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Old 05-10-2019, 08:14 AM   #1
47Merc
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Default 11 inch clutch flywheel question

I have a 1940 81A engine with a flywheel which is possibly not original as it has only ever been fitted with an 11 inch clutch and pressure plate.

The question is, would it have come from a later truck or similar or did they fit 11 inch clutches in 1940 and also would the flywheel be heavier than a 1940 car flywheel that uses a 9 or 10 inch clutch?

I am considering getting it drilled for a 10" pressure plate as the 11" one cannot be removed without taking the sump off but if it is too heavy for use in a car I'll look for another one or possibly get it machined and if thats the case how much metal would I remove?
Thanks
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Old 05-10-2019, 08:40 AM   #2
19Fordy
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Default Re: 11 inch clutch flywheel question

The part # for the OEM 1940 Ford 81A flywheel is 48-6375 which uses the 9 in clutch and pressure plate. It works really well.

IMO it would be worth the time and effort to install a 10 inch assembly, if possible, as 11 inch is overkill and weighty.
Whatever you do make sure your starter will work properly with the flywheel you install.
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Old 05-10-2019, 09:02 AM   #3
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Default Re: 11 inch clutch flywheel question

Depending on what you want out of your vehicle the 10in would be a better option for most applications.
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Old 05-10-2019, 09:51 AM   #4
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Default Re: 11 inch clutch flywheel question

The OEM pressure plate on my 1940 tonner was 11”. I opted to change to a 10” PP with the lighter springs and noticed a difference when double clutching the 4 speed trans especially in city traffic. You could lighten the flywheel by taking some metal off, but to what point? Unless you’re looking for performance I don’t think the cost is worth the effort. A suggestion. Should you decide to do the switch have the shop balance the PP and flywheel as a unit and mark an alignment location on each for reassembly. You’ll have to balance the flywheel anyway and balancing them as unit is a simple way of insuring that the clutch assembly part of the drivetrain is rotationally balanced.
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Old 05-10-2019, 06:51 PM   #5
rotorwrench
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Default Re: 11 inch clutch flywheel question

The 11 incher was used on 4-speed trucks including some special order 1/2 ton commercials.

It's OK for trucks but it's a big heavy old clutch so it slows rpm build due to that. It's definitely not for performance on a flathead.
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Old 05-11-2019, 04:28 AM   #6
47Merc
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Default Re: 11 inch clutch flywheel question

Thanks for the replys.
Is it just the pressure plate that is heavier (I can see the one I've got is heavier than a 10") or is the flywheel heavier too?
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Old 05-11-2019, 08:56 AM   #7
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Default Re: 11 inch clutch flywheel question

According to the specs from the 1940 Ford Reference Manual the flywheel in the 85HP and 95HP Ford, Mercury and Lincoln cars weighed 38.7 pounds. The flywheel in Ford trucks with an 11” clutch weighed 34.1 pounds. Assuming these numbers to be correct it looks like the engineers designed the flywheel and PP system in such a way that balanced the rotating weight of the flywheel/PP system with the torque curve demand of the engines. You may already have a flywheel on your car that weighs less than one on a passenger car. When I changed to a 10” PP there was no noticeable difference in performance other than the release and engagement of the pressure plate which was physically easier to do, especially when having to slip the clutch while in traffic. Other than a light cut on the face of the flywheel to freshen it up there was no metal removed from the flywheel on the tonner.
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Old 05-11-2019, 09:01 AM   #8
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Default Re: 11 inch clutch flywheel question

^^ I wonder if that is referring to the passenger type flywheel with the heavy raised ring around the outside??
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Old 05-11-2019, 09:32 AM   #9
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Default Re: 11 inch clutch flywheel question

There is more to it than just the total weight, the diameter is a factor. It is the weight at a distance, it is the inertia. The closer the rotating weight/mass is to the centerline the lower the inertia is. Inertia relates to acceleration, if you could care less about vehicle acceleration you might not notice the difference.

In something like a heavy truck caring a load, a large inertia with slow acceleration can be an advantage. A stripped down hopped up car with low gearing would want low inertia to help acceleration.
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Old 05-11-2019, 10:19 AM   #10
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Default Re: 11 inch clutch flywheel question

There were different flywheels but the clutch cover or pressure plate on an 11 inch Long type is heavier than the 10-inch for sure. A lot of the older cars with the thick rim and 9-inch clutch were pretty heavy. Most folks cut them down. On a light body car, the 9-inch clutch works just fine to lighten things even more. RPM builds like a rocket with the lighter flywheel. For a big old truck a person need the kinetic momentum to carry the torque better.
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