The MGA With An Attitude
RING GEAR Interference Fit - FW-101A

On 1/10/2007, Ian Hamer wrote:
"Anyone have knowledge of the interference fit between starter ring gear and an aluminum flywheel prior to shrinking on the ring? How many thou?"

It is the same for steel or aluminum flywheel, since you are heating the steel ring gear for expansion and shrinkage for tight fit. It also does not matter if the ring gear is forcibly installed with a large press or heated for expansion and loose drop on and shrink fit. Force fit and shrink fit are the same assembly, heat is used as a matter of convenience in lieu of a large press. In the case of aluminum flywheel you might heat the ring gear to avoid possible damage to the softer aluminum part by forcing.

There are five different standard classes for Force iNterference fit from FN1 to FN5, lightest to heaviest interference. The one most commonly used, FN2, is proper for a ring gear. From Machinery's Handbook, "FN2 Medium Drive Fits are suitable for ordinary steel parts, or for shrink fits on light sections". The ring gear is a light section part (thin relative to overall size).

The MGA ring gear mounting diameter is about 10-3/4" (by tape measure). Specified interference for FN2 fit in size range 9.85" to 11.03" diameter is 0.0040-0.0072" interference, to be split between male and female parts. For mass produced interchangeable parts it is intended to be:
Hole +.0020" to +.0000", Shaft +.0072" to +.0060".

Loosest fit would be Hole +.0020, Shaft +.0060, Interference .0040.
Tightest fit would be Hole +.0000, Shaft +.0072, Interference .0072.
This is based on standard hole size with tolerance on the plus side, amd making the shaft larger to provide the desired interference.

Measure the ring gear bore first at room temperature, as it may not be exactly 10-3/4". This is tricky because the ring gear may not be perfectly round, so measure across the diameter in several different places. If it is somewhat oval in free state you can pinch it slightly over the largest dimension to make it closer to round when measuring.

To the ring gear bore add 0.004 to 0.007 inch to get the finished size of the mounting diameter on the flywheel. When machining the flywheel, shoot for the middle of the tolerance range, as the next ring gear may not be exactly the same size. It is a bad idea to enlarge the bore of the ring gear to match an oversize flywheel, as you would no longer have interchangeable parts for future ring gear change. If the mounting diameter on the flywheel is undersize, it is trash, as it is not recommended to mount intended interference fit parts with adhesive.

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