shouldn't be affected. The coating people told us before they started that they can't control the depth, it's whatever it is, and you have to bore to suit they said. But we don't have another cylinder and we don't want the expense either. We want to compare the performance of these two pistons against a stock Yamaha uncoated piston, which is what we have in the engine at the moment.

I see....

Sorry, no clever solution to offer

Bore and sleeve the cylinder. One sleeve for each of the coated pistons.

Once you start to work the piston, you will fubar the ceramic coating that sounds to be the reason for the test in the first place.

Or, redline your prints and try to find fault with the ceramic coater for oversizing the piston. Hit them up for a new properly sized piston.

go with the Wiseco. Bore to correct fit and your YZ will run like a raped ape

...maybe glass-bead it off? Assuming it's visible... I know you can probably glass-bead canyons and valleys into the pistion too, but perhaps someone who's skilled in the art could get the coating off but leave the pistion more or less dimensionally correct..?

cheers
RF_G

The coating doesn't go on the ring area so the ring groove depth shouldn't be affected.

If the o.d. of the piston has increased then so has the ring depth.

Anyway, .0015 oversize = .00075"/side by hand is not out of the question. I would mount the piston to turn in a drill press or ideally a lathe if available and use the absolutely finest material available at a low RPM.

Wouldn't happen to be pistons for the Road Star Warrior? If so I would like to see the outcome.

Yes a regular machine shop can turn this down accurately. An automotive shop could set it up on the crank polishing machine or and regular machine shop could do it on a lathe. Specify the final dimension desired with whatever tolerance you need. So cost you less than $100 if it is a small local shop.

Sorry, no, it's for a Yam Blaster

I gather you haven't miked this piston?

This would best be done on a precision cylindrical grinder.

From High School Auto Shop- Aluminium pistons are cam ground - ie not round. Aluminium has a large coeffecient of expansion and I remember it having something to due with the large reinforced area arround the piston pin vs the thin skirt on the other sides. It is designed to be oval shaped when cold, but to become round after it warms up. Probably best to take it to a shop with the capabilities to cam grind pistons.

Neil

If your piston is indeed .0015" oversize and not .015" then an experienced lathehand can polish off the .0015" on a lathe with successively finer emerycloth.

Mirco.

As one other post already stated correctly, the pistons are not supposed to be round and you will loose your tolerances.

The bore however is round so you can very easily hone the bore even with a cheap sprungloaded machine hone device in your mill. This way I always honed my sinter bronze bushings (diameter 30, 35,40 and 45mm and 130mm long) after pressing in the housing and it works very well. Guess it will just as well on piston bores.

The outer profile of a piston is machined to be both oval and barrelled top to bottom on a cam or computer controlled machine. I am presuming the pistons are aluminium alloy. The ring lands are usually circular and are reduced in steps towards the top where the piston is hotter towards the top. Around the piston pin area the skirt is oval due to the different stiffness between the thrust and pin axis. Below the piston pin the skirt becomes less oval towards the bottom. The piston skirt is barrelled top to bottom to give the best hot fit in the bore and the top and bottom are usually reduced minimally on the thrust axis to give a wedging action for the oil film. Profiles are usually developed from design and running experience. Incorrect profiles can quickly lead to scuffing, bore wear or seizure.

DO NOT ever touch or modify the outer profile by grinding or with any grit if you expect to get reliable performance out of the piston. Even with the best cleaning you will never remove all the grit and it can damage the piston and bore. Hand finishing will never remove metal evenly all over. Talk to a reliable piston manufacturer who has a research and development department like Mahle or Kolben Schmit and does not just mass-machine profiles to order, they will tell you the same.

Rebore the cylinder is the quickest way to achieve reliable comparison figures as you then run both pistons on a new unworn surface honed correctly.

Im wondering how much the weight of your piston has changed. It might not be much at a few thousand rpm, but at higher rpms it could be a problem. Have you had your rotating assembly rebalenced after coating the pistons?

Not all pistons are oval/barrel shaped. You could turn it on a lathe to the dimension you require and file your ring gaps to make everything fit. Back in the day, if you had a piston that was too small alot of guys would knurl the piston which would give it a larger OD. You wouldnt be able to do that with a non cylindrical piston. I would mic out what you have and see.

Hi again and thank you all for your comments. We have miced the pistons and there is a 0.0015 difference between the two since the coating was applied, the Yamaha one that originally came out of the cylinder being the bigger of the two. Whereas, going by the difference in the oversize numbers, 0.5mm for the Yam and 0.020in for the Wiseco, the Yam one should be the smaller of the two by 0.0003in. So we're assuming the coating on the Yam piston is thicker, which is what the coating people told us might happen before we started. The Wiseco piston is a good fit in the one cylinder that we have for our test and so we now want to remove the skirt coating from the Yam piston but without damaging the coating on the dome and without altering the skirt dimensions. On reviewing the replies we received it seems the best answer may be to get the skirt coating ground off by a specialist or bead blasted if grinding won't work without damaging the dome coating and that's what we're going to shop around for now. Thanks everyone for your input.