Fine vs. Coarse Thread in Aluminum

Sandy · 07-09-07, 07:00 PM

About to do the fuel pickup flanges which are a standard SAE 5 bolt pattern in the RCR tanks. They looks to be about 1/4" think aluminum. Which is better coarse thread or fine thread for the smaller bolts (I think #6 or #8 as I recall).

Thanks

Sandy

Big-Foot · 07-09-07, 07:29 PM

I think I'd have a tendancy to lean toward the more coarse thread in the alloy.. I think you're talking about the difference between 24 and 32 TPI. You'd have more strength in the alloy with a beefier thread (not that 24 TPI is really beefy)..

Bill D · 07-09-07, 08:11 PM

I used a fine thread in mine. Rationale: more threads in the thin (1/4") aluminum

Sandy · 07-09-07, 08:39 PM

Bill - That's what I was thinking, but then I remember that aluminum is not as nice to tap as steel. Split decision

Sandy

Bill D · 07-09-07, 08:41 PM

Sandy
I had no problem. I used cutting oil on the tap though

Scott Calabro · 07-09-07, 09:30 PM

So bolts in a fuel pickup flange in an RCR go straight into fuel?
Best,
S

Scott Calabro · 07-09-07, 09:36 PM

I'm thinking then that if you removed a bolt there would be a hole in the tank?

How about some aircraft wet fuel tank self sealing nut plates for said bolts?

Best,
S

speed220mph · 07-09-07, 09:39 PM

Use coarse threads for threading into aluminum. Bolt strength is not the issue, it's the cross section of the tapped hole after threading, which is greater with a coarse thread than with a fine thread of the same size.

Now,when talking bolt strength, the bolt with the fine thread is the strongest, all other things being equal. Reason is the cross section of the bolt at the threads . . . it's bigger with a fine thread.

Jim S. · 07-09-07, 09:42 PM

Sandy,

Course thread is what you want in Aluminum due to the lack of strength of aluminum versus steel. Not to say that Aluminum is an inferior material, just stating facts. Normally the more threads you have (finer threads) the more holding power you have, but this is in steel, so Bill was on the right thought pattern, but in your case your talking about Aluminum. Course threads are the way to go. We (Eaton Corp) use course threads in all of our Super Charger cases for the reasons above.

Aluminum Tap Magic is great for tapping or machining aluminum.
MicoCut works well too for tapping Aluminum and steel.

Good luck,

Fran Hall RCR · 07-09-07, 09:42 PM

Nut plates are the perfect bulletproof solution...

The fuel sender/pick up supplied by Classic instruments is designed to be directly surface mounted with cork gaskets....

Sandy · 07-09-07, 10:10 PM

I found some nice SAE 5 hole nut plates as well for marine use, but easy to make if I had to. The RCR material looks plenty thick for threading into as well not too worried. I will end up using backing plates if I botch up the tapping of the aluminum. It is tight to get to once the bulkhead is on...

Coarse threads it be for now (I think the classics come with fine thread if I'm recalling).

I'm using the same type of pick up that Fran supples but I needed additional for return. Just decided to make my own (see build thread). Fran's (the ones from classic) kill a couple of birds with one stone so to speak. My fuel requirements need a pick up and a return and a fuel sender. I also wanted -8 lines for the hungry motor.

Thanks

Sandy

ragrod · 07-10-07, 01:14 AM

fine or course? I'm more worried about a good lube, WD40 works very well on Al. and drill alittle under size so you get 90%+ thread, most drill and tap charts are for 75%.
Randy

Sandy · 07-10-07, 12:51 PM

I have some TapMagic (I think that it) which I think was mentioned as good for Aluminum. I forgot to post the link of the backing plates for those that might want to have some.

FLS-U was the part, 5 bolt SAE Stainless, $6 bucks each + shipping.

Fuel Tank Senders, Gas Tank Senders, Water Tank Senders, Holding Tank Senders by WEMA USA, Inc.

Thanks for the tips.

Sandy

Jeff Hamilton · 07-12-07, 01:48 AM

If you use the Classic senders check to make sure they are installed the correct direction. You want 30 ohms full and about 250 ohms empty.
Jeff Hamilton

Sandy · 07-17-07, 04:57 AM

One gotcha with the Wema backing plates is that they are Metric. Will have to get some longer hardware and more of a pain for me since the Centriod fuel sender is too think for the extra long screws they provided... Always something

Sandy

Sandy · 08-13-07, 02:32 AM

Ok, did some hole cutting in the fuel tanks for the sender and other stuff. I was ordering some stuff from Enco, and saw a different style of tap, says it's a Forming tap vs. a Cutting Tap. I order a couple to give them a try on some scrap.

Enco - Guaranteed Lowest Prices on Machinery, Tools and Shop Supplies

To get the holes drilled in the ones closes to the bulk head in the RCR was a chore, I had to hack up a right angle drill out of an odd collection of parts. I'll get a picture for a laugh...

Sandy