Ring End Gap Out of Spec

[a ron]

Hey guys, been a while since I posted - finally revisited the Blaster top-end rebuild. I had the jug re-bored, and everything went together ok. After everything was assembled, I did a leak-down test and compression test and found that compression was about 95 PSI and the cylinder was leaking at roughly 22-25%. I thought maybe it was just a rough hone / dry cylinder, and perhaps the rings would eventually seat, so I installed the engine back in the quad.

The engine barely ran, so I took the jug back off to measure the ring end gap and found that it's at roughly .019", which is at least .005" out of spec!

The cylinder still looks basically new, and the Wiseco rings were the correct model/size (at least according to their packaging), so I'm thinking that my machinist screwed up by boring the cylinder too large. Is that what your guys' impression is too? How does a machinist screw something like this up?

I'm pretty bummed :-/

 

[rich1]

Really it depends how bad the cylinder was when you gave it to him. If it was out of round or had deep scratches then you have to bore it till they are removed. If the piston was cast or forged the clearances are different for both . He may have used the forged specs that are larger than the cast ones. Most of the time its best to let them bore it 1st then measure for the right size piston. However... If all else fails he may not know how to bore a cylinder correctly .

 

[Awk08]

i see no mention of a new piston that the cylinder was bored/honed to match up with ?
only that you had it bored or honed and used new rings ?
thats not how this works
to do anything more than a light hone to restore the cross hatching for new rings to seat, will require a new, next size piston and rings, not just rings on the old piston.

i'd say you need to take it to a different shop (that one should have known better) with the next size piston and have it rebored to match that new piston

you could also measure to see the piston to cylinder clearence and if it's within specs............

 

[a ron]

Most of the time its best to let them bore it 1st then measure for the right size piston. However... If all else fails he may not know how to bore a cylinder correctly .

Ohhh! I guess I did it backwards when I brought the new piston/ring set to have him match the cylinder to them. And what you said could be true, but still, he should've said something if that's the way he does it. I'm nervous that this guy doesn't normally deal with the tight tolerances in a small engine... but he's one of two machinists in town. The other machinist doesn't even deal with small engines.

i see no mention of a new piston that the cylinder was bored/honed to match up with ?
only that you had it bored or honed and used new rings ?
thats not how this works
to do anything more than a light hone to restore the cross hatching for new rings to seat, will require a new, next size piston and rings, not just rings on the old piston.

Sorry, I forgot to mention that I brought the new piston and ring set with the jug to the machinist. They were oversized by about .75mm to clean up some scoring. He seemed to indicate that he was going to match the two up and that he needed the new piston in hand to do it. I have a cheap caliper that says the piston is roughly the intended size.

i'd say you need to take it to a different shop (that one should have known better) with the next size piston and have it rebored to match that new piston

you could also measure to see the piston to cylinder clearence and if it's within specs............

I agree, I don't want to get involved with this shop again... but I'm almost pissed enough to ask them to buy me a new piston since I have a 65 dollar paperweight now.

Thank you SO much for the youtube link. I didn't know you can measure the cylinders without super expensive tools. I'm going to measure it to see if I can wrap my mind around how bad he screwed up.

Have you guys seen these kits? I'm considering just buying one and being done with it:

http://rover.ebay.com/rover/1/711-5...0001&campid=5337529790&icep_item=321317945039

 

[a ron]

Well, I have .009" of piston clearance. That's way too much. I think I could almost put the next size piston in there right now.

 

[Awk08]

Well, I have .009" of piston clearance. That's way too much. I think I could almost put the next size piston in there right now.

take the cylinder, piston and the feeler gauges to this guy, show him and ask just WTF he thought he was doing, and you'd like him to buy you a new piston in the next size, .009 is like twice the allowed limit, he screwed up and should fix it.

most pistons have the clearence listed in the spec papers in the box.

 

[rich1]

I agree with Awko8 try to make them fix it first .Almost all motorcycle shops these days do not do machine/boring work .They send it out to get it done. I talked to one place here and he said Blasters could not be bored since they are plated inside.I said ok....and quickly ran out the door !

 

[Blaaster]

Any machinist worth his salt, with piston in hand should be able to bore/hone to the correct size.

If it were me, I would be taking piston/cylinder back to him and demanding that he does it correctly!

As Awk said, it is not too hard to read the clearance printed in the installation instructions?

 

[Larry's Shee]

The proper way is cylinder gets measured, piston is ordered, cyl is bored/honed to fit that piston as there can be slight differences (.0001")

Agreed, how hard can it be, piston comes with spec sheet.

If you get this squared away you might be able to sell this piston, if it isn't scuffed bad, as it has very little use.

 

[a ron]

Thanks for all of your input guys. I think I put the pieces of the puzzle together a little better.

I went back and used a feeler to check clearance at the *skirt* instead of the top of the piston. The clearance at the skirt is exactly .1mm (.004"), which also happens to be the service limit.

It's possible that this guy might've asked someone, or consulted the service manual about the recommended clearance and added that clearance to the intended actual bore size instead of the actual piston size. Or maybe he interpreted the service limit as the recommended tolerance, who knows.

I can't find any mention of the recommended clearance on the Wiseco box or in the papers that came with it - but then again, their site says that they design the piston for the intended bore size, not the other way around. At any rate, I guess it seems less random and sort of reassuring to know that he didn't just randomly choose a gigantic bore diameter PFA.

I'm boned up on my tolerances now, so I know exactly what I'm shooting for. I want the cylinder to be .0025" beyond the skirt measurement. With the next size up it should basically be right on at 67.25mm actual bore diameter, using a Wiseco piston.