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MG MGA - GD series Engine - differences?

Hi all. I have been looking into the engine on my "A". I have been trying to find out what exactly the differences are between the GB series and the GD series.
So far all I have found is
1, High mount starter
2, Shorter drive shaft
3, Higher horsepower ( I don't know how they achieved this )

Does anyone know how the engines are different?

Thanks in advance.
.. CR
C.R. Tyrell

Early MGA with GB engine (1500 type) were rated 68-BHP. A bit more than a year into production this was upgraded to 72-BHP. I think it was done with better porting in the cylinder head, but it is not well documented. The engine designation remained GB. GA was the MG ZA/ZB Magnette engine (on the market before MGA). The GA engine got the same power upgrade as the GB engine.

In January 1959 the MGA gearbox changed from low to high starter position, and different propshaft (flanged output on the gearbox). This required a different back plate on the engine. There was no other (functional) change to the engine.

The engine and gearbox together is considered to be the "power unit", and it is the power unit that has the GB or GD model designation. From Jan '59 to Apr '59 the MGA had the GB power unit, until introduction of the MGA 1600.
Barney Gaylord

According to Neil Cairns (who wrote the relevant book) which is available here:

MGA 1500 Engine,(1955-59)
The ZA generously gave its engine and running gear to the car M.G. had been trying to get out and sell for a few years, in August 1955. The 1489cc 'B' series BP15GB, was a similar engine but with pancake air filters, and was the M.G. improved unit above specially produced for the MGA, having used the ZA Magnette as a test bed. A major difference was the MGA 'normal' sump, sitting behind the suspension cross member, and requiring a new oil pick-up, that it gained from the run of the mill Austin and Morris cars. It produced 68bhp at 5200rpm, later to rise to 5500 as the BP15GD with 72bhp at 6000rpm in 1956, with its improved crankshaft. In the low slung sports car it performed well, giving 0 to 60 in about 14 seconds, better than that of 16.5 for the TF it replaced. The 68bhp version was fitted to the ZA Magnette in 1956, and in 1957 to the Morris Minor based Riley 1.5. The crankshaft was very strong, made of EN16 steel, with 1.875" diameter big ends and 2" diameter main bearings, of generous length, being counterbalanced with shell bearings, now located by lugs at the cap joints. The valve springs had been beefed up on the power increase in the MGA & ZA, and were double, to permit higher rpm, where as on the lower revving Austin/Morris there were single springs. Normal BMC cars used a 7.2 compression ratio, for M.G. this was increased to 8.3 to 1. All MGA's use the same camshaft, with a skew gear on the rear end to drive a mechanical tachometer, valve timings are 16;56;51;21, with a lift of .355". This camshaft continued with the early MGB. The only two other cars that used this tacho-drive was the Riley 1.5 (15R,RA,RB engine,) and Riley 4/68 Farina ( 15RA & RB,) and 4/72 until 1966, (16RA,) both using a close copy of this MGA 1489cc engine in 68bhp trim. EX182, Le Mans 24 hour Engine,1955.
Used a balanced 'B' series engine of 1489cc as in the ZA Magnette, standard 8" clutch, with a modified cylinder head as per-BP15GD, two SU fuel pumps, twin SU 1.75" H6 carburetters, fitted with trumpets ( ram stacks) not air cleaners, with a cold air feed, oil cooler, mildly tuned to give 82bhp on a 9.4 to 1 compression ratio. The special Weslake cylinder head was lapped onto the block, so no head gasket was required. Rather a bland engine when you consider how famous the car is, possibly built for reliability rather than speed, ie to finish the 24 hour course. Two of the cars finished, 5th & 6th in their class. Alas no one remembers this, as a Mercedes crashed into the crowd killing many spectators.

MGA 1600 Engine, (1959-61).
By April 1958 the MGA needed a boost in power, and the M.G. 'B' series was re-cored giving the cylinders thicker walls, allowing it to be bored out to 1588cc with a 75.4mm bore and the 89mm stroke. The engine was otherwise unchanged from the 1489cc BP15GD, now being called a 16GA. This gave 80bhp at 5600rpm with 87lb/ft torque. It was called the MGA 1600, and the cylinder block was bored to this 1588cc size for the mga twin camshaft engine, and this capacity is unique to M.G. as no other marque used it. See also Twin Cam.
dominic clancy

Thank you for all the information. It just is very interesting to me that the HP was increased with no real documented changes.

Thanks again. So much information is there but so many fine details just never survived the passage of time.

Thanks again.... CR
C.R. Tyrell

if you look at The MGA Special Tuning Manual in Barney's website this shows how the factory modified the engine to get more power.

c firth

Thanks.. will do.
.. CR
C.R. Tyrell

This thread was discussed between 02/02/2016 and 06/02/2016

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