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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 11-05-2012, 02:04 AM   #21 (permalink)
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Tas, Nice X to Y parts.

Valve clearances:
1. Factory lash has a min and a max number. The normal aim is called "blueprint." Better known as is the [middle] valve lash number you aim for. Say nothing wears. We'll get back to that later. For now, we can see 3 distinct lash settings. Setting this number gives the best performance.
2. Factory maximum will be the wide side. Wide means longer seat time. Cooler running. Slow power. Waiting for the last diminishing return; is the valve sitting closed [longer] in the power stroke. This also means grunt. It is not as near the better [blueprint] performance number.
3. Factory minimum gap is to get away with the narrowest number. Hotter seat time. Faster is the event to occur sooner. You cut off the power sooner with the valve being tighter. The intake opens sooner to make things occur faster on the intake side. The power down is the exhaust opens sooner, so the intake cycle happens faster. This is more for top end work. High rpm, not street grunt with the wide gap.

Back to the blueprinted number is that middle shot at both grunt and top. Cool, you build carbon. The middle number, you burn off carbon and remain book hot. The tight valve is burning off the carbon and cooking itself too, is too short a sit time. Not recommended for the long haul touring stuff. Tight is for race.

Now the wear factor:

A. Cams are shrinking or being compressed like shot peening it. There is a gap or growth on the wide side.
B. Cam towers are compressing too via the valve spring tension. These contribute to a wide gap at the lash.
C. The valve tip can mushroom, not slide out of the valve guide, but hang up. This too causes a wide valve lash.
D. The valve is harder material, can hammer the seat past its narrow 45 valve area. This mushrooming on this end can cause the valve lash to go tight.
E. The hard valve seat can hammer the softer aluminum head. This moves the whole seat up into the head. This can cause the valve lash to narrow.
F. The valve itself can burn, lose its 45 angle, become mushroomed and move up towards; narrowing the valve lash.
G. The shims can hold their tolerances. They may not contribute to a clearance problem.
H. Under the shim bucket, the contact point over the valve tip; This is also some pretty hard material like the shim. This can hammer the valve tip, but would crack at that small a rise. I would assume this holds its contact point harder than the [mushroomed] valve tip and may not cause a clearance problem.

When tight, my experience shows they tick some than when loose.
The valve check is done by using a go-no-go system. If for argument sake, the narrow gap was .002" and a .003" could not fit in between, you are within spec. If for argument sake the widest gap was .004" and a .005" would not slide in, you are again, within clearances using the go-no-go method.
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Old 11-05-2012, 03:29 AM   #22 (permalink)
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Hello
Cool reply.......or hot, depending on valve clearance! What you say gives some credence to the anecdotal view that a hard ridden bike results in tightening valve clearances and a carefully ridden bike (eg touring) results in increased valve clearances, mainly I would think due to increased stress on the valve seat area at higher revs. Your comments make me comfortable with my re shimmed clearances (I measured them all) of 0.2 on all inlets and 0.28 on three exhausts and 0.27 on the fourth. Thank you for your thoughtful post.
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Old 11-11-2012, 12:05 AM   #23 (permalink)
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Hello
My Bonnie is back together and new PVL coil fitted. Completion of the work was delayed due to illness, I am ok now.........I think. Bike fired up immediately and runs smoothly. Will ride it on a hot day to see if my ignition fault reappears. If it does I will fit a Procom igniter. Thanks for all your comments, most helpful.
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Old 11-11-2012, 09:01 AM   #24 (permalink)
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Quote:
Originally Posted by TT140 View Post

A. Cams are shrinking or being compressed like shot peening it. There is a gap or growth on the wide side.
If the area of the cam you are measuring to is wearing then you've got big problems, the whole point of the clearance is that this part does not touch the shim.
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