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Engine basics => Engine set up tips => Topic started by: Annette Elmore 1954 on August 27, 2018, 01:06:24 AM
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I have made quite a few Delrin venturis but they've all been the old nva through type.
Can I tap Delrin to use the newer single sided nvas more common these days.
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Yes you can -- it's really sweet to tap a hole in it. But it won't be nearly as strong as aluminum. My gut feel is -- well, I don't know. Try tapping a few holes in scrap pieces and test them to destruction -- tighten until the threads strip, put a screw in & rip it out with pliers, etc.. Then you'll have a gut feel for it. You probably want to go up at least one size on the threaded hole for more strength.
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Maybe it's better to buy a good quality 3-tap set, and use only the #1 and #2.
That way you end up with a tight fitting thread.
But still, I don't know.. the material may stretch in time. L
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I have made quite a few Delrin venturis but they've all been the old nva through type.
Can I tap Delrin to use the newer single sided nvas more common these days.
Single sided? You mean like they screw in from one side? Those are very uncommon outside the Iron Curtain (although Fox had one). Usually these are also very heavy and stick out a lot, meaning they can be very good tuning forks. I don't think this is a terrible good idea to mount in Delrin, but there's only one way to find out. I suggest starting with a thread insert to begin with, like a Heli-coil or similar, which will spread the load of a (typically) 6-32 sized thread over a much larger area.
Alternately, if you can machine Delrin to the required accuracy, you can also machine 6061 aluminum, that seems like a much better choice.
Brett
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When threading a metal thread into a plastic body, or a plastic thread (like a nylon bolt) into a metal body, I use a starting tap (tapered) and NOT run the tap all the way through the part. Result is, depending on how far you run the tap, a loose (tap all the way through) to very tight (tap partial way through) and steps in between.
Norm
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If you're using real Delrin, it taps nicely. I've even used a "roll tap" in it. This makes the thread tighter, because the plastic deforms and then returns a little due to elasticity. Maybe you've noticed that when you drill plastics, the hole usually ends up undersize? Partly due to heat and expansion, and partly just deflection. Low rpm, high feed rate, peck drilling works best for plastics, as a general rule. Double drilling is best, along with allowing it to cool before using the finish size drill.
Like others suggest, 6061-T6 is going to be more reliable. The Tower .40 has the type of spraybar the OP wants to use. I noticed right off that the factory in Taiwan mounted them into metal! When I make a CL venturi for an engine with the offset "draw-bolt" holding the carb in (replacing the draw-bolt with a spraybar), I always make those from aluminum, but always wonder if I could get away with Delrin. FWIW, I dislike Acetal Acetate, which is generic Delrin. They are NOT the same. Real Delrin machines MUCH better. H^^ Steve