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MG MGA - 3 bearing B eng in 1500 MGA

Over the winter I am going to get my spare second-hand 3 Bearing MGB engine rebuilt to install in my MGA 1500, a few outstanding queries: -

gearbox Fitting - I am staying with my current gearbox which is actually off an early 1600 A. I believe that if I stay with the MGA clutch and flywheel all I need to do is fit my current (early high starter 1600) back-plate to my replacement B engine?

Clutch - I understand fitting the correct early B diaphragm clutch and/or flywheel involves significant extra work hence I intend to stay with the current A setup as I have always been happy with the A clutch. Is this approach fine? Can the A clutch take increased power?

Cooling - The B had an oil cooler, do I need to fit one? I have heard all the recent discussion on A cooling but I seem OK at present with my 40 year old early recon unit. I do live in Scotland.

Oil Filter - Is this the same? It looks it.

Starting Handle 'Dog' - As the B engine just has a bolt on the front can I fit the A Dog so I can still use the staring handle? Always useful for tuning, and dead batteries.

I think the rest all bolts back on and in terms of tune I just start with current A set up, is this correct?

In terms of who does the engine it doesn't look as if anyone will do an exchange on these units hence I am currently looking at MGOC, Moss, and Skiptune. Is there anyone else I should be considering? Any views on my listed suppliers would be useful, if negative email directly if necessary.

Finally I presume this upgrade is worthwhile and the power increase will be significant (30%+ on paper)?

All views/answers welcome. I have read Barneys, useful pages.

Paul Dean

Use your 1600 back plate that is correct match for the gearbox.

Clutch is not rated in HP, it is rated in torque. The 1800 engine makes more power by increasing torque at similar speeds. If you are into speed shifting under full throttle in the higher gears the A clutch might have excessive slip, and the friction disc might wear out too soon. For casual driving the A clutch would be okay. If you drive it for a while and find the clutch not to your liking, then you have to R&R the engine again later to do the MGB clutch conversion.

For cooling, the larger engine has about the same thermal efficiency as the smaller one, and it will generate about the same amount of heat with similar load conditions. The larger engine would generate more heat when running with 85% of full throttle or higher. This is usually a momentary condition of little consequence. If you tun it near full throttle for 10 minute straight, then you can expect to see higher coolant temperature, but not necessarily overheating. The A radiator should be okay. The B radiator has very little difference in cooling capacity.

Additionally, if you still have the original cell core radiator you would be well off using it or having it repaired or reconditioned if necessary. The original cell cores cool better than replacement VT tube cores. You can get a replacement cell core, but it is moderately expensive. It is also special order from only one or two possible sources, and they deal only with businesses, do not sell retail. So if you need one, get your local radiator shop to order it and use it to recore your radiator.

A and B oil filter assemblies are physically interchangeable.

The A crankshaft dog nut can be installed on the B engine.

Use the MGA carburetors and exhaust, but change the fuel needles to the ones original for the early 1800 engine.

It is an iron engine with a iron head, old tech, nothing special about it. Any competent engine shop could do the standard work for valve job, reboring, and/or regrinding the crankshaft. They don't necessarily need to know what car it came from, except to be able to order replacement parts. If you supply the parts they could do it without knowing the original car. Low tech, sort of a no brainer for the engine shop. Just be sure they put oil plugs back in the correct places (and not where they don't belong) and install core plugs properly so the won't pop out. You might be concerned about negative reports on engine shops.
Barney Gaylord

It is not a lot of work to change to the B clutch, which is stronger and less expensive an a A clutch.

You do need the following parts: an early B gearbox front cover and the matching release fork, and a complete B clutch kit. Additionally, you either need an early B flywheel, or you can have the A flywheel machined to take the twin peg lugs for the B clutch. The B flywheel is lighter, which is a big improvement, but you can have the original lightened instead if you can't find an early B one. AFAIK the later B flywheel does not fit the early crank.

You can R&R the front gearbox cover with the gearbox in the car if you are careful. If you have an early A cover here, the later one (A or B) with an oil lip seal instead of the scroll is a big improvement anyway.

On the cooling front, get the 3 bearing engine boiled when reconditioning, and do the same for the radiator. I run a supercharged engine with a standard radiator, and have a constant 70C engine temp except on long runs on hot days at high speed when it sometimes gets to 80C. 100C is 212 F. Ambient temp on a hot day would be 35C.
dominic clancy

Sorry Paul, just realized you are in the UK and don't need the Fahrenheit Celsius conversion, just trying to be transparent for our USA readers!
dominic clancy

Hi Paul,

Why not try Cameron Gilmour at

He is based at Errol in Perthshire and has been rebuilding MG engines since 1984.



They were on my list, at top actually, but I have been unable to get any response from them by phone or email. I have feeling I saw a message from them some months ago suggesting they were doing less, but I can't remember where I saw it. I will try again as half an hour up the road is a big plus.

Paul Dean

I did this conversion some time ago (a 'temporary' solution as I have a stock 1588 engine awaiting rebuild).
I can confirm all the above comments from Barney and Dominic.

I have the standard 1600 MGA clutch and flywheel. I am an 'enthusiastic' driver. I have maybe 5k miles on the car since the engine change - no problems at all with clutch slip or wear so far.
When re-assembling I found that the clutch cover interfered with the top of the gearbox bell housing. I needed to gently grind away the inside of the bell housing. The bell housing is not very thick there and it needed some careful measuring. I preferred this to sourcing a 'B' bell housing (I have 2 or 3 MGA gearboxes as spares).

As Barney says. Make sure that the radiator core is at least to the original MGA spec. With cars of this age it is very likely that the radiator has been re-cored by a previous owner. Any decent radiator repair shop will have details of heat dissipation from different core matrices. I do have an oil cooler fitted, and while it may well cool the oil, I'd be surprised if it makes any great difference to overall heat dissipation from the engine.

This is a very straightforward swap for a self-taught spanner monkey like me - you shouldn't have any problems.

D Brown

I have not been on here for so long but I have just had a phone call from an old customer of mine telling me about this thread . Yes I am still here and absolutly flat out with work and No I am not doing less If you have not been able to get me lately its because I have been recovering from a small operation.
The website has been blocked so you wont be able to buy any thing off it as its 13 years out of date but all the phone numbers and email are still the same.
It amazing how a little bit of miss information can travel so far.


Thanks for reminding me of Cameron Gilmour. I have now spoken to Cameron, just before seeing his entry above. As he says he has briefly been out of circulation and that must have been when I tried to get him.

Paul Dean

There is often an interference issue when using the MGB diaphragm clutch cover with an MGA transmission.

You should assemble the trans to the engine and see if it turns - I have seen them completely locked up because there was no clearance. Remove the trans and get in there with a grinder and remove anything that interfered (if you could turn the engine over when the trans was on it, look for bright spots indicating rubbing).
Bill Spohn

To finish off this thread.

Thanks for all the input particularly Barney's comprehensive advice and Andy for reminding me of Cameron Gilmour to whom I have this afternoon delivered the engine. I am sticking with the A clutch and standard A cooling for now. I am having the engine balanced and fly wheel lightened by 20%. Have removed my 1500 engine to recover back-plate and almost new clutch.

MGCC member will find Cameron starring in this month's magazine's Supplier Profile.

Paul Dean

Paul - There has been no mention of a brake upgrade. As you are here in the UK it might well be worth checking out what sort of upgrade is required for that bigger engine. The 1588 & 1622cc units needed front discs, etc. I'm sure the B had bigger brakes.
P N Tipping

The MGB didn't have bigger brakes than the disc brake MGAs. They were pretty much equivalent, albeit using a different caliper.

I second the idea of going to later MGA discs, though, as when someone upgrades the engine they often intend to drive it a bit harder and the drums on the 1500, while adequate for the day, need particular attention to friction material, otherwise they can fade.
Bill Spohn

I put the 5-bearing 1800 in my car, then put 1600 discs with the MGB calipers. I certainly noticed the better stopping power and no fade. I agree about the tendency to drive harder and faster. It allows you to keep up with the general flow of traffic and knowing that all those other cars have discs it is comforting to know that your stopping capability is also closer to theirs.

Steve Gyles

And at the moment I am really surprised to report that A disc calipers are cheaper than B ones. I logged onto Moss this morning to take advantage of the pre Christmas sale, because I had been thinking anyway of replacing the 25 year old calipers and was amazed to buy them for sixty pounds each (I don't have to pay VAT being outside the EU). The B ones are 50% more.

If anyone in Europe is desperate for a set of second hand Lockheed ones in the spring they will be available for the cost of the postage.
dominic clancy

Interesting discussion, perhaps I will crack and do the upgrade.

I nearly fitted front discs a couple of years ago but several very knowledgeable people said separately effectively the same things which was basically 'don't bother its not worth the effort as there is very little difference if drums are well set up and in some ways the drums are better'. When I say knowledgeable I am talking about a couple of reputable restorers, several owners who have had both, owner of an original works rally car, and at least one owner who currently owns cars with all 3 A brake configurations.

Clearly if I did fit them they would be A ones as B ones are effectively the same and have only really ever been fitted because at times A calipers etc haven't been available/expensive.

I have had a look at original road tests some of which shows braking test results and yes they do show discs being up to 10% better. Having had drums for 44 years I am not concerned over fade of drums, also an owner who had done the Stelvio Pass descent in a Classic Rally in a very hot rally spec 1500 said the drums didn't fade even then.

My insurers have quoted on basis of 1800 with drums but would probably be happier with discs.

Yours confused and willing to hear more.

Paul Dean

I have had As with both types of brakes. I did not see any difference in normal driving. I wanted the disc brakes to be better and maybe they were but I could not notice the benefit.
We do not have mountains and I do not race, I gear down when slowing, so have not experienced brake fade.
D Hanna

I think your drum set up should cope well.
Usually, if drum brakes are in good condition and you fit some decent quality brake linings you will get plenty of stopping power and a better initial bite and "pedal feel" than you would get with discs.

They can tend to fade more than disc brakes after really heavy use but you would probably have to actually "race" down the Stelvio to cause this.

My disc set up need a fair shove on the pedal to get the best out of them. My pedal has a very hard feel to it, worse since I fitted green stuff pads last year, and I am changing back to Mintex 1144 semi competition pads very soon to get the bite back into them.

Colyn Firth

This thread was discussed between 24/10/2013 and 18/11/2013

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