Fine Thread in Aluminium

OK, not looking for the answer to a problem this time so much as canvassing opinion on whether I might have a problem or not?

Am currently part way through the build of a fuel tank for a Moto Guzzi motorcycle and am pondering the attachment of the fuel taps. On a 'Guzzi the taps screw onto fittings in the tank which have an M16 x 1.0 male thread. An original 'Guzzi tank is steel and thus so are the fittings. My tank, however, is aluminium so it would be useful to make the fittings in aluminium (probably 6082) to allow them to be simply welded in. My concern is that M16 x 1.0 is a very fine thread and might strip rather too readily in aluminium. Am I right to be worried or am I just getting in a tiz over nothing?

S.M.

You're worrying about nothing. Most Jap bike engine mount threads and head bolt/case bolt threads that screw directly into the aluminium casing are metric fine. I hand tapped-M12 x 1.25 threads into my bike's engine casing and torqued them to 75 lbs-ft in 2007 and they are still solidly in there today.

Edited By Pete Rimmer on 10/12/2018 22:02:17

Short answer, yes

Whatever fitting you are going to put in the tapping only needs tightening to make a seal. If its an 'O' ring seal it only needs to a little more than finger tight.

There is probably more chance of buckling the sheet metal tank if it was overtightened rather than the thread stripping.

Ian P

Thanks for the prompt responses guys. One less thing to fret about!

S.M.

Bear in mind if you make the weld-on boss and drill/tap the thread before welding it's likely that the thread will be a bit tight after welding especially if it's on the small side. You might have to re-cut the thread after fitting.

Have five bob each way and make the hole up the middle of the fitting small enough that it could be tapped to accommodate a male-threaded petrol tap as used on older Brit bikes etc. in an emergency.

Many thanks for the heads up Pete but on that score at least I am up to speed. Have encountered post-weld thread tightness issues on the tanks I make with welded in necks for screw in filler caps. Caught me out royally the first time I met it - got a cap well and truly stuck and for one brief moment of panic thought I'd wrecked a brand new tank! Calmed down, started thinking and solved the issue but it imprinted the phenomenon fairly firmly on my brain...

S.M.

Edited By Stirring Moose on 10/12/2018 23:23:16

Hopper, that's an interesting plan. Think the thread in question would be a 1/4BSP which, if memory serves, requires an 11.8mm tapping hole. Will have to double check exact measurements but that could all work out. Thanks for that.

S.M.

My modified Guzzi Cali III steel tank was converted to std Brit 1/4" BSP.

I just like 1/4" BSP in all my tanks (except my Yamaha ty trials bikes) because it's a great thread for the application, and, more importantly, such a wide choice of different style petrol taps available.

Nothing to worry about i just love 1mm pitch its the best thread available, used daily for pretty much any application. Anyy thread can be created, easy to work out or produce have done it M29.6 X 0.75mm etc takes well over 4500 psi where your car tyres around 33 psi.

M16X1 = 15mm hole
1/4"BSP = 11.5 hole = 13.15 o/d
3/8"BSP = 15mm hole 16.66 o/d 19tpi = 1.33mm pitch

I wold just look for taps suitable and go from there.
Problem arises with undersize fittings and susceptability to stripping threads as theres less grab. Would think on 6082 would need 7mm of actual thread, that would take tinnes of pressure.