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Old 14-11-12, 04:29 PM   #9
embee
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Location: Warwickshire
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Default Re: Valve grinding on an sv650?

As Sid suggested, lapping is a bit of a mixed blessing really. Cutting is a better engineering solution since it should (!) achieve seats with straight profiles, the problem with lapping is that it generally achieves reasonably well mating surfaces but they can be any profile, like 2 pieces of jigsaw if you like, full of ridges.

Valve and seat insert angles are specified with usually something like 0.5deg tolerance (normally 45deg nominal but not always), but the seats are toleranced "plus" and the valves are "minus" such that the contact surface is always at the fire side (cylinder) so you don't get a narrow wedge gap exposed to combustion, which leads to gas leaks, accelerated erosion, overheating etc. Lapping removes this condition.

Providing the mating is reasonably good they will bed in when first run anyway, production valves are not lapped (for reasons above), they sort themselves out.

If you are capable of dismantling the engine to that stage I'd say you are more than capable of lapping them yourself, then you'll get it right rather than relying on an unknown person who may or may not know what they are doing.

Use the finest lapping compound you can, use a few strokes back and forth (preferably a hand sucker type lapping stick rather than a machine oscillating type), turn it 1/4 rev, continue etc. After 1 rev clean off the paste, inspect the surfaces, should be smooth matt grey appearance. Try lapping with no paste and see if it produces a hint of polish to indicate where it's mating.

Using too coarse paste is a common mistake.

People use engineering blue to check seats but unless you are familiar with getting just the merest hint of blue on the surface it can very easily hide the reality (too thick a blue layer just looks even all round regardless).
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Last edited by embee; 14-11-12 at 04:30 PM.
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