The Triumph Dolomite Club - Discussion Forum

The thermostat housing bolts on my Sprint are virtually shot. So I though I'd get some stainless ones. I looked in the ROM, but I can't find where it gives their sizes. They're 5/16 UNC by the look of them, but what's the correct length?

Graham

I've found the part number which is HU0858 and I've looked it up in my Standard Triumph hardware catalogue it is indeed a 5/16" UNC thread. It has a length of 1".

Hope this helps.

Absolutly smashing, thanks. I'll order some today.

Graham

The usual steel bolt into ally thread in presence of water issue...

I'd think about helicoiling the thermostat bit of the inlet manifold as well...I expect the threads are well worn by now.

Jonners

Yes, they probaly are a bit. I'll have a good look when I pull the ones that are in. Not sure what happens with alluminum alloy, steel, and watery antifreeze. I could be lucky, and the steel's rotted sacrificially. But I'm not relying on it.

Graham

Please be careful with stainless bolts as they have a very low tensile strength and although you are not using them on structural or load work stainless bolts should never ever be used on suspension or any such places that come under load.

you may have to coat or grease the stainless steel bolts to prevent galvanic corrosion with the aluminium manifold

I have not checked the tensile strength of the originals, if they are stamped at 8.8 or lower,
Titanium may be a better option, there are aluminium ones as well, however I am not sure
if they will stand up to temp and warping.

Any info would be useful to us all.

Don't worry too much The original bolts are probably 40 years old or there abouts. If a new set of decent steel bolts last as long it will be about 80 years old! Good call on the helicoils, though I think they are steel, so may have issues ? But some sealant to keep the water out may do the trick.

I'm not too worried about the reactivity of stainless with aluminium alloy – it has got to be much, much less than between carbon steel and aluminium, and that was nearly good enough. Actually, the problem appears to have been the reaction between the bolts and the coolant, and that should be much less too. Neither, in this context, would I worry if the stainless bolts only have 80 percent of the yield strength of the carbon steel ones: about 20 percent of the carbon steel screws' CSAs appear to have just washed away with the coolant.


Graham

The correct length set screws arrived and the rain stopped long enough yesterday to see if the holes are up to taking them.

I only tried one, but there's just not enough thread in there to risk it - only about two threads (just over 1/10th inch) of the bolt are exposed above the washer, with the bolt just pushed into the hole.

It looks like I could use bolts a 1/4 inch longer - the one that's in that hole is a 1-1/2 inch bolt, washered up to about 1-1/4 -, but I'm going to helicoil the holes as suggested.

Graham

Of course studs of the correct thread would also do the job....
Then there is not so much stress on the alloy manifold with undoing them...the thread stress from undoing just goes on the
nut at the top which will then not suffer from corrosion from the coolant.
Much better solution all round the more I think about it...

Jonners

I thought about studding the holes, but then went for set-screws. Now I'm less certain. I would think it's still a good idea to coil the manifold, though. Anyway, I've ordered the kit.

I would need to check, but I'm guessing I'm looking for studs with about 1/2 an inch of thread into the manifold, below the unthreaded section, about 3/8th before the upper thread, and another 3/4 - 1 inch threaded above that. Unfortunately, at the moment I have no vice[s - believe that, and you'll believe anything], so I would want to get ones that don't need shortening. I wonder if they would want to be those mixed 5/16ths UNC/UNF type, with 5/16ths UNF nuts? I'll go look what's about.

Graham

Do you intend keeping your car?


I use Evans Waterless Coolant (the modern cars' version). The conversion cost is such that it will take maybe 6 or 7 years to make a saving over using antifreeze.
This completely eliminates all these corrosion problems, doesn't need changing, doesn't pressurise the system, avoids cavitation, etc.

Based on my experience I believe it also eliminates the weakness that is the hot spot at the back of a slant four engine. I would like to conduct
temperature tests on a few engines to see if that is actually true.


If you are going to helicoil the inlet manifold, why not use M8.....?



Ian.

trackerjack wrote:Please be careful with stainless bolts as they have a very low tensile strength and although you are not using them on structural or load work stainless bolts should never ever be used on suspension or any such places that come under load.

'very low' is an exaggeration, 80% as Graham mentions is the rule of thumb I was advised to use by an engineer at Westinghouse Brakes whose job was specifying fixings for the crap slung under trains. He could spend months on FEA, testing and reports for the bolts on one underslung load. He also advised, not that he needed to, DO NOT use them for suspension shear loads. There are grades of stainless up to it but your wallet doesn't want to go there.

sprint95m wrote:If you are going to helicoil the inlet manifold, why not use M8.....?

No need to swear Ian.