MM nut and bolt thread patterns and sizes.

[RobThomas]

Might be worth turning into a sticky and having a compiltion of the MM threads and bolt sizes?

I'm stripping the engine down to get it out and have found the 4 long nuts holding the exhaust manifold on to be 5/16 x 22, which is 5/16 BSF (The one size I don't seem to have in a tap). It is dangerously close to 5/16 x 24 which is commonly found in tap and die sets. It would be easy to screw up the manifold studs if I'd have used the wrong one. 5/16 BSF is also the standard factory thread for the small bumper coach bolts, evidently.

I'll happily add to this list as the engine gets stripped (although I'm off to Cuba for a week starting on Tuesday)

[mike.perry]

The threads on the engine should be metric fine. Check the threads with a metric gauge in case they are a very close match.

[RobThomas]

Hmmmmmmmmm. That is why I posted the info. I was expecting a Metric pattern but they are 5/16x22 which is BSF and I've tried a few Metric bolts and taps but none come out at the same figures. Metric 8mmx1.0 comes out at 25.4 TPI and 8mmx1.25 is 20 TPI whilst 5/16 UNC is 18 TPI and UNF is 24 TPI? These are definitely 22 TPI

[mike.perry]

Are the ends of the studs which screw into the engine the same threads as the nut end? I will try to check my Vedette engine tomorrow.

[mike.perry]

I have just measured a spare manifold stud and it has a 1.0mm pitch on both ends and the long nut screws on to it. Is your stud the same thread on both ends or has someone rethreaded your block?
See Derrington P 4.

[RobThomas]

I've just pulled the No.3 stud from the block. It is 5/16 BSF on the outer end with a solid-ended tube nut but 8x1.0mm metric on the inner. I'll snap a photo later. This is in the '49 tourer but it is a later USHM3 block and came from Carlo a few years ago in a running car. I guess BMC standardised at some point onto Metric components or maybe moved to British nuts for some reason? Could it have had something to do with the military tank engines units?

I wish someone had told me that they go straight into the waterjacket!! I now have wet shoes.

[alanworland]

I thought Morris bought a shed load of tooling from a French company - and used a lot of it on the sidevalve! Metric threads but with whitworth hexagons.

[mike.perry]

The military manifold nuts only have a hole and thread in one end, unlike the ordinary manifold nuts which have holes in both ends, they are also painted light green but to the best of my knowledge are 1.0mm pitch. I have not got my military engine at the moment so I cannot check but as I have one car engine manifold nut on it I am pretty sure.

[RobThomas]

I've been up to the barn and pulled one of the studs from a genuine MM '52 engine and it has 8mm metric fine threads on both ends. I think that makes it almost a certainty that the Tank engines are an odball mix of metric and UK threads. I've bought 16 inches of 15mm AF brass hexagonal rod and some EN8 steel 8mm diameter rod to make some Metric-at-both-ends studs and super-long nuts so that I can save a lot of aggro in the long run. £10 for the lot.

[mike.perry]

It still sounds strange, I am sure that the manifold nuts on my car, generator and Vedette engine are all the same thread. I have checked the car and Vedette engines, it will be a while until I can check the generator engine.
I am trying to figure out the parts numbering systems for the car and generator engines. Taking the conrods as an example both engines use the same part nos, rod = SA1507/4, bolt = X316o4, nut = X31648. The exhaust manifold stud on the car engine is LA7084 and the nut is LA7085, on the generator engines the numbers are X31317 and the nut has a military code,, FV105794 as it is a special part.
Parts that should be the same on both engines eg valves and springs both have X prefixes but different part nos, so a different part no. does not necessarily mean a different item.
I think I will stop there before I confuse myself any more.

[MMaurice52]

I've been up to the barn and pulled one of the studs from a genuine MM '52 engine and it has 8mm metric fine threads on both ends. I think that makes it almost a certainty that the Tank engines are an odball mix of metric and UK threads. I've bought 16 inches of 15mm AF brass hexagonal rod and some EN8 steel 8mm diameter rod to make some Metric-at-both-ends studs and super-long nuts so that I can save a lot of aggro in the long run. £10 for the lot.

I have been measuring with inch thread pitch gauges trying to find the correct pitch on the water jacket cover bolts of my SV engine! I spent most of the morning confused. I'll check them tomorrow with the mm pitch gauge.

[mike.perry]

My generator engine which has been rebuilt and is now in the car has had new manifold studs inserted with 1.25mm pitch threads. A hex bar approx 5/16 Whit spanner size was drilled, tapped and cut to make the manifold nuts. the inner ends had to be turned down to fit between the pipes

[RobThomas]

Just done the water jacket and water pump on the sidevalve. 8mmx1mm Mteric fine on the jacket and 5/16x22 BSF on the waterpump studs. 2 of the jacket bolts on mine had been replaced with UNF studs and nuts, thus knackering the threads.
Hope this might be useful in the future for someone.

[mike.perry]

That is the problem, people do not realise the threads are metric and force bsf/unf nuts on to them with the result that the threads get well and truly b******d. There should be warning notices on the engine

[wimor]

We have here (NL) the problem that people not realize that other than engine Series MM components are BSF and force metric nuts……………. But where b******d stand for?

[RobThomas]

Oops. Late reply, Wimor. It stands for 'bugg ered'. (There is some anti-swearing software on this website, I think)

Anyway, I was suprised to find that the studs and nuts on the carb to inlet are 6mm x 1.0mm. I would have thought that they'd have gone to UK sizes for something that far away from the engine. I ended up buying threaded rod and making longer studs so that I could add an alloy spacer between the carb and the manifold. It may have been designed to have some sort of spacer in there but I've not found any in the boxes of bits. Does anyone have a spacer on their MM?

[mike.perry]

I PM'd Wimor with the answer to the abreviation!
The early USHM2 engines did not have a spacer between the manifold and the carb and suffered from fuel vapourisation.
The USHM3 engines had a heat proof (bakalite?) spacer which I am assured by those that have fitted it to the early engines, cured the problem, plus re routing or insulating the fuel pipe past the exhaust. Longer studs / bolts are needed to fit the spacer.
I have fitted alloy spacers to the Traveller Midget engine to clear the LCB exhaust.