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MG Midget and Sprite Technical - Front drum to disc conversion

Converting a Frog from Drum to Disc and I seem to have got a disc setup from a late 1275 or a 1500. The track control arms are AHA9958 and AHA9957 but I think I need BTA648 and BTA649.

Anyone want to swap newly powder coated AHA9958 and AHA9957 for any BTA648 and BTA649?

The track rod ends for a Frog have big diameter tapers but the AHA9958 and AHA9957 need smaller diameter tapers. The TRE with smaller diameter tapers won't screw onto the rack track rod of the Frog.

I could drill them out but I'm fresh out of tapered drills!!!!!

Rob
Rob aka MG Moneypit

Can't you use the original Frogeye steering arms?

They should bolt straight on.
Dave O'Neill2

it depends on VIN number IIRC - the early Frogeye ones are different again.
David Smith

I have the not so early Frogeye arms with the drums, ATA4130 is one of them. Any reference I have read has the BTA648/649 or AHA9958/9957 listed as correct for discs depending on whether the early or late 1275 steering rack is fitted. The rack changed about 1972 the later had more turns lock to lock and I think a bigger turning circle.

Never thought of trying the original arms, just assumed they would be wrong with discs. I'll give it a go tomorrow.

Rob
Rob aka MG Moneypit

Rob
If you have stock Frogeye rear slave cylinders you'll have to change to Morris Minor
Alan
www.masckent.org
Alan Anstead

Does that apply even if I stay with the original Master Cylinder?

Rob
Rob aka MG Moneypit

Apparently yes
If you mail me via www.masckent.org I'll dig out the info
scan it to mail and send it to you.
Alan
Alan Anstead

In my Mechanical Service Parts list for the first MG with Discs it says in the remarks column where the drum brake type track control arms (ATA4130/ATA4131) are listed "W.S.E. use BTA648 and BTA649 in pairs only" so what does "W.S.E" mean? Any idea?
Rob aka MG Moneypit

Rob I've found a spare pair of BTA648 & 649; email mgsmithy AT yahoo dotcodotuk if interested.
David Smith

Thanks David. I'll email you shortly.

Rob
Rob aka MG Moneypit

Rob

You have e-mail
Alan
Alan Anstead

The early disc braked cars up to 1966 used a tandem master cylinder which had a 3/4" bore as opposed to the drum braked cars which have a 7/8" bore. The smaller bore means the brake pedal requires less brake effort and is strongly advised when changing to discs. As Alan has said the 7/8" bore rear brake cylinders also need changing to the later Morris Minor 3/4" bore to maintain the correct brake balance. If not under heavy braking the back can come round!
Bob Beaumont

Alan, thanks for the email. Yes, I see the reasoning. I have to emulate the slave cylinder bores of a disc braked Midget on a Frog which has different rear brake backplates. It all makes sense now it's explained.

What about the clutch slave cylinder? Does that also need changing because it is my understanding that Drum and Disc master cylinders have the clutch and brake bores identical. i.e. drum brake MC's have two 7/8" bores and disc brake MC's have two 3/4" bores. Is that correct?

Rob
Rob aka MG Moneypit

Rob,

I converted to disc brakes when I rebuilt my frogeye and just used the standard drum brake master cylinder and didn't have any issues, the pedal didn't seem to require excessive pressure to lock the brakes. I think the rear cylinders I was supplied may have been Morris Minor, even though I asked for sprite ones at the local motor factors, so maybe that was a lucky swap, the originals were beyond use. No issues using the 7/8" bore master with the 1098 clutch slave either.
David Billington

I've opened pandora's box by searching the archives on this subject. So much conflicting advice, I wish I hadn't looked!!

Thanks for your input David, as many have said in the archives it works with the original Frogeye MC.

I replaced most of the braking system about 2 years ago so the components should all be ok, but I want to change to Silicone brake fluid (which is also in the same box owned by Pandora!) so maybe I should renew all the brakes and get a new 3/4" MC and all the other bits too and be done with it.

Confused of Wilmslow
Rob aka MG Moneypit

The clutch slave for the early disc braked cars was not changed from that used on the drum braked cars until the 1275 engine with a diaphram clutch was used. If are using a 1275 engine and retain the original slave and change to a 3/4 bore master there is a possiblity of clutch overthrow. However I did not experience it when I changed over. The later 1275 slave has a bigger bore.
Bob Beaumont

One of the principle benefits of disc brakes is that the pads remain in (very light) contact with the discs when released. So the movement required to bring the brakes on is slight and less fluid movement is needed. And of course they never go out of adjustment.That is why disc braked cars use a smaller bore m/c; they don't need to move so much fluid. The consequential benefit of the smaller diameter m/c is that there is a greater mechanical advantage and less foot pressure is needed for the same effect.
Guy W

Bob, that is very interesting because I plan to use a 1275 engine with a 1098 gearbox and a 3.7 diff. And the 1098 gearbox uses the earlier clutch slave which will work with either MC.

I found two back-plates from a 1500 which I plan to fit with the dual acting rear brake slave cylinders (which are half the price of the single acting Frogeye ones) so I think I am sorted.

Just need a realistically priced 3/4" MC which I am waiting to hear about.

Rob
Rob aka MG Moneypit

This thread was discussed between 11/01/2014 and 14/01/2014

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