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rdalley

New top end, no compression HELP

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I bought a '06 RM85 for my son and it needed a new top end.  Had 90psi.  I pulled everything apart and the cylinder looked good.  No damage but was pretty shiny with faint cross hatching.  I don't have the tools to measure the bore so I cleaned it up real well with scotch brite and ordered a Pro-X B piston (47.95mm) which Rocky Mountain said was stock size.  When I got everything assembled with new gaskets and o rings I decided to check compression again and I barely get 30psi.  I tried both the new and old head o-rings and no change.  What am I missing?  I measured the old piston and its the 47.94mm so even if the bore needs to be replated shouldn't I get better compression with new piston and ring?  

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Just wondering... did you put it in the correct way around? 

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3 minutes ago, tim2007 said:

Just wondering... did you put it in the correct way around? 

Yes, double checked that, arrow toward the exhaust and the intake reliefs on the piston towards the intake side.  

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Hmm. How about comparing the new and old pistons, is the distance from the pin to the top identical? Actually, the whole things should be identical... 

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Ill have to take it all apart to compare but initially I thought to myself that the old piston looked exactly like the Pro-X that I was putting in.  Im thinking maybe the cylinder is worn out.  Just puzzled as to why theres less compression with the new parts.  

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To be at 30psi the cylinder would have to be incredibly worn, seems rather unlikely to me.

Does the 85 of that year have a powervalve? If so, is it stuck open?  Though even then i'd expect more psi.

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It does have a powervalve, I disassembled and cleaned it per the manual and reinstalled it.  Everything seemed to be working correctly.

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4 hours ago, rdalley said:

Ill have to take it all apart to compare but initially I thought to myself that the old piston looked exactly like the Pro-X that I was putting in.  Im thinking maybe the cylinder is worn out.  Just puzzled as to why theres less compression with the new parts.  

If the cylinder was worn with the old piston, then the old piston will seal better since cylinders wear more in an oval shape rather than round as we hope.  Likely the old cylinder is slightly oval and this is perfectly round letting a lot of blow by.  You may need a new cylinder or just a bigger piston, but you’ll need more tools to determine this accurately. 

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14 hours ago, M3t said:

If the cylinder was worn with the old piston, then the old piston will seal better since cylinders wear more in an oval shape rather than round as we hope.  Likely the old cylinder is slightly oval and this is perfectly round letting a lot of blow by.  You may need a new cylinder or just a bigger piston, but you’ll need more tools to determine this accurately. 

That makes sense.  I bought a cheap set of telescoping bore gauges last night.  I know they aren't 100% accurate but it looks like the cylinder has worn too big and is oval so iff to re-plating it goes.  Anyone recommend one company over the other?

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A quick search yields many options:

http://www.usnicom.com/Services/CylinderReplating

https://www.millennium-tech.net/serviceInfo.php?id=1

http://www.powersealusa.com/

I'm having mine done by Millenium as recommended by others here.  Yet others might suggest going with alternates.  Everyone's got an opinion.   :)

For my 250, Millenium doing the replate for $225 and, because there was a bit of actual damage/scarring in the cylinder, a repair ($75).  Still far cheaper than a brand new cylinder although add perhaps $50 for round-trip shipping (Millenium doesn't cover shipping, not sure if others do).

Send your new piston with the cylinder to help ensure proper bore size, etc.  Millenium 'suggests' doing so.

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Talked to Millenium today.  I will be sending it out to them.

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4 hours ago, rdalley said:

Talked to Millenium today.  I will be sending it out to them.

Good choice.  I've heard nothing but good about them.  

The telescoping gauges wont be perfect but it'll be a hell of a lot better than one's eyes lol.  Good stuff.  Glad you got it sorted out.  Let us know how it goes!

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Did you hold the throttle wide open when kicking for the comp test?

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