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Valve grinding and lapping
Vixen:
This question has probably been asked many times before.
I am about to lap the valve seats on the Bristol Jupiter. Each valve head is 1/2" diameter and there are 36 valves to be lapped. So I have a few questions.
What is the current wisdom regarding the best medium to be used ?
What grade or grit size will give quick and consistent results ?
What is the best method for rotating the valve, I am sure they do not made dop sticks that small?
Thanks for your advise
Mike :noidea:
Admiral_dk:
I haven't done any our model size, but I have done quite a few full size without the "dop stick", using a piece of plastic tube. This is a very annoying way of doing it, but it can be done, just remember to pull on the hose while doing it .... Hmm - that didn't sound polite :facepalm2:
.... and come to think about it - I'm sure it would have been quite a lot easier if I had just used a short piece of hose to "connect" the valve with a round stick ... :noidea:
Looking forward to see your solution ... and a future run of the engine :Love:
petertha:
Hi Mike. The best reference I can think of is Terry's 18-cylinder radial ~ post #185
http://www.homemodelenginemachinist.com/showthread.php?p=251041&highlight=valve#post251041
I've saved some other links related to cutting/prepping the valve seats too, could be part of this build project or one of his priors.
I built a prototype head for the radial I'm working on & replicated his valve seal mini vacuum test apparatus. I'm no expert but I think it has to be a worthwhile exercise. Very subtle lapping changes can make or break resultant seal, so personally I would to improve my odds on the workbench.
petertha:
Not sure if this is applicable to your particular heads if for example ring type valve seats are already in the head. On my radial, the valve cage is kind of an integrated seat/flow chamber/stem bushing part. It gets glued in the head with HT Loktite then the port passages are drilled into the upper cup. Anyway, some pics of my lapping pot & vacuum valve tester. The vac test uses a 'standard' cup so the bleed down comparison to valves is apples & apples. is. any lapping mods are to the valves themselves. Ask me a year from now if it actually works & if the engine runs LOL!
petertha:
I bought this progressive grit selection of oil based diamond lapping compound from AliExpress at very reasonable cost. I've also messed around with a variety of different compounds, mostly typical machinist brand name aluminum oxide. But even the smallest pastes are rather biggish quantities for the small amounts I use, it becomes a $$ outlay just to have a few grits to try. I saw a tube of valve lapping compound at an auto supply store, cant recall details but remember it was quite coarse for our purposes.
From what little lapping I've tried with the diamond, it cuts excellent using a aluminum lap. There are plenty of lapping experts on this forum that will chime in, so I'm watching too. One thing I am wondering about is if some diamond might impale itself in the valve seat itself (because it is sharper & doesn't break down?), or if it can be suitably removed. It appeared to wash away with solvent on the aluminum lap bushing.
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