7mm thread on rocker arm adjusting screw?

[katy]

After about 2 year of work on the our ’61 Morris (1275 engine, from?), I decided to get it running again. Decided to check/set the valve clearances and found one of the locking nuts on the adjusters to be cracked. I didn’t like that and decided to replace it. Being that the nut was 7/16” AF, and it looked like fine thread, I assumed it to be ¼”-28 UNF and grabbed such a nut to put on but it didn’t fit. After mucking about and finally measuring it, the thread turned out to be 7mmX 1.0
FWIW, the rocker arms are pressed steel.

Question is: Is it normal for these cars to have 7mm threads on the rocker arms, with the nuts being 7/16” AF, but all the rest of the engine fasteners are inch sizes? Or is this engine an anomaly?

It seems to me that 7mm are an odd thread, mainly because I needed some a few years ago and had a hard time finding them, located bolts but no nuts, so I drilled out some ¼” nuts and re-tapped then to 7mm. Of course my metric tap and die set didn’t have any 7mm components so I had to go out and find a 7mm tap.

So again I cobbled up a nut to fit the rocker arm.

[bmcecosse]

It certainly should NOT be a metric thread - never mind the strange non-standard 7mm thread you mention..... And on pressed steel rockers the nuts would normally be 1/2" AF. So you either have something special there - or someone has been messing about. Check the others - seems like someone has forced a non standard nut on that adjuster..... Are the pressed steel rockers all centred perfectly over the valve stem tips ??

[les]

I agree they're an odd size, although I have never thought to measure them, maybe I will!
I think the pressed steel type are a smaller diameter.

[Trickydicky]

I was intrigued by seeing the M7 size, enough for it to get out my Zeus tables which state that M7 * 1 is a standard metric coarse screw size. Further investigation revealed that M7 * 0.75 is the metric fine size.

Must say though that in all my time in engineering, M7 in either coarse or fine threads, I have not seen it commonly used. Generally it is either M6 or M8.

[bmcecosse]

Is the tpi of either metric thread close enough to be confused with the expected UNF thread?

[les]

M6 & M8 are whats known as 'prefered' sizes, making M7 unusual. I would say the thread in question is of fine pitch and I am pretty sure it's not unf. Although I have not attempted to find out what thread it actually is, on the face of it would not argue with the figure mentioned of m7x1. Hope we can arrive at a definative answer now this has been raised!

[Trickydicky]

M7 * 1mm pitch in good old english sizes is .275 dia and .0394" pitch.
The nearest sizes are,
0BA which is .236" dia and .0394 pitch
1/4 BSF is .250" dia and .0385" pitch
1/4" UNF is .250" dia and .0357" pitch

I would say that it would be most likley to be 1/4 BSF. What do the rest of the guys think?

[bmcecosse]

The correct thread for the gap adjuster is 5/16" UNF. Never been any different - although at some point the 7/16" AF nuts were increased to 1/2" AF but still on the same thread. I'm sure someone has forced a wrong nut on the thread - damaging it.

[katy]

It's not just the one nut, all 8 of the nuts and adjusters are the same size. And they are definately m7x1, they're all the same.
The nut I fabricated using a m7x1 tap fit perfectly.
Doing a little bit of digging I find that the standard AF for m7 nuts is 11mm, which is only .00443" smaller than 7/16".

Are the pressed steel rockers all centred perfectly over the valve stem tips ??

Yes, they're all centred, as should be.

Normally I would never bother with what size the threads are, just loosen the nut, make the adjustment and retighten the nut. But on this one a wrench didn't want to fit, so after getting it loose I took it off to check it and that's when I found it to be cracked. So when tightened down it expanded just enough to prevent a 7/16" (AF) wrench from fitting. Actually more than one wrench as I tried at least 3 different ones before I resorted to an adjustable.

I have a spare 1100 engine which I believe has the same rockers on it. I'll check it out and try for some pics w/measurements.

[bmcecosse]

Rockers from a 1098 engine will not be correctly centred on the valves in a 1275 head. But the adjusting screws etc of course interchangeable... They will NOT be M7 x 1. So the only explanation is that the two threads are incredibly close to each other in size and thread pitch.

[katy]

Pulled both valve covers and the rocker shafts, rocker arms and adjusters all look the same to me. Only difference is that some of the supports are different.
Well here's some pics:
1) showing one of the adjusters from the 1100 engine and a caliper showing that it's .275" diameter, which is as close to 7mm (.27559") as dam is to swearing.
2) showing the same adjuster with a thread pitch gauge at 1.0 mm.
3) showing the top of the 1275 engine.
4) showing the top of the 1100 engine.
The 1100 engine head casting is 12G295
The 1275 engine head casting is 12G1316 which I've been told is an Austin America smog head which is basically the same as a 12G940 head.
The 1275 engine serial # is 12V/671Z/L8989[frame][/frame]

[katy]

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[katy]

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[katy]

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[chesney]

Could do with a different manifold!

[bmcecosse]

It is NOT a 7mm thread.....why on earth would Austin use a metric thread - least of all a non standard 7mm thread ??? It just so happens that the 5/16 UNF thread is very close. But not close enough - DO NOT put metric nuts on the adjusters!! The 295 head on that 1098 engine is well worth having!! The pressed steel rockers are the same on both engines - but the spacing is different to put the rocker tips directly over the valve tips on the 1275 engine - where the valves are pitched differently in the head to make room for the much larger valves.

[IslipMinor]

I believe they are a 'non-preferred' UNF - 9/32" x 26TPI - I would be very --- very surprised if it is metric, and 7mm is a very non-standard metric thread. I also tried a 1mm pitch thread gauge and it is very close, but not exact, there is a slight 'roll' of the gauge along the length of the tappet adjuster thread.

[katy]

The manifold works for us.

[bmcecosse]

Oh dear - can take the horse to water, but can't make it drink....

[katy]

Islip Minor got it right, although it appears to be m7X1.0 it is actually 9/32” x 26 tpi, I found it under a chart of “British Standard Fine Screw Threads – Basic Sizes” in an old book (1945).
Boy, it sure is close to m7X1.0 metric, but what an odd size to use.
Now all I have to do is figger out where to get a proper nut for it, although the m7 nut I made seems to fit and work pretty good.