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MG MGA - Balance

Is it OK to rotate the flywheel 60 or 120 deg away from the "1/4" position? This is to expose unworn teeth on the ring gear to the starter.
Also, the clutch can be installed in one of 2 positions on the flywheel. Does it matter which, as I can't see any difference or index marks.
Art Pearse

The engine can stop at the compression stroke on any cylinder. If the ring gear teeth are worn, rotating the flywheel will not help at all, you need to find another flywheel or replace the ring gear.

The clutch plate positioning on the flywheel has no significance. If I remember correctly the A flywheel has three dowels, the B has two, sp the a clucth can go in three orientations, the B in two. as it's a rotating mass it has to be balanced in any orientation, which is why there is no "wrong" position (as long as the friction plate is correctly installed with the flywheel stamping tpwards the flywheel)
dominic clancy

Put the flywheel back in the original position.
As Dominic says the engine can stop in four different positions anyway.
The clutch can go back in either of two postions (it has two dowels, not three).

M F Anderson

Hi Art. If the ring gear teeth are badly chewed up, it would be best to replace the ring gear, or use a better flywheel. Changing the ring gear isnt that difficult, but the ring has to be heated enough so that it can slip onto the flywheel. A home propane BBQ gets hot enough to expand the ring gear enough to fit it to the flywheel. Cheers, GLenn.
Glenn Hedrich

The teeth are not badly chewed, just a little according to popular opinion on an earlier post. I then smoothed the rough bits with a file. So I will re-use this gear. It was suggested that rotating the gear by 90 will expose the "best" teeth to the starter. (the bad ones are in only 2 positions 180 apart. My question was concerned with the rotation of the flywheel with respect to the crank to achieve this "better" position. As I see it, the flywheel is independently balanced so it should not matter. If there is any doubt to this theory I will install it as it came off. I will also install the starter and test spin it before putting the transmission on and into the car.
Art Pearse


I have assembled a block from parts of different motors. The flywheel had some rough teeth and I smoothed them off with a file and then emory paper. I did all the teeth and it worked well. When I put the fly wheel on the crank, I did rotate it 90 degrees before it was balanced for the same reason you mentioned. When I was able to spin it with the starter, I was happy with how it sounded. There was no major change in the sound as the gear rotated. The flywheel and the crank were balanced separately, and then fine tuned as a unit and marked. Hope this helps your decision.

George Raham
TD 4224
G. L. Raham


There is still this confusion. The engine does not stop in the same position each time. It stops on the compression stroke of any of the four cylinders.
Put the flywheel back in the original postion. George said that he moved his flywheel, but then he had it balanced after he did it.
Unless you are having the crankshaft and flywheel assembly balanced as a unit, do not move the flywheel.
The factory changed the balancing policy for the Twin Cam in August 1959. They probably changed the policy for the pushrod engine as well, but we cannot be sure, or if they did, the date or engine number.

See attachment.


M F Anderson

I would leave the flywheel in the original position just in case it was balanced that way. My ring gear showed wear in two positions, 180 deg. opposite from each other. This is normal as there is a piston comming up on the compression stroke every half turn of the crankshaft. If it is bad enough to justify moving it, then I would replace it. However, the photos of your ring gear do not look worn enough that I would bother changing anything.
Ed Bell

To clarify my above comment, I am referring to my work on a "T" series block. I added an addendun to my thread that explained the situation. It appears that it was not included in my thread. I hoped my comments would be of some help to Art.

George Raham
TD 4224
G. L. Raham

A four cylinder engine will stop in one of two positions when pistons are about half way up the stroke against compression. Heavy wear on the ring gear teeth will be in two places about four inches long 180 degrees opposed. The flywheel on the 3-main bearing engine is mounted with six bolts equally spaced and no dowel pins, so it can be repositioned in 60 degree increments. This 60 degree rotation will place fresh ring gear teeth in proximity to the starter pinion (and it can be done twice to triple life of a ring gear). I have done this multiple times with good results and no perceptible balance problem.

The mga twin cam flywheel is same part number as all MGA 1500/1600, so replacement flywheels would be the same for push rod engines as for Twin Cams. Every MGA flywheel I have ever seen has the "1/4" marking regardless of year of production, even after the CSM says they are no longer required to be matched sets.

In spite of the fact that MGA flywheels are marked "1/4" for the initial relative position with crankshaft, the flywheel and crankshaft were always balanced separately in production. For early production cars there was apparently an additional operation for checking balance of crank and flywheel in assembly. I have no idea if the assembly was ever retouched for balance during this process. For all I know the "1/4" mark may have been applied to the flywheel before it was ever mated to a crankshaft. If any adjustment was deemed prudent it may have been done by swapping flywheels rather than by retouching balance of either part.

I have on multiple occasion swapped crankshafts and flywheels independently between various 3-main bearing engines, including replacing broken crankshafts and using whatever flywheel happened to be handy and in good condition. I have also installed early MGB flywheels on MGA crankshafts and vice versa, no concern for orientation or parts matching, and no problems.

If you change either part independently to be no longer matched as the original pair, the original "1/4" marking becomes entirely irrelevant. It is of course difficult or impossible to know if the parts in your vintage engine are original parts in original mated pair after decades service and many changes of ownership. If you are at all concerned about this, the only way to be sure of good balance "together" is to have both parts checked for current balance condition, and adjusted accordingly if necessary. When the two pieces are balanced accurately it is not necessary to further balance the assembly (or have any concern for orientation).

On one occasion I did ask a shop to check and/or balance a crankshaft and a flywheel, each about 40 years old at the time. Both parts were returned with a smile and no fee, checked and verified to be within reasonable balance with no touch up required, and no special orientation of flywheel on crankshaft would make any difference. It does not logically follow that all original parts will be similarly well balanced, but I have never seen otherwise.

Our old friend John Twist once said, "I have never seen an engine failure that could be attributed to a balance problem". I agree and support the same statement. For a no holds barred race engine, by all means balance everything meticulously to better than original standards. For a stock engine in a street car, just put it together and enjoy the drive.

My one exception to this is for connecting rods. Connecting rods originally had a total weight variation of +/-1 ounce (total range of 56.7 grams), but were matched in sets with weight variation within 3 drams (5 grams total variation within a matched set). Individual parts were stamped with a weight range number, so a single part could be replaced without subsequent balancing operation. At minimum do be sure that your con-rods all have the same weight range stamp number.

For any con-rods passing through my hands I will weigh both ends of each con-rod on a digital postal scale, then grind a bit as required to match weights within 1 gram. A production shop may not do this without special request, and you may not want to pay for this special operation. I do this simply because it is a nice touch and takes only 1/2 hour for the full set.

As a side reference, most new piston sets will be matched with 1 gram (sometimes within 1/2 gram). Field service and aftermarket parts are commonly more accurately matched or balanced than original production parts.

Now I will pick up my proudly amateur advice and go back to lurking. If you don't like the free advice you can have your money back with a smile. For those with a little faith, $.03 and you're welcome. For those who never believe anything $.02 and bless you.
Barney Gaylord

Again, Barney is completely correct. I have rebuilt numberous MGA and MGB engines, 3 main and 5 main. All the rotating components are idividually balanced from the factory.
James Johanski

Thanks everyone. I should have mentioned that the engine I am working on is a 1622 Austin A60, now converted to MGA. This replacement engine probably came with its own flywheel. There is no matching 1/4 mark on the crank flange.
Art Pearse

I beleive you can also consider a gear reduction starter. They stick out past the ring gear and pull into the ring gear rather that thrust out into it like the original starter. I found that most original gear teeth are chewed up in the front but are in decent condition in the back.
WMR Bill

My starter already pulls into the gear. It is the standard Lucas unit. The starter pinion sits aft of the flywheel. Isn't this normal?
Art Pearse

Sorry Art, I may have had that backwards. I think it is the gear reductions starter that push out and the original starters pull in.
WMR Bill

This thread was discussed between 16/08/2009 and 18/08/2009

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