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Old 04-18-2017, 09:27 AM
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Default CR250 2001 Crankcase gap

     

Hi everyone, I'm in a bit of a predicament with a rebuild of my CR250 2001.

To cut a long story short, the piston decided to give up one day and showered the combustion chamber as best it could. Unfortunately a piece lodged in the oil gap of the big end of the connecting rod and as it went round seems to have scraped and deformed the bottom of the crankcase.

Ive had the cylinder re-plated and will be putting all new components in, but I'm dubious about the state of the crankcases... where they have been impacted from inside, it seems they have rolled out and away from each other slightly. I read on Dogger's build that if you can get a 0.002 (0.051mm) feeler in between them then they'll need to be lapped.

Thing is - I live in the middle east and those kind of services aren't readily apparent, and even so, if they were lapped wouldn't that then mean the bearing seats of the crank and the gearbox shafts also need relieving? And even then, wouldn't that also mean the cylinder studs would be closer together?

Ive taken some photos of the cases but cant post links until I have 5 posts under my belt. I tried some feeler gauges at the base of the case, the 0.01 (0.25mm) feeler doesn't penetrate all the way.. and I have to go down to the 0.0035 (0.09mm) before it goes all the way through. I think there is a slight lip on the inside edge of the sealing surface which prevents the thicker feeler going through.

Would the gasket (measured the OEM, its 0.5mm) take up the slack or am i going to get horrible leaks if I just reassemble as is?

Sorry - that ended up being quite long... thanks all!

Last edited by Invisibligger; 04-18-2017 at 09:25 PM.
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Old 04-19-2017, 11:27 AM
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Some photos may help the diagnosis....















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Old 04-19-2017, 11:38 AM
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I'm no expert on this subject by any means but I would think that would be okay. In lieu of being able to find a shop to lap the cases, perhaps you could use a sheet of glass and some abrasive material to lap them yourself?
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Old 04-19-2017, 11:50 PM
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Thanks for the reply Cowboyona, I have considered the do it yourself approach but I'm reluctant to irreversibly take any material off unless its absolutely necessary given how hard these things are to fid now in serviceable condition.
But also, if it came to that - Im guessing I would need a custom thicker gasket made to compensate?
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Old 04-20-2017, 10:10 AM
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Lapping the cases by 0,1mm would not hurt bearings or studs, you could try to heat up the cases at that point where the gap is and using a rubber mallet to carefully set them them back into position. Using 1mm gasket will off set the bearings etc by a lot (0,5mm) so personally would not recommend it.

Normally you should not have any problems using the original gasket (0,5mm) in combination with copper spray gasket maker.

Personally I would try a mix of these things
1) Heat up the cases at that point and try with a rubber mallet to reduce the gap. It needs a lot of attention and caution here as of where to heat and where to hit though!
2) Lap it until the gap comes down to 0,05mm
3) OEM gasket and copper spray.

Do not forget to replace the water passage o-ring that goes in between the cases.
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