Eric,CR500 Piston&wall clearance?

[Grimwikked]

I have a 98 CR500 that is standard bore.My local shop honed the cylinder and told me it was in very good shape.My piston to wall clearance is .0045.I will be using a wiseco piston.Do you think this is too much clearance or will it be fine with a proper warm up?:scream: I dont want to blow it up,hehehe any info would help.
wiseco says no more than .0035,i have known bigger clearances and they were fine.....Thanks Tim.

 

[cujet]

My experience with Wiseco pistons is that some additional clearance is needed for most folks. The stock CR500 clearance can be up to (I think) 0.0035 inches with a cast piston. I would suggest that an additional 0.001 inches would be the ideal Wiseco clearance.

Chris

 

[Pete Payne]

Cujet,
I completely agree with you on the clearance. On the CR 500 I have seen many pistons crack on the intake side. Some people say it is due to too much clearance, (.0045 is not too much) but is due to the excessive squish velocity of the stock head geometry.
Sorry, I know this is a little off topic , but I have hear stories off the dealers telling guys this ( the clearance is too big , and if we don't bore the cylinder to tighten this up , the piston might crack due to too much clearance ) in order to sell the bore job to the guy. And then they don't clearance the Exh. bridge properly and open up another can of worms.

 

[EricGorr]

One time I had a guy come back at me with a question about how I set up his cylinder bore, using .0045 inch on the cylinder wall and .007 inch at the bridge. He said he wanted me to rebore the cylinder to the next size because his Honda manual stated .003 inch as the maximum clearance and he was concerned about vibration. I laughed and said "If you're so concerned about vibration why are you riding a CR500!".
Like this could actually feel another .001 inch through the bars. That reminds me of another funny story about a magazine editor that claimed he could feel a vibration from my 150 kit, from a piston that was 11 grams heavier than the stock piston. I guess his but-dyno was tuned to the fine seeting that day:p

Pete makes an excellent point, on a CR500 its the bridge relief that means way more than the piston to cylinder clearance because thats the main reason why they seize after a fresh bore job.

 

[Grimwikked]

:) thanks for the info guys.I thought it was ok,but the guy at the local machine shop has that sell you a more expensive repair anyway attitude.hehehe..
Eric a buddy of mine got a KX500 big bore from you and i wanted to say it was a fine job!!!I do alot of dirtbikes for the local kids and friends(I love to take them apart as much as ride them)and i will be recomending you as a performance option.....as well as a repair service.Thanks for all your help,Tim

 

[TravisCR500R]

One more thing....

Dont forget to drill your exhaust bridge holes on your piston. At least Wise-co reccommends it, i do not know what the other piston makers say. I did mine on a CNC a tad over kill, but what ever you can do. Dont hold it between your knees and use a hand drill !!! DOH you would have to be a KWAD rider to be that dumb.
Hey Pete...My 85 cracked the piston on the intake. I have heard alot about that happening too. Do you suppose there is a way to redefine the squish/quench area of the head? Obviously machining it, but got any dimensions? Have you fiddled with the design at all? About all ive done to my head is polish the Pee-Pee out of it, got rid of the rough stuff. It should keep the carbon off the head though somewhat.
One more thing, a good deburr job on your holes and all sharp edges will help as well.
travis R

 

[Pete Payne]

Sorry for the slow reply.I have had good results using the spec that Eric Gorr uses . Set the squish band to 15mm wide (1 side of the chamber) and blend it to the combustion chamber by cutting it on a lathe. The blend angle is 25 degreee's. Hope this helps.

 

[NO HAND]

Re: One more thing....

Originally posted by TravisCR500R
My 85 cracked the piston on the intake. I have heard alot about that happening too. Do you suppose there is a way to redefine the squish/quench area of the head?

When a piston is way too worn out often it will break at the skirt on the intake side. At least this has been my observation on most skirt breaking horror scenes.

 

[CRmann]

What are those grooves above exhaust ports on a CR500 cylinder? One says they are some kind of decompression things. One says they are some kind of pistonringsavers. Are the grooves necessary at all?

 

[Grimwikked]

those chamfers are a compression release.If you want to be able to start your 500 easily then they are good.I think you would have to bore the cylinder more than .080 to get them out!or resleeve.

 

[Pete Payne]

Those 2 grooves are a bleed for compression and they are normally gone after the cylinder is bored .020" . I just bored one for my brother in law . We only went .020 over just to tighten the bore up and get it to be round again. This removed those grooves completely.