Lapping valves in the real world

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Cocker Cox
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Lapping valves in the real world

Post by Cocker Cox » Sat Jul 18, 2020 10:26 am

I’m lapping the valves in on my B25SS at the moment, and it seems to me that there’s a balance to be struck between perfection and good engineering practice. I’ve spoken to a pal who worked on the mechanical engineering side for an American oil giant, so he’s nobody’s fool. He says a small amount of valve pitting is an acceptable industry standard and that the ‘inverted-petrol-tight-head’ test is ‘unlikely’ even with perfectly acceptable lapped valves. He also pointed out that the valve heads expand and the seats contract when hot, so what’s the point of grinding a seat to death in order to get that perfect seal? My own thought is that I want the valves and seats to last as long as possible before replacement, so want to do the minimum lapping to produce an acceptable seal. I have produced the required light grey line on the seats and valves; replacing the valves and springs, inverting the head and filling it with petrol produced a slight wetting of the ports over an overnight soak test; the level drop of petrol still in the head was hardly noticeable. There are a couple of tiny pits on each seat but none on the valves. Good enough? I think so, but I’ll lightly lap the valves in once more and see if it affects the ‘petrol-test’....

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SteveS
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Re: Lapping valves in the real world

Post by SteveS » Sat Jul 18, 2020 4:29 pm

Hi Cocker

There will be others that demand perfection but I'd be happy with what you have achieved. Get the bike on the road and enjoy it
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Jeff K
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Re: Lapping valves in the real world

Post by Jeff K » Sun Jul 19, 2020 8:00 pm

Compared to a Norton, The BSA and Triumph valve seats are softer. At the start of the 1970's, I purchased a factory Triumph valve seat cutter set. No problem on BSA or Triumph, but it would not touch a Norton seat on anything as new as the commando Models. I later replaced that with a couple of Van Dorn valve seat grinders. Then later on I purchased the Neway Valve cutter sets. Even with the proper equipment , I would always still use a felt pen to mark the valve face and valve seats and then do a light hand lap to be sure. If you are that close now, after a few short trips they should bed in and seat well.
Jeff
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