Stripped flywheel bolt

Ross F1
For peace of mind, remove the crank and have it drilled and tapped 45° around.
This would also give you fresh ring gear teeth for the starter
Bruce
 
If I were going to pull it all apart, I'd just replace the crank. I have three others on the shelf. I don't think I'd want to add any more holes to the flange, and the dowels would be problematic.

Not sure how that would change the teeth? I use a mini-starter anyway.
 
Or just replace the flywheel. Easier than messing with the crank.
Has anyone here welded up a flywheel hole and retapped it?
 
I just did some calculations and if you can get some proper good quality bolts, it does sound like 12x1.25mm would be a good fix. If I were doing it this way I would do at least 2 bolts and might even do all 4. Don't drill out any holes in the crank as they are already above the minor diameter for 12x 1.25. You will have to drill the flywheel to 12mm.

Holding the tap square to the block might be a little tricky. Maybe drill the flywheel first then thap through. If doing it in vehicle it may be hard to ensure good alignment and removal of chips. They will just fall to the bottom of the bellhousing though, so no big deal.

You might be able to rig some sort of v block or something to hold the drill square.

Mart.
 
Mart, I am planning to make a drill guide to keep the bit square, and will go thru the flywheel. The tap drill for a 12mm x 1.25 is a 27/64, which is .422". The pitch diameter on the 7/16"-20 is .405", so I'm only removing about 8 thou all around.

This is an 8BA, so chips will just fall to the floor. I have the offending hole on the bottom.
 
Hi Ross, fix it with the correct heli-coil thread. It's actually quite simple if you make a simple drill & tap guide. We've made dozens of these "guides" over the years. We have most "slotted" but you can do it by the center-to-center method on the bolt locations.

Thanks, Gary in N.Y.

P.S. Here's a couple shots of a few with one showing the tooling mounted. The same tool is used first as the drill guide then moved and used as the tap guide. The tools have all been tapped with the respective heli-coils taps.
 

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Got 'er done! Actually doing it once I had all the pieces in hand, fixture made, etc. took a lot less time than all the hand-wringing! Par for the course...

Gary's excellent drilling fixture inspired me to make one out of square tubing. Worked like a champ. The 27/64" tap drill leaves a little bit of the old thread, but since the 12mmx1.25 is almost exactly 20 TPI, it actually helps keep the tap going in the right way. There is almost no room behind the flange, so I rigged a piece of heavy sheet metal with a magnet to keep from drilling into the oil pan. The tap just barely made it thru to the full-size threaded part.

Thanks for all the help, glad to have this behind me!
 

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Ya just GOTTA love this forum.....The wealth of information and willingness to help each other is unmatched.....anywhere.....I have no plans to drill a crank flange.... but.... the methods described will certainly come in handy in one of my upcoming disasters...I'm sure....
 
Just the one, Mart. I'll post a pic of how I will compensate for the heavier 12mm bolt, not that I think it's really an issue. Amazing how close the 12mm is to a 7/16".
 
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