Jump to content
Sign in to follow this  

To replace or not to replace?

Recommended Posts

... The piston, that is.

So I have my engine all torn apart and everything looks pretty good. The only bearings that don't feel buttery smooth are the one for the water pump and one side of the crank. Both have a slight gritty feel to them so I'll be replacing both (and the other crank bearing) The rest of the engine looks really good and hardly worn at all. No serious scoring on the cylinder, there was a little corrosion on some of the studs but that's not a big deal...

So I'm looking at the piston, and aside from some carbon buildup on the top is looks really good. The wrist pin had a perfect fit, and there is only one or two scratches down the side. So should I replace the whole thing or just go with a set of rings? I know these engines are pretty good as far as life goes, but it would be nice to know it's at 0 hours and not unknown. The cost for the piston, wrist pin, and bearing is about $160, compared to about $30 for just rings. I'm sort of thinking I should just order it anyway, even if I don't use it just to have a spare, and maybe a second set of rings.

What do you guys think?

Share this post


Link to post
Share on other sites

if you're splitting cases to do the crank bearings i'd probably throw the new piston in just because.

what's the history on the top end? did you just do a top end recently? bought it used? what's up? because, to me anyway, if the crank bearings need replacing i'd need to do the top end as well for peace of mind.

what's the ring gap? in spec? sorry for the lack of cohesion in my thought process. i'm still having coffee.

Share this post


Link to post
Share on other sites

I would sent the piston to these guys and have the piston coated. I have had 5 done by them. And All I can say is that they do a great job and a fast turn around.

http://www.swaintech.com/store.asp?pid=8537&catid=19694

My motors run a lot quieter and cooler after having it done.

Then just put a new set of rings in it.

I have nothing to do with the company just a happy customer.

They can also built up a piston when it's a little to small. They are doing my ty350 piston as new piston is not available

Share this post


Link to post
Share on other sites

I think you're right, Schrode. It was used so I have no idea what's on it. I might as well do it right now and not have to worry later!

Thanks for the link WVTrials, I'll have to look into that!

Share this post


Link to post
Share on other sites
I think you're right, Schrode. It was used so I have no idea what's on it. I might as well do it right now and not have to worry later!

Thanks for the link WVTrials, I'll have to look into that!

You have a Montesa?? I know my Gas Gas has a coated cylinder.. Nikasil or some other spelling I believe.

http://en.wikipedia.org/wiki/Nikasil

And if I'm correct, you should NOT change rings as they will not seat properly without the honing marks to cut them in properly... And I'm not sure you can hone a Nikasil cylinder properly...

Better check into that a little deeper.

Looks like there has been some TT discussion on the subject.. I have not read this but you can..

http://www.thumpertalk.com/forum/showthread.php?t=353346

Share this post


Link to post
Share on other sites

Thanks for the heads up, 2PLY! I was going to hone it, but it looks like the consensus is to just go at it with some scotch brite to deglase it. That's cheap and easy! From my understanding, just about every modern dirtbike cylinder is nikasil plated or similar and people toss in new rings in them all the time so I don't think I'll have an issue there. The idea, I think, is to take off the glazing so the new rings will seat properly.

Share this post


Link to post
Share on other sites

Yeah, I was kind of scratching my head over the whole flaking or rounding port corners part, but if scotch-brite works then that's probably what I'll do. No use buying a hone that I'll probably only need once and don't even really need...

Share this post


Link to post
Share on other sites
if you're splitting cases to do the crank bearings i'd probably throw the new piston in just because.

what's the history on the top end? did you just do a top end recently? bought it used? what's up? because, to me anyway, if the crank bearings need replacing i'd need to do the top end as well for peace of mind.

what's the ring gap? in spec? sorry for the lack of cohesion in my thought process. i'm still having coffee.

+1 with schrode you got it apart might as well for piece of mind and not too pricey!

Share this post


Link to post
Share on other sites

The problem is that the new rings need to be "cut" by the original honing to shape correctly to the cylinder walls.

I believe the correct way is to buy a new top end... re-conditioned cylinder, piston AND rings all at the same time. I was faced with this choice once and ended up biting the bullet for the whole kit.. There is some new information about engine "break-in" and what it is really all about and that is applying enough power without synthetic oils so that the rings cut better before what little honing is gone.

I might be wrong but you might want to read this web page and then research a little further before you decide.

http://www.mototuneusa.com/break_in_secrets.htm

Share this post


Link to post
Share on other sites

I've read that article a few times (I think it was from one of your posts the first time!) and that's how I broke in my YZ250 rebuild with no problems. Although I think he is specifically talking about 4 stroke pistons and the dynamics for 2 stroke rings are slightly different... I think the same principals apply though.

I know lots of people over on the MX side of things replace their pistons and rings a lot more often than they get their cylinders redone. Maybe it is just less effective to deglaze rather than replate but still effective.

It is my understanding that a regular hone won't cut into the nikasil because it is too hard, and the "factory" cross hatching is done with a diamond hone. I am not sure of the state of the cylinder after it has been run for a wile but my reasoning is that the rings will get cut before the plating does. I think although the plating will wear slightly, it will mostly be filled in my the "glazing" that is a byproduct of combustion and also a little steel. Sort of like a machine file being filled up with softer material. It won't cut very well (break in the rings) if you don't clear out all of the softer material (deglaze).

That is my theory at least, if anyone wiser would like to enlighten me then please do so!

I would like to have the cylinder replated, but it looks so good that I think it would be $200 that I really don't need to spend, so I will opt to deglaze and run with that. I haven't decided if I'll hone or scotch brite it though. Dad was telling me back in the day, he always honed Porsche cylinders and they were nikasil plated... Of course, I haven't measured the ID of the cylinder yet, fortunately my dad has the tools and know how to measure it correctly. In the off chance it is out of spec it'll have to go off the the platers.

Share this post


Link to post
Share on other sites

Given everything looks so good, and assuming the bike didnt have any piston slap prior to this freshening up, I would just scotch brite it with a mild acid, slap in a new set of rings and check it out. It will probably run great. If not, you can always pull the jug easy enough and go all new then. If you are a big peace of mind type of guy, then I could understand just doing it now, but I doubt you need anything but rings given the info below. (did you measure the end gap?)

Share this post


Link to post
Share on other sites

I haven't measured anything yet, it's been kind of crazy the last couple of days. I did manage to place an order of parts from Sandifords earlier. I ended up getting a piston, pin, bearing, and two sets of rings (among other small parts) That should keep me going for a good long while.

Share this post


Link to post
Share on other sites

OK, I got a chance to measure some stuff.

The cylinder ID was 2.843 inches, that's spot on between 2.8428-2.8431 that is the spec for the "B" ctlinder.

The piston measured a little undersized at 2.8407, compared to the spec of 2.8411-2.8414 for a "B" piston. I guess that means I'll be putting in a new one.

And the ring gap was .018" for the top ring. The manual didn't have a spec for ring gap though...

Also to note that my piston to cylinder clearance was a little large due to the undersized piston, coming in at .0023 where the spec is .0014-.0020.

Share this post


Link to post
Share on other sites

Rule of thumb for nominal ring end gap is .1mm/ 1" of bore. Equals about 0.011" in your case - so I would say your end gap is too high and rings need replacing. The piston to cylinder clearance is fine in my book if that's worse case on a cold day.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×