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Message started by Oldfeller on 12/02/09 at 03:02:21

Title: Saving worn piston/bores
Post by Oldfeller on 12/02/09 at 03:02:21

Pistons commonly wear out on our bikes and things get slappy when the mileage gets high. If your bore is still well within service specs for out of round (coke bottle wear effect) and your piston is not scored consider rematching your existing parts to the correct running tolerance using a piston coating on the piston itself.

Bumping the piston up to a correct running tolerance to the worn bore is much less expensive than reboring the cylinder and buying an oversized piston. You can save nearly $100 by recovering your existing parts by coating the piston.

There are many shops offering this service (hit your browser with "piston coatings") so shop around to find the best deal closest to you.

Swain has been in the business since it started and their low friction coatings seem to be able to bridge more gap than some of the others can do.

http://www.swaintech.com/store.asp?pid=10964

Note: if your piston comes back a little bit tight to the bore or has a high spot on it, fine 600-800 grit wet or dry very carefully applied to the tight spot can make up a proper fit again.

It can be sanded gently where needed, it is just a very tough slippery baked on paint job after all ....

Title: Re: Saving worn piston/bores
Post by srinath on 12/02/09 at 12:36:38

Correct, I am planning this on a GR ... and you coat the top of the piston with a ceramic coating that reflects heat.

However there is a catch. The piston gets fatter with coating, the ring land stays the same old size. The bore is a wee bit larger, obviously that is why you can coat the piston. What happens is that you're rings will be too small for the bore. If you're going up 1 size in the rings, you need to make sure you can fit that size rings to the worn piston. Not all rings will fit into a .25 under piston, that way you ahve to replace both the piston and the ring set. So coatings are irrelevant.

Having said that, the coating on the sides take in oil and have an anti gall and self lube quality, the tops send heat back into the chamber, valves and heads also can be done that way so everythign runs cooler and its all lubed better and wont gall and tear.

Title: Re: Saving worn piston/bores
Post by Boule’tard on 12/02/09 at 13:02:10

Do you recommend honing the cylinder at the time you coat the piston? I wouldn't trust myself with the hone stones, but maybe a quick once-over with a bottle brush would be good. I'm assuming the old "cross hatch" pattern we were taught in auto mechanics hasn't been rendered obsolete by new materials.

Also, aren't there cylinder coatings ("nickasil" or whatever) that would have a similar effect, in case you needed to take up even more slack? If so, is it a good idea to mate a coated piston with a coated cylinder?

Title: Re: Saving worn piston/bores
Post by srinath on 12/04/09 at 08:16:34

If you nikasil the bore, you're building the bore back to original size.

Now you cant coat the piston unless you got a undersize piston.

The cross hatch is still important. It will hold oil and get the rings seated. In nikasil bores they still hone it, except they use a diamond hone to do it. It so hard.

I actually ahve heard something but am yet to find out if its true or not.
Modern sport bikes dont ahve steel liners in the cylinders. They are nikasil coated right on the aluminum wall. I have heard since there is aluminum piston in an aluminum cylinder, you can run a lot less clearance. Almost down to 0. No differential expansion, just that they heat at different rates, and come to different operating temperatures, but other than that they are not needed to be clearanced at all.

I have an xs motor I may coat the pistons on, the bore will just be lightly honed. There are people who have dimpled the piston skirts and had good results, anything you do to keep oil where stuff slides up and down is good.

Cool.
Srinath.

Title: Re: Saving worn piston/bores
Post by srinath on 12/04/09 at 10:09:43

OK apparently the coating is 3-5 thousanths of an inch thick. That translates to 2 10th's of an mm in diameter.
A piston is usually .05 mm under the bore size.
So a 75mm bore will need to be 75.20 to take the 74.95 mm piston coated with the moly/graphite.
Will need hand gapped rings for the .25 larger size. The 75 rings wont work.
Also from what I have heard, the over sized rings are made of higher tension material. Stock sizes are sorta, flaccid.
Cool.
Srinath.

Title: Re: Saving worn piston/bores
Post by Oldfeller on 12/05/09 at 03:33:57

Nikasil on a bore our size is quite expensive. You can primo coat a used piston (which has generally more wear on the skirt than you would think -- measure it and the bore then request the build up that you need to get back to running specs) for less than 1/4 the cost of the nikasil job on the bore alone.

Main advantage here is selective fitting -- you can hand sand the piston to a perfect fit if you spec a tiny bit of extra skirt build up and are able to do very fine hand work like bearing fitting.

Our bikes actually run OK with like twice the spec'd skirt clearance (and they do for many miles before they start to slap knock) so you have more windage room for a roughly toleranced spray on build up job than you would think.

Remember, any piston scoring up in the rings area means either a new piston or a weld up job by a pro -- both are quite expensive so if you do a high mileage pull down do it just as soon as you think you hear a slap knock then you might avoid the hundred plus dollar cost of fitting a new piston.

Title: Re: Saving worn piston/bores
Post by Christof13T on 09/23/17 at 06:44:00

Holy Ancient Thread Batman!

Yeah I know...

So Oldfeller...
Do you happen to know how the cylinder to piston clearance is measured?
Is it .0020-.0024" per side? or across the whole I.D. of the bore?
.0020-.0024" per side seems too loose...
.0010-.0012" per side seems too tight...

My `06 with 4k miles on it just went slap happy.
The jug can be saved with a punch job, and if I know what the target numbers are my buddy and I can do the cutting ourselves.

Thanks for any wisdom you can share on my dilemma.