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MG MGA - Flywheel Weight

Hi,

Does anyone off hand know the weight of a stock MGA 1500 flywheel?

Thank you,

Tom
Tom Baker

My recollection is about 28 lbs.
Bill Spohn

Hi
Bill's right, I have several, standard is 28-30 lbs ( approx 13.5 kilo's ) You can safely reduce them to 10 kilo's, probably less. I'm sure somebody has!

Terry
Terry Drinkwater

Picking up on this theme. What are the advantages/disadvantages of lightening the flywheel? Is there an optimum weight to go target for general 'fun' motoring? What part of the flywheel should the weight be taken from? My engine is the 5 bearing 1800, running an MGB clutch (if that is at all relevant).

No immediate plans to do any weight reduction (except on my own stomach) but just for my own knowledge and perhaps a project in the future.

Good one for you Terry!

Steve
Steve Gyles

Steve:
Assuming you have an MGB flywheel to go with your 1800, it is already significantly lighter than a stock MGA flywheel. In fact, switching to an MGB flywheel is the easy alternative to shaving an MGA flywheel.

I understand that the disadvantage of making the flywheel too light include rough idle and run-on.

Regards,
M.D.
'57 Coupe
M. D.

I always thought that the MGA flywheel was slightly over heavy. MGA owners will know that you have to wait a while for engine revs to fall away before a gear change. This weight flywheel contributes to a stable platform but may not be to everyones choice. If the MGB flywheel is lighter then it could perhaps be swopped over to the MGA. Some owners however may wish to keep their flywheel but has anyone successfully skimmed it and from what area?

Regards John
J H Cole

MD

I believe I have an 18GB flywheel. I run the engine with the 5-speed box conversion.

Steve
Steve Gyles

I have mine lightened to 15 lbs, and there is NO rough idle, or runon!
And I have a reground cam.
Ya gotta ask yourself, why did they originally have that heavy of a flywheel?
Well because that's what they did in those days.
It really isn't necessary.
Safety Fast
Dwight

Dwight

Hi All

Flywheel weight affects how Quickly the engine can spin, race engine have light flywheels as well as lightened cranks, rod, pistons etc. A fast road engine benefit from reduced weight, but this can be a trade off at lower engine revs i.e. when in slow traffic as use loose your 'flywheel effect'. My old 3 brg 1800 had a flywheel which weighed 10 kilo's, I read somewhere that standard 3 brg cranks could whip with a light flywheel. My current 5 brg engine has a flywheel reduced to 7.5 kilo's, reduced from 11 kilos, I think its the same a Steve's 18 GB engine ( inertia starter type). As mentioned by Dwight, I have no problems with tickover or slow running, apart from the occasional MGA pilot asleep at the lights !!!
Sorry Steve !! This bumper thing will end up on my gravestone!


Terry
Terry Drinkwater

Steve, weight removal from a flywheel has the most effect the nearer to the outter edge. I don't remember all the math, but IIRC the effective mass goes up with the square of the radius or something like that. It's been way to long since I was in school to recall the specifics.;-)
Bill Young

Bill, you are correct. What the flywheel does is store kinetic energy which is mass x velocity^2 and the outer edge of the flywheel is moving fastest. If you want a strictly "light flywheel" then you should remove metal from the middle - this will preserve the inertia and smooth running. However, the objective is to reduce inertia , so remove the metal on the outside. Confusing? Another benefit of reduced inertia flywheel would be better braking, as it is part of the job of stopping the car to reduce the engine rpm.
Art

MD said in his post that one possible downside of lightening is run-on. But from what Art is saying, surely this cannot be so. If the inertia is lowered, there is less energy left on ignition switch-off for the engine to run-on. Or am I missing something?

Steve
Steve Gyles

I should clarify my post:

The potential downside that I mentioned was for lightening the flywheel "too much", and was not based on first hand knowledge but what I've read on this bbs over the years. I'm not sure what too light would be, but I guess you would know when you start stalling at idle or runing-on! I have a lightened flywheel on my coupe, and I have problems with neither idle nor run-on.

I think I may have read about those downside risks in reference to alloy flywheels. An archive search might shed some light on the reasons for the idle and runon symptoms.

Regards,
M.D.
'57 Coupe
M. D.

Most stock MGA flywheels are about 28 pounds. Late production MGA 1600 MK-II flywheel is about 20 pounds. This was carried over to the early MGB 3-main engine. The first 5-main engine was still mated to the 3-syncro gearbog for 1965-196y production. That flywheel is also about 20 pounds and can be used to install a 5-main engine in the MGA with original gearbox.

In 1968 the flywheel got a little larger diameter with the 4-syncro gearbox. I think that flywheel is a little lighter weight but having about the same rotational inertia due to mass at the larger diameter.

I have driven an MGA with the 9 pound alloy flywheel, competition clutch, 4.55 final drive gear, and fast road cam. That combination is tough to drive around town. If you're not very careful to rev up and feather the clutch on startup it goes from Vroooom to awsh** in an instant. With insufficient pre-rev it kills the engine, with just a little more revs it spins the tires, and there's not much in between.

Unless you intend to be into competition regularly, I would not recommend the alloy flywheel. The stock 20 pound flywheel is fon on the street. The 9 pounder is definitely not. You might like to shave a litle more off the 20 pounder, but not a whole lot for street use.
Barney Gaylord

I agree with Barney - really light flywheels are not much fun on the street. I'd settle on something like the 3 main B unit for all around street driving.

On one of my race cars I used an alloy wheel around 14 lbs. with a steel face pinned to it - it may surprise people to discover that this was actually an homologated option in SCCA for MGA and 3 main MGB engined TVRs. I do not recall ever seeing these offered for MGBs. Sadly, this rare flywheel is (I think - must look at my spare crossflow engine to see if it is on that one) currently circulating the Nurburgring in the TVR under new ownership.

I also run a lightened steel wheel on the Twincam of about the same weight. It is hard to get people to do this sort of work, as they fear legal retribution when a badly machined flywheel comes apart and separates someone from their ankles and feet.

This steel wheel, on a car with more than 300 degree duration cams, works fine as long as you don't lug the engine. In town, it would be a right pain, as Barney said. The Twincam even without this flywheel revs up faster than a Porsche owner makes excuses when beaten by an MG, so you really wouldn't need a lighter wheel for them anyway.
Bill Spohn

This thread was discussed between 13/11/2005 and 16/11/2005

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