360 Piston/Bore In Spec?

[kMoto]

Hey, I have a question for the engine guys out there...


I tore my engine down this past weekend and I'm trying to decide what to do with my top end. Also, I relocated to another state recently and I'm shopping around for a good machine shop at the moment. So, I would like to get some opinions here before I take it in.


Piston skirt OD: 66.85mm and 66.96mm


Cyl. bore OD: 67.00mm for both


I measured the piston skirts directly @ the bottom of the piston and I measured the bore @ the bottom as well.
I used a standard Mitutoyo Digi caliper to take the measurements.


The pistons ring gaps were all with in spec, measured at the bottom of the bore.


Any input would be appreciated. I know the the measurements are pretty close to the service limits. If it were a perfect world I would replace everything oversized and bore, but as usual, money is tight.

 

[teazer]

You already answered the question. It would be better to use new pistons and rings in a fresh bore. Yours are close to the limit, but there's no rule that says you absolutely must have everything at optimal specs. If the bore is not scored and the ridge at the top is acceptable and the pistons look good, I'd have no issues with running them for another season or two if the budget was already stretched

The other side of that argument is do you really want to strip the motor again in a year or so to get a rebore and fresh pistons?

If you're more OK with the extra work than the extra cost, use them. If doing a second time is a bigger issue than the cost then get it bored. For a street bike that wasn't a daily high mile driver, I'd save the dollars for now.

 

[crazypj]

measure the pistons about 8~10mm from bottom edge, 90 degrees to pin

 

[kMoto]

I guess I did answer it. I'm pretty confident making these decisions usually, but the everything here is right on the edge of swaying me one way or the other. The pistons do have some wear on the sides. You can kind of see it here..http://www.dotheton.com/forum/index.php?topic=21538.70.

And I looked at the bores again this morning and there was some really shallow scoring towards the bottom of both bores. Just barely enough to feel.


I did do a compression test back about 3 months ago or so. I didn't record them, like an idiot, but I do remember them being slightly low.

I guess piece of mind works it's way into the equation as well.

 

[kMoto]

pj, I did take another set of measurements exactly like you said, they were slightly different, but I left those notes at home. I'll post them after work for what it's worth.

Thanks guys.

 

[kMoto]

Well. SON OF A B!#$%. It now looks like ebay made my decision for me. I just went back to purchase a set of NOS +0.50 piston/ ring set and they were gone. I found a set at a dealer to, but they were definitely not in my budget.

I have no issues with tearing the top end back down next year. So I'll prob just re-ring it and get it running, hawk eye'ing ebay all the while.