Re: flywheel

Posted by Joe on March 20, 1998 at 09:47:44

In Reply to: flywheel
posted by jim on March 19, 1998 at 14:14:15

Jim
I heartily agree with John's comments that I don't like cracks on something that is spinning at 2200 RPM just below my feet.
However, I have seen these cracks on my flywheel where the clutch disk is applied and information I saw in one of the Model A books indicated that this is normal and is caused by the heat generated by the clutch.
Apparently on rapid play of the clutch, the surface where the disk touches gets locally very hot, expands and puts itself into compression as a result.Some yielding and deformation of the surface results and when everything cools, the cracks begin at the "intergranular" weak points.Normally these cracks do not propigate beyond their local area where the heat is applied.
Normally, a flywheel is faced for a clutch only if the surface is torn up or shows wear marks such that it is apparent that the clutch is touching at only one sector. (this can be caused by a warped flywheel)When clutch disks are allowed to get really worn to the point where the rivets touch the active surface, the rivets can sometimes cut into the face.This is what flywheel facing is mostly used to cure.
You may remember from your high school physics that centrifugal force is dependent upon the square of the distance from the center of rotation.In reality the center portion of the flywheel is subject to considerably less centrifugal force than the exterior diameter.Less than a third of the distance in from the edge of the flywheel, the centrifugal force is less than half of the maximum amount that shows on the edge.Thus these cracks are at a location that is really not highly stressed and are probably not a result of the stress.
I would say that if your cracks don't seem too deep and don't appear to extend into the band where the pressure plate is bolted onto the flywheel, you're probably all right.You can get a better visual examination for this if you bring your flywheel to a machine shop and have them use either a dye penetrant& blacklight or magnaflux examination, or you can do this somewhat less effectively at home yourself by washing the flywheel carefully in kerosene and then wiping it clean.Then pop your flywheel in the oven set on "warm" for a few minutes and see if any kero oozes out.
Be careful of any answer, or opinion you get from a machine shop specializing in automotive machine work.If you ask them if refacing should be done they are most likely to reply in the affirmative and indicate that if you don't do it you are headed for big trouble.Refacing a flywheel is technical in that it normally cannot be done at home but it is "bread and butter" to people in the trade, and they count on doing it to put food on their table and their kids through college.There is something known as "technical intimidation" and it is used effectively by many (probably most) people in a service oriented postion.
Fortunately, flywheel turning and facing is relatively cheap, except that it leaves you with a thinner flywheel (you are simulating the effect of years of clutch useage) and is not likely to remove all the cracks anyway.The other downside for the Model A owner is that it affects your pressureplate and clutch setup and adjustments and you'll have to compensate for a "deeper" face on the flywheel.
I would probably leave it as it is if it looked o.k. according to what I described.
I hope all this does not add fuel to a fire.There may be others who make the Model A their business who routinely reface and resquare their flywheels "just to be sure it is right".It's hard to argue against this.Also, the person who takes action against a perceived safety threat is never wrong.But do we really have a safety threat in this case?You'll have to decide this for yourself.
I hope a little engineering background has helped you make up your mind.
Best regards,
Joe

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