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MG MGB Technical - Harmonic Crank Balancer

As we approach 40 years, and 100k Miles on our 1974 B/GT one of the items I want to replace this spring is the Harmonic Crank Balancer, and timing chain. Moss USA list an Australian made balancer unit for ($284.95 US), and also what I assume is the standard replacement with dampers for around ($100.00 US) less.

It appears (to me at least) the Australian item is targeting the owners with modified engines, and racers. My two questions are; first, does anyone know who makes the "standard" replacement, and second has anyone installed that version and are they pleased with the results?


Larry C. '74 B/GT & '69 Midget
Larry C '69 Midget

Well I have one that has done 300,000 miles + and the engine has been rebuilt twice - still running on the original balancer and there is nothing wrong with it.

Yes the rubber may appear cracked where it shows between the the two parts but trimming that up while cleaning the pulley in the rebuild shows the rubber is still flexible and well adhered.

Unless yours is obviously disintegrating I would leave well alone!
Chris at Octarine Services


Thank you for input on this, and your experience. The car is entirely original, and never been apart from new. My wife, and I are planning a cross country drive in this car. The original thought was to replace the original single row timing chain and gears with a new double row. If I did that work, I would replace the harmonic balancer at the same time.

Visibly the damper looks good, so I will leave it well enough alone.


Larry C.
Larry C '69 Midget

I would agree with Chris...Ive had a lot of B motors apart but never seen a damper go bad. The MGA motors I usually try to find an early 3 main MGB motor and use those as the A had just a pressed stamped steel pulley.

Your GT is a 74.5? It would have a double row timing chain already unless Im mistaken. I thought they went to singles in 1975.

Steven Devine


I'm not sure I'm right, but didn't the single chain start with the first of the 18V engines, which is sometime before the change to rubber bumper?

C R Huff

Steven & Charley,

Our 1974 B/GT has the single row chain. No real signs of a problem, but rather replace this now than have a problem on the road. As for the harmonic balancer itself, I'll have a good look at it when I remove it from the car. If it looks good I'll reuse it as Chris suggests.


Larry C.
Larry C '69 Midget

Hey Larry
Charley and you are right I was confused...
18v are single rows.

Its been a while since Ive had a 5 main apart...

I remember pulling the double row pulleys off the cars at the junk yard and putting them in my single row cars.
Steven Devine

The double row sprockets were dropped at the start of 18V production in August 1971 as were the double valve springs.

If you do decide to change to double row chain & sprockets then be aware that it shifts the valve timing and you will need to reset that using offset keys or a vernier cam pulley.

There is no NEED as such to make the change - it is not "better" unless you are running stronger valve springs or high lift cam / rockers. The single chain & sprockets is more than capable.

You may want to check the tensioner for wear and replace the single chain if it is worn - but they rarely fail.

Chris at Octarine Services

"($284.95 US)"

Gordon Bennett! 25-40 quid here.
Paul Hunt


I'm sure that's not the only item Moss (USA) has overpriced in their catalogue. If I decide to change it out, I'll shop around a bit.


Larry C.
Larry C '69 Midget

The 18-V-584-Z-L and 18-V-585-Z-L engines used the duplex camshaft drive chain (BMC Part # 2H 4905) and sprockets(crankshaft drive sprocket BMC Part # 12A 1553, camshaft driven sprocket (BMC Part # 11G 203). In October of 1972 the camshaft drive chain and both the drive and the driven sprockets were changed from a Duplex (dual-row) camshaft drive chain system to a Simplex (single row) camshaft drive chain system (Drive Sprocket BMC Part # 12H 4201; Driven Sprocket BMC Part # 12H 4200, Camshaft Drive Chain BMC Part # 3H 2127) on the North American Market 18V-672-Z-L, and 18V-673-Z-L engines. Although the 18V-672-Z-L and later versions of the 18V engine sacrificed dual valve springs (BMC Part # 12H 1679, Outer, BMC Part # 12H 176, Inner) used on the earlier engines for single valve springs (BMC Part # 12H 3352) in an effort to reduce production costs, it should be remembered that due to changes in their valve timing, these later engines reached their maximum power output at the notably lower engine speed of 4,800 RPM instead of the 5,400 RPM of earlier engines, and thus spring surge was deemed to not be a problem that would require the extra cost and complexity of dual valve springs.
Stephen Strange

This thread was discussed between 10/03/2014 and 16/03/2014

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