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MG Midget and Sprite Technical - Dual master cylinder and 1275

I've just discovered that the 'new' dual master cylinder on my 1963 car is 7/8 diameter and not 3/4 as it should be for the 1098cc disc brake car. The master cylinder was new and had been fitted in the USA but had been stood for years so I thought I'd rebuild it only to find my 3/4 rebuild kit won't fit.

My car is going to have a 1275 and rib case gearbox with standard 1275 clutch and slave cylinder.

So before I splash out £140 on a new 3/4 master cylinder I thought I'd better check that it will work ok with the 1275 clutch. Obviously it should be fine for the brakes as the 3/4 is a match for the disc brakes but can someone confirm that it will be ok with the clutch?

And can someone tell me why the 3/4 master cylinder is double the cost of the 7/8 one when apart from the bore it is identical?!!
John Payne

There's a hole in my footwell that needs filling!


John Payne

John, cant answer your question but if you buy a new one make sure its a Caparo one not a TRW one which is what most of the usual suspects supply.

Trev
T Mason

I have a 1275 engine clutch and rib case in my Mk11 (1098) with the dual cylinder. It has been like that for years but the previous owner did it so can’t say whether there was any adjustment of pushrod lengths etc . I can’t remember off hand what slave cylinder it has on it
Mike Fairclough

If you use the 3/4 bore master then use the 1275 1"clutch slave cylinder. It avoids the clutch overthrowing as the bore is larger than the 948/1098 slave. The clutch on a 1275 is different from the earlier cars as its a diaphram one rather than springs. be sure to use the correct release bearing as each of the 3 engine sizes use different bearings.

The 3/4 bore was only ever fitted to the later Mk2 and Mk3 sprites so there is less demand. The 7/8 bore ones are used in the MGA as well as the Drum Frog/ early Mk2 so more demand equals cheaper!!
Bob Beaumont

John

Ear
Over this year Powertrack Brakes had AP Caparo (I.e an OE Lockheed spec) dual 3/4 inch master cylinders in stock.

If not, you can get a 7/8 inch one resleeved to 3/4 inch. You will need to track down some correct length pushrods.

Suppliers to check out: Powertrack Brakes Ltd, Coasting Ltd and Past Parts Ltd.

http://www.powertrackbrakes.co.uk
http://www.coastings.co.uk
http://www.pastparts.co.uk

Info on pushrods and resleeving:
http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm
http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm


Cheers
Mike
M Wood

I used the original pushrods and adjusted them accordingly. All works well.
Bob Beaumont

Cheers for the info everyone. So it looks like I need to splash out £130-150 for the correct MC and find some push rods. Moss don't list the pushrods (well they only list one type) so as the ones I need are shorter I'll probably just cut the old ones down using the lengths given in Dr John's article.

Looks like it will be a while before I have any brakes!
John Payne

I went to the expense of getting a three-quarter inch dual when I swapped a 1275 into my 1962 all those years ago, but now I can't really see the point. On my Frog I've simply adjusted the clutch pedal travel to give the correct throw.

That should work, shouldn't it?
Nick and Cherry Scoop

Nick, As I understand it, you'd fit a 3/4" mainly if you're converting to disc brakes (as I am). I got a TRW one in a Moss sale a while back so am stuck with it now (not fitted yet). I hadn't thought of different pushod length but will probably cut down the old ones as Bob has and as John proposes. Another useful thread for me about something (again....) I hadn't fully thought through.

Restoration isn't straightforward is it!
Bill Bretherton

Thinking about it again, as mine is a 63 car originally with discs then maybe the push rods are original and correct. I'm guessing they fitted the 7/8" MC because the right one wasn't available then?

I'm off to do some measuring!
John Payne

I don't understand this point of push rod length effecting the clutch throw. Surely the throw is determined by the amount of fluid displaced with a full stroke of the pedal, so will relate only to having the correct bore of master and slave.

The push rod length, and adjustment, can only be to match or determine the start point of the clutch pedal, adjusted to take up slack if it is low to start with.

If the clutch release lever has a pull-off spring (as fitted to Morris minors) then the push rod length may also determine how much clearance there is at the clutch release bearing. If not, then the release bearing should only be pushed back by the clutch pressure pad springs, plus maybe a fraction to allow for contact wobble as it spins.
GuyW

I'm in luck. The push rods are the right length for the disc brake MC so no adjustment needed. I've also checked my new clutch slave and that is 1" diameter.

I can see now why there is a lump of steel bar welded to the clutch pedal, the bigger MC must have been overthrowing the clutch so to compensate the PO had added a stop to the pedal which would hit the bulkhead!
John Payne

John, my clutch pedal also has a peg welded to the back to limit travel. Maybe they were all like that?
GuyW

John

For a while I recall the 3/4" bore cylinders were simply not available (unless there was an odd one on a parts shelf). (pre Ebay Days) The 7/8 has always been around and as it looks identical its perfectly reasonable the incorrect one has been fitted sometime in the cars life.
Bob Beaumont

I'll hang fire with the hacksaw then Guy, thanks! The welds look a bit amateurish which made me think it was an addition, I'll leave it and see how it goes.

Yes, apparently the 3/4" bore ones were unavailable for a while and good secondhand ones commanded a high price.

With everything in the brake system being new I'm going to go for silicon fluid mostly to avoid the instant paint strip of cellulose by DOT4.
John Payne

Mine has the welded lug on the back as well , like you I thought it was a previous owners work but then I saw others like it . Presume it was done at factory
Mike Fairclough

Well, my new frog clutch system was throwing too far, so that the pedal became hard well short of its stop, and pushing further might have damaged the clutch.
So I adjusted the pedal horn stops to get the right throw at full pedal depression. Of course it means that the pedal at rest is not as high as it can be.
It works so far.

Bill, so far as the disc brakes are concerned, the three-quarters mc would be a bit easier on the leg, but why would it be considered necessary?
Nick and Cherry Scoop

I had a problem with overthrow when I initially used a 1098 slave. The 1275 slave cured it. It also made the clutch much more comfortable to use. Mine has the lug welded on the back (along with a couple of spares I have in the garage) but I managed to get all the pedals to line up with careful adjustment of the pushrod
Bob Beaumont

Nick, the 7/8 cylinder has a residual valve whereas the 3/4 one has a restrictor valve. The latter is better for disc calipers I believe as it stops the pads locking on momentarily when the pedal is released.
Bill Bretherton

I fitted discs to my frogeye with the standard 7/8" master cylinder as I didn't know about the residual pressure valve and was never aware of having any problems. It was later replaced with the 3/4" when it needed changing. I think i was fortunate that the local factor provided the smaller MM rear cylinders when I replaced those before it went on the road.
David Billington

Davd, I have no practical experience of this so maybe it doesn't really matter if you don't mind the increased pedal effort.
Bill Bretherton

Good point, Bill. I had forgotten that difference in the valves.
By the way, I always found that drums required less leg effort than discs, so perhaps the 3/4" with the restrictor valve really was a well thought-out modification. And why not? They were quite good at what they did.
Nick and Cherry Scoop

The UK made GMC112 3/4 cylinder is still available here, it's the quality one now branded as 920E (which is just the new name for Caparo, exactly same tooling used by exactly same staff though!).

http://www.jagspares.co.uk/Abingdon/partdetail.asp?partno=GMC112

I used it with 1 inch slave on my 1275.
Clive Seager

Just a quick update. The new master cylinder arrived today from David Manners (cheers Clive) and all looks good. It's got AP cast in the side and looks good quality - though so did the TRW one to be honest. It cost £130 including VAT (and fairly expensive delivery) which is around the price of the TRW.

Just got to paint it and then get it fitted.

Thanks for all the replies.
John Payne

I retained the cast iron look by spraying with petrol resistant varnish.
Bob Beaumont

Yes Bob - but you've seen John's under-bonnet standard of finish. You've only got to look at the 'New Radiator' thread above.
Nick and Cherry Scoop

Thanks Nick, I wouldn't win any concourse awards though! I must admit after Bob's comment I remembered that my old original Mk1's MC was unpainted and thought maybe I should have left it. But by then I'd already slapped some nice satin black on it!
John Payne

While I'm here I'll just mention that my old 'new' 7/8 master cylinder is now on eBay if anyone might be interested in a cheap spare!
John Payne

Apart from the risk to the diaphragm, over-stroking a 1275 clutch brings the thrust ting on the diaphragm into contact with the clutch plate, and pushes the plate onto the flywheel. Result:clutch drag, feels like it is not disengaging. If over-stroking, releasing the pedal a little finds the sweet spot where the clutch is properly disengaged.
Paul Walbran

This thread was discussed between 09/11/2018 and 23/11/2018

MG Midget and Sprite Technical index

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