I'm looking for advice/information on bolting the pressure plate to the flywheel on a 70's 390. All the torque references I've found online list ~35 ft. lbs. of torque for these 5/16" x 18 tpi bolts. Torque charts for this size grade 8 bolt show below 30 ft. lbs., the range I've found varies from 18 to 29 ft. lbs. depending on conditions (dry/oiled/plating). My problem began with one bolt breaking before reaching the 35 ft. lbs. of torque. I have compared 6 torque wrenches and have determined that an inaccurate wrench is not the problem. I obtained another set of bolts from a salvage yard and two of those broke. I still have enough remaining to do the job. I assume that these bolts are grade 8, right? I also have a set of new 3/8 x 16 pressure plate bolts and I am prepared to drill the original mounting holes in the flywheel and pressure plate to proper size and retap the flywheel for 3/8 bolts. I've got a capable drill press and the correct drill and tap to do this. I've worked as a machinist several times over the years and feel I'm capable of reworking the flywheel and pressure plate.

Questions:
Is the 35 ft. lbs. correct?
If so why is it greater than the torque charts I've found for a 'generic' grade 8 bolt?
Could it be I have bolts that have been over-torqued in the past and have been fatigued by that and many years of stress from use?

As soon as I can get this job done I'll be ready to set this remanufactured engine in my 1967 F350 flatbed and begin the 'break-in'.

 

Looking at the 72 shop manuals, Ford didn't give a spec on the clutch cover bolts. If you're running stock bolts, I'd keep it under 30, more around 25-28. If you're running ARPs, they give a torque spec that is usually higher, around 32-35.

 

350329-S2 .. Bolt-Clutch Pressure Plate to Flywheel / 5/16" -18 x 13/16" long / Grade 2 National Course Thread / Obsolete

1965/67 F100/350 352 / 1968/76 F100/350 360/390.

GREEN SALES CO. in Cincinnati OH has 8 = 800-543-4959.

MAPLES FORD in Warsaw MO has 11 = 660-438-5110 x 229.

MORLAN-SHELL FORD in West Plains MO has 13 = 417-256-3193.

 

Wow, I assumed that the bolts would be grade 8 or at least grade 5; given their application.

Both of the sites below give 35 ft. lbs. as the pressure plate to flywheel torque.
http://boxwrench.net/specs/ford_352-428.htm
http://www.mainemustang.com/mustang-info/ford-engine-bolt-torque-chart
What I've found online gives the torque for a grade 2 - 5/16 x 18 bolt to be ~8 to 10 ft. lbs./oiled, which certainly explains why I broke three bolts.

The new (Mr. Gasket) 3/8 x 16 bolts I have are for pressure plate mounting on GM products. They are grade 8 and have the unthreaded portion of the shank for piloting the pressure plate to the flywheel. I took a friend to Springfield MO Friday for outpatient surgery. While I was there I searched for a 5/16, three or four flute drill, a three, four, or five flute counterbore, or a three or four flute core drill. I could not find any of those to use for properly enlarging the existing 5/16 bolt holes. I am going to do a trial on a scrap piece of 1/2 thick steel; drill and tap for 5/16 and then drill and tap for 3/8. If I can maintain a +/- .005 location on my test piece then I am going to go ahead and enlarge the holes in the pressure plate and flywheel.
Thanks for the replies, I hope I have better luck with this than I did with last night's Power Ball...

 

But what's the source for their information?
If you drill the flywheel, you'll have to drill the pressure plate too. It's hard to hold a .005" tolerance in manual work.

 

I would not do over 15-17 lbs for grade 8 bolts as they will break at about 20-22lbs. A 460 motor use 3/8 Bolts and this is about the Torque You would do on them.. Go over 17lbs and you may wish you didn't Look at the max Torque rating for grade 8 bolts it's in your manual for all sizes.. Don't go by some ones editing error ...orich

 

I used 3 of the original pressure plate bolts for piloting the plate to flywheel and then put grade 8 bolts in other 3 locations with loctite red applied. After the 3 grade 8 bolts were torqued down I removed the original bolts and replaced them with grade 8 bolts also.

I have been able to post a photo to the 'Introduction' forum, so now I'll try to get a couple up here.

I found the truck on craigslist. Here's the photo from that ad.
https://www.ford-trucks.com/forums/p...ctureid=132025


So much room to work in!
https://www.ford-trucks.com/forums/p...ctureid=132026


I have cleaned and painted the engine compartment since this photo was taken. I think I have all the parts necessary to go ahead with installing the reman 390 into the truck.

 

I clean the threads and reuse old bolts a lot but there is always the problem of using one that has been over torqued and has stretched , then break when it's used again. I use new bolts if they are in an area that's hard to get to.