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I've got a 1915 engine I rebuilt a few years ago. I purchased a new 8 dowel flywheel for my existing 8 dowel crank, but it doesn't fit.

I measure the crank end and it measures 53.75mm diameter, the flywheel hole measures 53.17mm.

Does anybody know what is going on here? Which is the correct diameter?

6v or 12v?
The new flywheel probably has an oring to seal the end of the crank to the flywheel.
(goes in the flywheel)
I'll bet the old one doesn't/
The non-oring early flywheels took a steel gasket between the crank and flywheel...
Unless it was wedgemated then it needed nothing.

I can't recall what year that was but it was when they went from 6v to the 12v flywheel.

Early style 12v flywheels exist.

Thanks for the Info. It's a 12v, 130 tooth

The old flywheel also had the rubber O-ring inside.

The problem is that the flywheel mount inside diameter is so much smaller than the crank, I can't get it to go on, no matter what I try.

I didn't get out the mike, but a quick caliper-check of three stock O-ring crankshaft snouts came up at 2.107" (53.518mm) so it looks as though your issue may be caused by a crank and/or flywheel that's improperly machined (assuming some slight discrepancies in our measuring techniques).

The change from gasket- to O-ring-style seal came midway through the `66 model year. To provide a place for the O-ring to live, the later cranks were machined to a smaller diameter for ¼" or so from the end. The endplay shim I.D. was reduced by 1mm at the same time, a common mistake is to use the early large-I.D. shims on an O-ring-style crank - those will sling around and slice a groove right into the snout until they hit the dowel pins.
In this case I see no solution (short of having the crank snout turned down) other than taking the flywheel to a machinist and having its I.D. opened up to allow it to fit over the crank snout. You could probably get away with doing it at home with a Dremel since it isn't really critical that if fit perfectly, so long as the flywheel can go on fully, but if you choose to try that I recommend that you first place a steel gasket over the mating face to protect it from damage if your hand slips. When done, use a whet-stone to dress off any burrs that you may have created at the inner edge of the thrust surface (lacking one of those, stroke/spin the flywheel over some solvent-flooded wet-or-dry sandpaper on a flat surface like a piece of window glass).

Thanks Marc. I appreciate you taking the time to measure some cranks. Not having access to others I wasn't sure which was out of whack, now it seems clear it's both the crank and the flywheel. Not sure how that happens, but guess that's just the kind of luck I have.

I have been contemplating the dremel solution, as the only machine shop in my town doesn't "want to do that kind of work". So it will eaither be the dremel, or drive to the next town and see if someone there will do it.

You might want to update your profile in case someone knows of an ACVW friendly machine shop near.

Viking131313 wrote: ↑Sun Apr 23, 2017 7:15 pm...I have been contemplating the dremel solution...

You shouldn't get in much trouble if you protect the mating surface - it doesn't have be perfectly round, just big enough that they'll go together fully. That's easy to check by just putting some Prussian blue on the end of the crank before the 'wheel and tightening the gland nut down hand-tight - it'll be evident that they've made good contact when you take it back off. Grease or even lipstick will work too.
If you make any nicks in the thrust face they won't hurt anything, just remove any raised material that might interfere with your endplay setting. To be extra cautious you may want to wrap the O.D. where the seal rides with some tape too.

Yep, used dremel and a fine grinding stone. A couple gentle passes in around the inside and it fits like a glove.

If you are trying to mate an orphan crank to a new flywheel w/ 8 -dowelling prior - they will NOT match.

The dowels do not home in into the holes. You will have to 'blue' the holes in the flywheel and scrape to fit the flywheel to the crank. You will need a bearing scraper tool.

I just did a job like that to an orphan crank. I did the dynamic balancing on it too.

nsracing wrote: ↑Sat May 13, 2017 7:41 pm If you are trying to mate an orphan crank to a new flywheel w/ 8 -dowelling prior - they will NOT match...

Not the issue in this case but a good point in these days of pre-8-dowelled cranks and flywheels. All use the "SPG" pattern (with one slightly-offset dowel hole) so it looks as though they should go together fine, but it's a pretty rare case when all eight line up perfectly and with the correct interference fit. To do a first-cabin job of 8-dowelling, the crank and flywheel should be bolted together along with the SPG drilling jig (using a gland nut with notches ground out of the head) and drilled at once, then each hole reamed to the proper final diameters. The dowels should have an interference fit of ~.0015" in the crank and ~.0003" in the flywheel for the union to have maximum strength, anything looser and there's not much benefit compared to the stock 4 dowels. Properly drilling a flywheel to match an already 8-dowelled crank is basically impossible but through the use of transfer points installed in place of the 4 "added" dowels one can come close - it's common to have to step those four up to 11/32" in order to get a perfect fit.