Britstuff
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- Jan 8, 2021
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Hi:
It would seem that inlet valve guide seals are in place to stop oil being sucked down the guides into the combustion chambers. The question I have is can oil travel DOWN the exhaust valves guides, (via suction, gravity or whatever) to the combustion chambers, or out the exhaust and cause the dreaded Commando white smoke?? Conventional wisdom being that upwards pressure of exhaust gases would prevent such from happening??
The reason I am asking is as follows:
I have a 1974 Commando 850. Recent re-bore and hone with +.020 pistons and rings and rebuilt head with new valves and guides etc. So far have about 100 miles on it. The left hand side cylinder has started to smoke, spark plug is fouled with oil. Right side appears to be fine with a nice brown spark plug tip. I do not use the side stand, and I have an oil tap in place to prevent sumping, (both of which it seems can lead to smoking).
I recently removed the head to see if I could work out what is going on. I thought maybe a wrist pin circlip had come loose and damaged the bore, but no, the bore looks fine. Inlet valves are very clean, inlet valve stem seals are apparently still in place and doing their job. Exhaust ports are oil fouled. I removed the exhaust valves to check for wear etc, both are tight but the shafts were surprisingly oily. No exhaust valve stem seals are currently fitted, (as standard).
Presumably either my piston rings have not bedded in properly, and or oil is running DOWN the exhaust valve guides. Trying to find out if anyone has seen oil fouling caused by oil migrating DOWN the exhaust valve guides.
Thank you,
Kind Regards,
James
It would seem that inlet valve guide seals are in place to stop oil being sucked down the guides into the combustion chambers. The question I have is can oil travel DOWN the exhaust valves guides, (via suction, gravity or whatever) to the combustion chambers, or out the exhaust and cause the dreaded Commando white smoke?? Conventional wisdom being that upwards pressure of exhaust gases would prevent such from happening??
The reason I am asking is as follows:
I have a 1974 Commando 850. Recent re-bore and hone with +.020 pistons and rings and rebuilt head with new valves and guides etc. So far have about 100 miles on it. The left hand side cylinder has started to smoke, spark plug is fouled with oil. Right side appears to be fine with a nice brown spark plug tip. I do not use the side stand, and I have an oil tap in place to prevent sumping, (both of which it seems can lead to smoking).
I recently removed the head to see if I could work out what is going on. I thought maybe a wrist pin circlip had come loose and damaged the bore, but no, the bore looks fine. Inlet valves are very clean, inlet valve stem seals are apparently still in place and doing their job. Exhaust ports are oil fouled. I removed the exhaust valves to check for wear etc, both are tight but the shafts were surprisingly oily. No exhaust valve stem seals are currently fitted, (as standard).
Presumably either my piston rings have not bedded in properly, and or oil is running DOWN the exhaust valve guides. Trying to find out if anyone has seen oil fouling caused by oil migrating DOWN the exhaust valve guides.
Thank you,
Kind Regards,
James
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