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Valve piston clash

davidjoyner

Well Known and Active Forum Website User
Non-VOC Member
Help. It all started with a broken valve seat - rear inlet. Had the heads redone and put it all together and ran for about 200miles. Hard to start so checked and found about a 16th to eigth valve clearance and assumed oil feed problem and something worn. Stripped the timing chest all OK. Heads off looked all OK. Re-assemble ...grrr. and rode for another 200 miles- the last 3 miles on one cylinder - incidentally sounds like a comet but with the power of an allotment wheelbarrow!
This time the rear exhaust tappet adjuster stripped, well they were due for replacement so in goes a new one. Set the clearance on TDC power stroke and NOW as you crank the swinish thing over the exhaust valve touches the piston and it locks up. Have to back off the tappet adjuster about a 16th to clear.
Haven't touched valve timing - bikes been running for 200 miles WHAT am I doing wrong??
 

timetraveller

Well Known and Active Forum Website User
VOC Member
David, you write that you had the 'heads redone'. Did that include new valve seats. And if so were the old ones badly pocketed. If so then the valves are now starting off further down their guides when closed and thus more easily able to hit the top of the piston. This is more often a problem with Mk IIs but with either poor piston top machining or poor valve timing it is possible with anything. If the answer to the first two questions is yes and you are using either Mk Is or IIIs then I suggest that you check the valve timing. You should be able to see where the valves are hitting the piston tops. When I was racing and helping to tune other peoples bikes I found that most often it was the edge of the valve hitting the raised side of the cutaway, outer side of the piston. Eventually I always cut these back to give more clearance. It was never the flat part of the valve hitting the flat part of the cutaway.
 

nkt267

Well Known and Active Forum Website User
VOC Member
Another thought. If you had the heads done were the guides changed? If the guides were not shortened at the top then with Mk2's the collar could be hitting the valve guide..John
 

davidjoyner

Well Known and Active Forum Website User
Non-VOC Member
Another thought. If you had the heads done were the guides changed? If the guides were not shortened at the top then with Mk2's the collar could be hitting the valve guide..John
Yes everything was replaced. It's done about 400 since the heads were done.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
My guess is that NKT 267 has it right but have you tried taking out both spark plugs and then turning the engine over by hand, not foot. You should be able to feel the problem. If it is the valve hitting the lower guide then it will be where the valve is at its highest lift. If it is the valve hitting the piston then that will be near top dead centre near to where the overlap on the valve opening occurs. It sounds like you have probably caught it in time to avoid serious damage further down the cam/follower etc. train. Lucky that a Vincent is so easy to take apart. Good luck with it.
 

davidjoyner

Well Known and Active Forum Website User
Non-VOC Member
Vale seat loose in the head,? Is it the adjuster hitting the inside of the cap, Cheers Bill.
Slammed the door shut on t' shed and took the head off! It's the blasted ET 26 ally bush that has developed oval holes - seems to have lots of oil - I don't use metering wires, so what caused this and why does it let you over adjust the tappet to allow the valve to touch the piston...and...and What a wonker - I needn't have taken the head off either more grr
 
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