The Triumph Dolomite Club - Discussion Forum

The garage, replacing the rack, I didn't remind them (again) 48 lb.ft - need I say more. Luckily only one wouldn't take 48 pounds, and I had a spare with manky corners.

So, will the 3/8 unf alloy wheel nuts take a coil well enough? I can't think of a reason why they wouldn't but I thought I'd ask.

Graham

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The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

I think the drill for a 3/8-24 unf helicoil is 25/64ths. Which is 3/64the smaller than the 7/16ths that the later nuts are threaded for - I assume it's the same blank as I understand the wheels are the same. So now I need to look at what the thread depth is. Think it's about 0.866/24 inches, if my geometry is still good. But what's that in 128ths?

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

Well, 25/64ths is the drill size for 7/16ths UNF as well. And since the 24 tpi helicoil tap should cut shallower threads than the 20 tpi 7/16ths UNF tap, it should leave more meat in the walls. At least I think it will.

So I reckon I shall invest in a 3/8ths UNF coil kit.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

I have used helicoils for 3/8 UNF wheel nuts with success, both in the 1970s and more recently. Always because of uninformed mechanics.

The drill supplied with the kit is 9.9mm, just under 25/64.

I have gone Up to 60 lb-ft with no problem, just to be sure, then backed off again.

Peter

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1978 Dolomite Sprint - Inca

Thanks Peter,

That idea of going to a higher torque and backing off might offer a way to answer the related question I just posted, about what the limit on internal thread diameter might be.

But then the question would be what test value to use?

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

Graham,
I don’t know what the minimum diameter will be. There isn’t much spare capacity in the nut to start off with.

I only know from testing on alloy generally that the pull-out strength of a helicoil is greater than the strength of the original alloy thread.

I only tested with 60 lb-ft to be confident that 50 lb-ft would be OK. I never had enough nuts to test to destruction!

It goes without saying that it is important to make sure the newly drilled hole is axial with the old one.

Peter.

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1978 Dolomite Sprint - Inca

Well, almost. I have been thinking about that. I suspect that if I take them to a shop with the capacity to do that to high accuracy, it'll exceed the cost of replacing with good second hand ones - there're some on eBay at £7.50 each for 3/8ths and £5.00 for 7/16ths.
I've only got a vice and a black and decker. But since I'm putting a 9.9mm (25/64ths) drill into a hole already 9.5mm in the case of the stripped one, I think it should be good enough to make a nut for the spares box. Slightly less confident if I decide to do the 8.6mm id ones.

As to the capacity in the nut walls: they are 17 mm od, and we're looking at 0.468" or 11.38 mm free outer diameter of the helicoil threads - which I think must be larger than the inserted diameter, but I'm not sure by how much (I'll measure the tap to be sure when I get it).

And that's compared to an outer thread diameter of more than 11.11 for the 7/16 unf thread - if anyone has a 7/16ths bottoming tap to measure, that would be interesting. That gives a wall thickness of about 2.95mm each side with the 7/16th. So coiling at 3/8 UNF give a wall thickness that must be no more than 0.14 mm less, or 6 thou off about 118 thou.

If someone can give me the 7/16ths UNF tap diameter, and I measure the 3/8 UNF coiling tap at anything like 12 thou bigger, I think I'll be fine with that.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

Yes, I think if you use soft hands the old hole will guide you in OK.

If your nut is a write-off then nothing to lose.

My 7/16 UNF tap has a thread tip diameter of 11.15 mm.

Peter.

_________________
1978 Dolomite Sprint - Inca

PS. My 3/8 UNF tap is 9.55mm 24tpi and the helicoil tap is 10.79mm 24tpi

Peter

_________________
1978 Dolomite Sprint - Inca

Forgive the edit please.

So the 3/8 UNF helicoil tap really is smaller than the 7/16ths UNF thread tap. By nearly 15 thou as well. That's good, as the helicoil set is on order. Only needs to do 2 nuts successfully to pay for itself.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

That’s correct. Even with helicoil, there will still be more meat in it than a new 7/16 alloy nut.

And, even if your 3/8 nut is stripped smooth, it will still have to be drilled out slightly before tapping.

As you say, two nuts and you break even, and kits normally have 8 or 10 inserts in them

Peter

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1978 Dolomite Sprint - Inca

So, given the equipment and skills I have, the answer seems to be no.

I was able to drill out a hole that was 9.5, maybe 9.2 in places, to 9.9, though that wasn't easy into this hard an alloy, with the low torque curdless B&D I have. I hadn't any paraffin, so I used turps sub as a lube, which seemed fine. The drill went down true enough, but the tap didn't go in near as square as I'd want. Despite that would mean the nut would go on wonkey, I thought I'd just try to get a coil into that thread, which otherwise looked fine. But I just couldn't get one to start - never had this problem with helicoils before, though I've only done UNCs, not UNF, and that may make a difference. So after I'd bust the crossbars of two coils, I gave up.

I will look at getting one or two of the less totally buggered ones done professionally, but, even at £7.50 each for second hand ones, I can't see it being worthwhile. Maybe when there's none anywhere, it'll be worth it.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).

Every alloy thread I have done has been UNC, so I could imagine UNF being a challenge.

As for drilling for helicoils, attempting to do it with a hand drill is always pot luck. Really you need a pillar drill, to make sure you get it absolutely square.

Still you have lost nothing by giving it ago.

Have you tried http://www.grayston.biz/wheelnutmenu.htm

They do a wide range of wheel nuts.

There’s a guy on the Scimitar Forum that makes Stainless wheel nuts.

Think his name is a Bill and he’s based near Brighton.

I’ll try and find a His contact Details later.

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2009 Mini Clubman Cooper S Daily Driver.
1980 Dolomite Sprint with a touch of BLTS
Balanced Lightened and Tweaked 13B Rotary and SuperCharged.
Back in my possession 22 September 2019.
Rebuilding the Sprint time taken so far, 111Hrs@15/12/2020
212Hrs @31/12/2021
352 @ 28/11/2022
455Hrs @ 20/10/2023
480Hrs @ 14/03/2024
This is time taken at the Sprint not necessary time worked.

Working on a ratio of just 7Hrs a day not including driving to the Sprint.
That equals to 68 days that doesn’t include weekends.
Member TDC no 0471

Project 13B Sprint now back on.