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le3045 · Member

i have made a brass valve for my qb78 and it has 1100 psi on it i have lapped it with valve lapping compound the poppet is delrin but i cant get it to seal any suggestions from the experts thanks Lee Crying or Very sad

rsterne · Moderator

If you know that the sealing face on your poppet is 100% square to the stem, and you need to lap the seat, you can try this....

Cut a disc of 220 grit sandpaper and punch a hole in it the size of your stem.... put some double-sided tape on the back and stick it to your poppet, with the sanding surface towards the stem.... Trim the outside diameter to the size of the poppet head.... Drop the poppet into the valve, chuck the stem in a drill, pull gently while running the drill forward then back a few times.... This should sand the sealing surface of the valve smooth and square.... Change to 400 grit and repeat, then 600, then lap with toothpaste.... clean and assemble....

Bob

le3045 · Member

thanks so just the toothpaste is all the delrin needs but the valve needs the paper ill give it a try tomorrow i just assumed that lapping compound would be all that was needed just out of curiosity could you use a o-ring seat or would the pressure be too much thanks Lee

rsterne · Moderator

The O-ring would collapse too easily, and take a lot of hammer energy to open.... Delrin is perfect for what you are doing....

Bob

Alstone · Moderator & Site Supporter

What I usually do is polish the brass seat with a good brass polish, make sure the delrin is square on the shaft put it in the valve then rest the valve on a vice with the stem between the lose jaws and give the brass delrin case a good tap with an hammer, then spin it around with a bit of brass polish to lap it in,
Make sure every thing is square before tapping it with the hammer.

Al

lorraine · Member

We have had super luck using the polish made for lexan windscreens on motorcycles, It turns the seat and the poppet glass smooth, It takes a little more time but the results are much better. Remember your valve job is only as good as the valve stem to stem bushing bore fit.