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MG Midget and Sprite Technical - Valve rocker adjuster

Does anyone know the size of the adjuster thread in the A-Series rocker? I'm reconditioning some old cast rockers and one of the threads is burred. It seems to be very slightly larger than ¼" UNF.
L B Rose

Ah, there seem to be 2 sizes at least. Later ones were 5/16", so these early ones look like 9/32". Unusual size but taps are available if it's correct.
L B Rose

Don't worry folks, I have worked it out. It is indeed 9/32". Later ones are bigger.
L B Rose

From memory it is not a unf thread.
Peter
Peter Burgess Tuning

Also the rockers aren't cast, they're forged steel.
David Billington

Peter
Your memory serves you well
Just measured an early rocker adjuster screw that I just happened to have where I could find it, and, it is----- 7mm x 1.0mm pitch

willy
William Revit

That is interesting Willy, some time ago my engineering whizz kid (in his 70s) mate tracked it down as a British Cycle Thread so I thought I would Google it, seems the fine Metric threads are very close probably interchangeable at that!
https://en.wikipedia.org/wiki/British_Standard_Cycle
Peter
Peter Burgess Tuning

If 9/32" it may also be 9/32 BSF as that is 26 TPI also. The thread angle would need to be checked as BSF is 55 degree and BSC 60 degree.
David Billington

Hmm
Cycle thread is usually 26tpi as well
My thread gauges are only about 1/2" inch long, if they were a bit longer I'd be able to pick up the difference between 26tpi and 1mm but as it is both are fitting but I would say the 1mm fits best from what I've got here
William Revit

Just had a look on a Morris Minor resto. site and they list them as 9/32"unf
sorry if I've led you astray with the metric thing but that was what I came up with
It's probably more likely not to be metric I guess
willy
William Revit

Hi

Some useful info on the A Series 5/16 rocker arm posts (studs):
http://www.minispares.com/product/Classic/51K885.aspx
UNC thread one end and UNF the other.

Cheers
Mike
M Wood

I'll dig out a set on Monday and ask my engineering mate again. He may have it written down still as we were trying to source nuts and I seem to recall 1/4" UNF are not correct(we have 1/4" UNF in stock for A series heater take off studs). Sometimes websites and folk make assumptions when they haven't measured anything.
Peter
Peter Burgess Tuning

Peter

Point taken about not relying on websites for spec. Having said this Swiftune list the following rocker adjusters:
http://www.swiftune.com/Product/1619/516-unf-rocker-ball-stud-adjusters.aspx

Cheers
Mike
M Wood

Thats the 5/16ths ones which are UNF, I think we were considering the 9/32" smaller ones?
Sometimes books and websites can be wrong but become 'gospel', take for example Vizards descrption of the 12G206 head, he states the valve sizes as being the same as the 12G295 Cooper head, this is not so as the inlet valve is smaller being the same size as the 1098 12G202 head. It makes me think he could not have measured many. I tried to sell a 12G206 casting on ebay years ago and was berated by 'experts' telling me I was stating the wrong valve sizes!
Peter
Peter Burgess Tuning

Just tried a 7mmx1.0 nut on this stud I have and it feels good But in saying that the nut is only short at 5mm thickness
With a longer nut the difference between 25.4tpi for the(1.0) metric nut and 26tpi for the imperial would show up but with the short nut it fits ok
I'll be quite interested to see what Peter's guy comes up with
Got a feeling it's not metric though
I got this set of studs to replace those in my friend's morrie racer a couple of years ago and can't work out why I didn't use them
- Might have been the wrong thread ---lol
Willy

William Revit

Somerford list three different parts/part numbers for tappet adjusting screw, see items 70, 72 and 74: http://www.somerfordmini.co.uk/eshop/index.php?main_page=page&id=116 (one of these numbers appears on Morris Minor websites). Apparently at least two different lengths.

Best wishes
Mike
M Wood

And Mr Del Monte, he say, it is 9/32" BSF thread
He has even made me some new nuts from hexagon steel bar just incase we need them as it is a rarey size these days.
Peter
Peter Burgess Tuning

Tracy Tools has 9/32" BSF taps listed and also 9/32" UNS 26 TPI. Need someone who can get a good measure of the angle of the thread to know which is correct.
David Billington

Hi David
My mate bought the tap he used to make the nuts from Tracy tools. The nuts are a perfect fit on the rocker screws and the oe nuts are a perfect fit on the 9/32" BSF tap.
My mate has all the thread gauges known to man I think :)
Peter
Peter Burgess Tuning

I can pop one on a contracer and check the angles, but it won't be for a few days.

http://www.youtube.com/watch?v=4RtXqBLUupM
Dave O'Neill 2

Dave,

Nice that you have access to that as it looks like just the tool for the job to give a definitive answer to the thread angle question. Look forward to hearing the result. Can you get a front wishbone trunnion checked to see what those threads are? or is that pushing it.
David Billington

Always dangerous to 'assume', but as the A-Series engine is based on imperial 'Unified' threads, I would have expected the rocker adjuster nuts to be a Unified thread as well. That would mean 9/32" x 26 tpi UNS, i.e. Unified 'Special'?

With a thin half-nut as the adjuster, there are approximately 4 threads in it, so I would doubt that the thread angle difference between 55° and 60° thread angle would be noticeable in thread 'fit'. The effect would be that a UNF, BSF or BSC thread with 26 tpi could well all fit, so probably would an M7 x 1.0, as the OD is very close, as is the tpi.

Only one of them is the right answer, but which one?? I'm voting for 9/32" x 26 tpi UNS!

Richard
Richard Wale

Would the original drawing for the adjuster be in the BMIHT archives at Gaydon? That should provide the definitive answer.

I've not been to the archives for about 20 years, but I have just found this on their website:

Q. Can you undertake private research for me?

A. No, but we can research a single piece of information for you by a Web Research Request. Alternatively we can produce archive material for you to conduct your own research. This is by prior appointment with our Reading Room, giving a week’s notice. Our opening hours are Monday to Thursday between 10.00am and 4.30pm, Friday between 10.00am and 4.00pm. To make an appointment, please visit the Reading Room website [https://www.britishmotormuseum.co.uk/archive/general-advice-to-reading-room-users] or email us at archive@britishmotormuseum.co.uk
Jonathan Severn

Peter
I accept your findings on this and have binned the screws I have here as being suspect,and maybee from a different manufacturer -which is probably why I didn't use them in the first place---can't remember
I messaged a guy I know the other day who manufactures such things to get his take on it, It'll be interesting to see what he has to say, but I respect very strongly that your man is correct

willy
William Revit

Ah, I seem to have opened a can of worms here. It explains why the box of assorted taps and dies I inherited from my dad, and going back 70 years, didn't contain the right one. Maybe the only definitive answer will come from Gaydon.

Les
L B Rose

OK, I managed to do some checking yesterday.

Put it on a projector first, and it confirmed the angle at about 55º.

Dave O'Neill 2

Blowing it up slightly, shows the threads are truncated, so not too helpful for measuring the OD.

Dave O'Neill 2

Then on to the contracer.

This shows the angle as a shade under 55º and a mean pitch of 0.0389" or 25.7 (26) tpi, so it looks like it is 9/32 BSF.

Dave O'Neill 2

Bu**er me, does that mean my mate who has been in engineering since he could first walk (four generation family business) was right? I'll not tell him mind, it might go to his head :)
He has dozens of internal and external thread gauges, amazing really, goes back to Victorian stuff I think and Terry is a hoarder.
Peter
Peter Burgess Tuning

This thread was discussed between 04/11/2016 and 11/11/2016

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