Top End Question

XR250rdr · Post by **XR250rdr** » December 29th, 2008, 6:57 pm

I tore down the top end on my 500 after the compression was noticeably dropping and found the piston and cylinder to both still be in spec. My original plan was to send the cylinder off and have it plated, but that turned out to not be in the cards just yet.

So yea, the question is in the poll.

100hp honda · Post by **100hp honda** » December 29th, 2008, 8:51 pm

if piston is still fine why would you buy new one of the same size ?

JBaze · Post by **JBaze** » December 29th, 2008, 9:16 pm

How many hours on your top end? Rather than go through the expense of just re-ringing or using the same size components, I think I would just go for the next size piston and rings and get the cylinder bored.

XR250rdr · Post by **XR250rdr** » December 29th, 2008, 10:44 pm

100hp honda wrote:if piston is still fine why would you buy new one of the same size ?

A guy I race with recommended I do so. I wanted to see what you guys thought. My original thought was to just re-ring it.

JBaze wrote:How many hours on your top end? Rather than go through the expense of just re-ringing or using the same size components, I think I would just go for the next size piston and rings and get the cylinder bored.

If I had to take a wild ass guess...at least 60.

Since everything is still in spec I'm hoping I can keep it on this bore for a while longer.

JBaze · Post by **JBaze** » December 29th, 2008, 10:51 pm

Alot of the bike shops will bore a cylinder for like $20 or $40. Piston kit will cost what it costs whatever the bore it's on. I would just go ahead and do it.

kkvslayer · Post by **kkvslayer** » December 30th, 2008, 5:50 am

if it ain't broke why fix it?Hone,re-ring or throw a new piston kit in,depends on how much cash you got to burn I guess

redrocket190 · Post by **redrocket190** » December 30th, 2008, 8:55 am

You are saving the cost of a piston kit, re-bore and shipping for the expense of second gasket set and your labor taking the head and cylinder off. Replace the rings and do the honing....

Post by **AlisoBob** » December 30th, 2008, 9:30 am

I voted " Ring It".

britincali · Post by **britincali** » December 30th, 2008, 9:41 am

Me too.

JBaze · Post by **JBaze** » December 30th, 2008, 9:57 am

The only reason I have for a complete rebuild is, On my 91 500 had the same thing happen, so I just re-rung it and put it back together, every thing looked good. I went to a race the next weekend and the piston came apart on the start straight. So I am paranoid about that kind of shit.

Post by **AlisoBob** » December 30th, 2008, 10:02 am

In my new Smokey Yunick autobiography.... there is a interesting story on honing.

Smokey invented allot of stuff we take for granted, like torqueplating.

"If the cylinder is round, the rings are seated...... period!"

One day he got the idea to use such fine stones, the cylinders gleamed like chrome.

AFTER running the engine for some time, when dissasembled, the cross hatch was visable....

WTF!!!

He ( correctly) summised that the final "honing" was really just folding the peaks of the cross hatch over, like folding a box closed.

He revised his honing schedule, to remove these peaks, instead of folding them over... and " Plateau Honing" was born....

This was like in 1966 mind you...

Thanks Smokey!!!

MICK · Post by **MICK** » December 30th, 2008, 11:37 am

If you inspected the piston and it looks good, within spec even as you mentioned. I don't believe you are going to have any problems running it for another season, maybe two?

My top end has about as much time as yours. In the next couple weeks that's all I'm doing is putting new rings in it. I know the piston won't be as tight as I'd like it...but what the hell. If the piston looks sharp I'm not going to replace it. I'm just not ready to do the full rebuild like I have in mind...

lewisclan · Post by **lewisclan** » December 30th, 2008, 1:38 pm

AlisoBob wrote:I voted " Ring It".

me to now get to it

TYSTYX · Post by **TYSTYX** » December 30th, 2008, 3:07 pm

I say ring it !!

100hp honda · Post by **100hp honda** » December 30th, 2008, 4:01 pm

XR250rdr wrote:
100hp honda wrote:if piston is still fine why would you buy new one of the same size ?
A guy I race with recommended I do so. I wanted to see what you guys thought. My original thought was to just re-ring it.

i cant see anybody giving that kind of advise, especially since you said your piston is just fine. if you bought a new piston, theres no way to know for sure what its going to measure. it may be looser than the one you have now, or it may be too tight. theres also a chance it may be exact size you need

XR250rdr · Post by **XR250rdr** » December 30th, 2008, 7:32 pm

Thanks for all the input. I'm going to hone and re-ring then reassess things when racing takes a break for summer. I'm getting tired of riding the couch.

100hp honda wrote:
XR250rdr wrote:
100hp honda wrote:if piston is still fine why would you buy new one of the same size ?
A guy I race with recommended I do so. I wanted to see what you guys thought. My original thought was to just re-ring it.
i cant see anybody giving that kind of advise, especially since you said your piston is just fine. if you bought a new piston, theres no way to know for sure what its going to measure. it may be looser than the one you have now, or it may be too tight. theres also a chance it may be exact size you need

There are all sorts of different schools of thought. He said he had good luck doing it that way but it didn't make total sense to me.

mega491 · Post by **mega491** » January 2nd, 2009, 1:20 pm

Did anyone mention to check the cylinder for taper? Obviously the piston to wall clearance in the case of taper out of tolerance will change at different points of stoke causing the ring to constantly flex and eventually break......

iggys-amsoil · Post by **iggys-amsoil** » January 2nd, 2009, 2:42 pm

XR250rdr wrote: If I had to take a wild ass guess...at least 60.

Since everything is still in spec I'm hoping I can keep it on this bore for a while longer.

I voted Rings however, with 60est., if its a cast piston I'd think I'd get a new one, I don't trust them.

britincali · Post by **britincali** » January 2nd, 2009, 2:47 pm

Just re-ring it and be done with it, my AF has probably the lowest compression of everyone on this board and still runs tits

4Z · Post by 4Z » January 2nd, 2009, 8:58 pm

I voted for replacing top end, but I like spending other people's money...

m178 · Post by **m178** » January 2nd, 2009, 9:09 pm

I like spending other peoples money too. It's fun. I still voted ring and ride it!

iggys-amsoil · Post by **iggys-amsoil** » January 3rd, 2009, 1:48 am

britincali wrote:Just re-ring it and be done with it, my AF has probably the lowest compression of everyone on this board and still runs tits

Says the white man that don't jump.

Hey aren't you suppose to be in Dudemont?

XR250rdr · Post by **XR250rdr** » January 5th, 2009, 4:40 pm

mega491 wrote:Did anyone mention to check the cylinder for taper? Obviously the piston to wall clearance in the case of taper out of tolerance will change at different points of stoke causing the ring to constantly flex and eventually break......

I checked that too. It was within factory spec.

XR250rdr · Post by **XR250rdr** » January 6th, 2009, 1:17 pm

EDIT: found a set of rings

XR250rdr · Post by **XR250rdr** » June 19th, 2009, 11:09 pm

So to drag up an old topic to give closure.

I re-rung the piston and the bike ran strong (except for some low throttle jetting issues surrounding a PWK swap) for two races. The next time I rode it the left crank bearing apparently had developed some play and it sucked air past the seal. It started to run away on me then stuck the piston about a mile from camp and I had to push it back.

I guess I'm glad I didn't throw a brand new piston in it.

BannedCR500Riders.com

Top End Question

Which option do I go with?

Top End Question