broken flywheel bolt

The 58 American and big car TSMs call for 100-110 ft/lb for the OHV and flathead (flywheel to crankshaft). There is no torque given for the pressure plate to flywheel bolts. When no torque is listed, I generally use a standard length wrench (or ratchet) and pull it 1/4-1/2 turn after it snugs up. Not exactly scientific, but if the tightness was critical there would be a torque listing. They just need to be tight enough to hold the pressure plate cover on and not come loose. The pressure plate is under constant "pull" from the flywheel, so the bolts aren't likely to work loose.

Using the torque wrench, which has a longer handle than a standard ratchet/wrench, you're able to put a lot more force on the bolt than you realize -- can't tighten by "feel" that way! I've never had a problem with bolts coming loose. 

IIRC the pressure plate to flywheel bolts are 5/16", not 3/8", and the flywheel to crank bolts are 1/2". Max torque on 5/16" bolts is 20-22# for plain grade 5, 24-27 for plain grade 8 (coarse-fine threads). So you were at least 13# over max rating. I got torque specs from this chart: http://www.offroaders.com/tech/Bolt-Identification.htm. Scroll down for torque ratings for DRY  threads. At the bottom of the chart it states what values to reduce torque by when threads are lubricated, and what lubricant is used.

Since the bolts have been overtightened by about 35% (assuming grade 8 fine threads), they should all be replaced. Check the heads and replace with whatever rating the factory bolts are. They will either be grade 5 or 8. There's little point in going to a higher grade. That could, in fact, reduce the holding power unless you also go up to the higher torque or the higher grade due to bolt stretch (elasticity) --  the higher grade bolt doesn't stretch as much. I don't know how much the engineer was counting on that elasticity for holding, but it seems to me that a bolt with less elasticity wouldn't hold as tight as a bolt with more elasticity if both are tightened to the same value -- the less elastic bolt shouldn't put as much pressure on the assembly... make sense??


Edited by farna - Dec/02/2007 at 1:55am

Well being that it is cold out I guess I will attept to remove the bolt I will take a look and see if the dremel way will work. If not I will have to drill and tap it. Also concidering replacing all of them with Grade 8 hardware.

Right - if the threads are clean and it didn't break because it bottomed, it might come out really easily.

So I spent all weekend stuffing the little 6 back into my big rambler. I got almost every thing bolted down when my Dad asked If I had torque down the clutch to the flywheel. I said "it doesn't list a torque spec" and he replied “it doesn't matter as long as it is even" So I went to torque them to 40 pounds. I got 4 of them done and on the 5th one it seem to require a lot of force and the next thing I new I snapped the head off. First question is how much of a problem will it be if there are only 5 bolts instead of 6? 2nd question is if it is a problem can it be fixed in the car?