Torque for rear main nuts?
 

In my books I read 2 different torque values, one says 80 ft-lbs, another says 50 ft-lbs. Which is correct? Or are they both wrong?
Tnx, Ken

Re: Torque for rear main nuts?
 

80ft-lbs sounds more correct to me but I cannot confirm it's correct

Re: Torque for rear main nuts?
 

I haven't seen the lower number and have used 80 on all the T and A mains. While I do use the 80, if the slot is really not lined up, I grind a little off the back of the nut and try again or swap the nuts around. There is a reason that wrenches are made in different lengths, they are designed so that a wrench of X length can only be pulled so far in normal use. I have done a little checking, giving a good steady pull on the main nut wench till it stops moving yields about 80 foot pounds when checked wit a torque wrench.

Re: Torque for rear main nuts?
 

I don't use castellated nuts, may be OK on mains but definitely not on rods, but then I'm a racer. For the nut in the valve chamber I have shortened a socket and by using an old-fashioned flex type of torque wrench I can torque that nut too.

Re: Torque for rear main nuts?
 

Playing Devil's Advocate, what does it matter if the torque is 60lbs, -70lbs, -80lbs ...or anything in-between??


I have a cap grinder, and I have found most core caps are really not flat in the flange area and so 'a thou or two' off the cap helps the cap mate parallel with the shims and main saddle of the block. From there is really is more about keeping the cap from "walking". Theoretically if the cap mounting surfaces are in a different plane by a little and the engine builder torqued the bolts at 60lb when it was line-bored -and the owner torques them at 80lbs this time, I suspect you will see some deflection & distortion on the cap. In Ken's situation this may not be possible but we always try to use a torque wrench to loosen the bolts as a way to know what to re-torque to.

Re: Torque for rear main nuts?
 

We torque to 80 lbs.

We hold the nut, and torque the square end of the bolt, on the front, and center. The rear main cap, on the nuts.

We pull 80 lbs, and then pull the key hole, no nut grinding. If the caps are not flat, they will stress the babbit. We have always used new lock washers, on the front, and center bolts, and keys on the rear mains. A whole lot easier adjusting, the mains, latter.

After the caps are perfectly flat, they have to be hand fit to the block. Should be void of any movement, even slightly, when trying to rock the caps.

Herm.

Re: Torque for rear main nuts?
 

Thanks, I'll go w/75-80 ft lbs.

After cleaning out the oil/anti-freeze goop and replacing the cracked/stitched head (as posted in an earlier thread), the engine developed a serious rear main oil leak which I need to look in to. It's definitely the rear main as I pulled the inspection plate and could see where the oil was coming from.

The 50 was in "Restorer's Model A shop manual" by Jim Schild

Re: Torque for rear main nuts?
 

In the old days bolts that were slightly oversize, big enough to make them a drive-fit into the block, were available and used to keep things aligned. When inserts are used, the caps are often pinned with locating dowels.

Re: Torque for rear main nuts?
 

Torque is a matter of "stretch." Stretch is proportional to torque (roughly.) That is until you stretch beyond the "yield point" of the bolt.

Most torque values of torque-tensioned bolts are designed to be within this yield point. Some modern bolting systems (My TDI VW fer instance) uses "stretch bolts" as a way to establish a more consistent amount of "residual stretch" (stretch remaining after yield)and compensate somewhat for different amounts of stretch found resulting from different degrees of lubrication for the torque-tensioned bolt system.

Stretch bolts are of course supposed to be stretched only once. Bolts torque-tensioned supposedly can theoretically be re-torqued many times, although most restorers prefer important bolts be torque-tensioned only once.

I have done both. I have repeated torque-tensioned bolting with success, and also had my failures. And new bolting is not a guarantee of torque-tensioned success.

One thing is pretty certain: success in torquing is an almost foolproof guarantee against failure in service. Most tensioning is done WAY beyond the tensioning required of actual gasket compression or strength of joint against shear or tension.

If the tensioned bolting survives tensioning action, it will survive to end of life.

Joe K

Re: Torque for rear main nuts?
 

Ideal torque stretches the fastener to 75% of its tension yield strength to reduce the alternating stress range.