Lost Compression on my Twin Cylinder

Dec 30, 2019
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Need some advice. Rebuilt my son's twin cylinder 340 Cat (2500 miles) twice now. First time it had lost compression but (surprisingly) did absolutely no damage to cylinder walls or pistons so just replace rings and honed x pattern on walls as done many times before on machines. He got 300 miles and compression went to 60 and 90 PSI but again absolutely (from looks) no damage - not a scuff or scratch. Checked ring gap (.045) and cylinder to piston clearance (.003). On first rebuild the rings were worn a lot and sticky so could imagine reduced compression but this time rings were nice and loose and not worn with good spring. Could it be a bad break in the second rebuild?? Any hints appreciated. Happened at 30 -40 below but we run in that all the time. Thanks for any help. Stan from Tanana, Alaska
 

Blk88GT

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What did the plugs look like when it went down? Piston top?

Have you gone through the carbs? Fuel quality? Are you using ISO at that temp?
 
Dec 30, 2019
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What did the plugs look like when it went down? Piston top?

Have you gone through the carbs? Fuel quality? Are you using ISO at that temp?
Plugs looked nice and tan. Piston top had a little brown on it. Single carb had bowl drained. always new fuel. very little ISO used
 

up-high

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Plugs looked nice and tan. Piston top had a little brown on it. Single carb had bowl drained. always new fuel. very little ISO used
Sounds like might have been a cold seize. Motor didn't get to warm up before it was at full throttle. What was he doing when it seized?
 
Dec 30, 2019
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Son was traveling on a flat hard packed road at reasonable speed. Friend who runs a performance shop said those cylinders are Nikosil coated and have a limited life and often don't rebuild good.
 

Coldfinger

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Lots of variables.

What kind of oil? What ratio?
I heard you are not supposed to hone nikasil, not that it is the problem here.
My understanding is a nikasil cylinder can last thru multiple sets of pistons and rings.
What brand of piston and rings?
 
Jan 6, 2020
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Lots of variables.

What kind of oil? What ratio?
I heard you are not supposed to hone nikasil, not that it is the problem here.
My understanding is a nikasil cylinder can last thru multiple sets of pistons and rings.
What brand of piston and rings?
Put in new crankshaft seals and you'll be off to the races!
 
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