O/T Grade 8 bolt modification

I need a few 1/2 in. rod bolts to finish a engine rebuild. The original bolts are so old and have been torqued to death. The thread is actually distorted to the point where the nut slips over the first few threads. If I get grade bolts and mill off 1/2 of the head so the bolt will fit the rod flats, will this bolt still have the strength is needs to hold the rod cap? If I could but new ones I would. Thanks for any help Stan

RR Merlin for a P51? Ford 300 six? Brigs and stratton 3.5HP? makes a difference in the answer.

Can you leave enough meat on one side to stay away from the fillet radius under the head, or must you mill it flush to the shank? The quality of that fillet radius is a large determining factor of the fatigue strength of the bolt.

So this is to make up studs? I'd just go to the auto parts store (the old one with a Dorman counter, not the box store) and see if they have hardened studs in the correct size. I do agree with willie, though. Depending on what it is in will make a difference as to how and where you can scrounge parts.

Why not buy the correct bolts (arp from jegs) and avoid disaster? For Gods sake don't use the pot metal bolts from China.

As noted, it makes a big difference what kind of stresses the bolts will see. And, as noted, buy some ARP bolts.

By all means do it.

Please post pictures of the contents of the oil pan after the bottom end lets go immediately thereafter...

Motomoron said:

By all means do it.

Please post pictures of the contents of the oil pan after the bottom end lets go immediately thereafter...

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Oh come on... That didn't even knock holes in the block. I love the twisted rod though.

I'd have to agree with all of the above. ARP engine bolts are rated for 190,000ksi. Well beyond that of grade 8. I think the whole hardware kit for the bottom end on my 350 was under a hundred. Even if it's increased to twice that now days, it's money well spent.

What type of motor several asked. It is a 1938 4 cyl Allis Chalmers tractor 4 1/2 X 5 in stroke. The rod bolts are 1/2 x 20 3 in long. and are torqued to 55 lbs. They were over torqued some where in their past by their condition. The parts are no longer available. Thanks again. Stan

The 55lb/ft of torque is a bit misleading as to the loads incurred by the rotating assembly. That torque is merely what it takes to keep the nut from coming off the bolt when cold. It's not representative of the loaded forces acting upon the fastener.

That size should be easily procured from a number of fastener companies. ARP being one of them. Just because that motor isn't specified, doesn't mean that the fasteners aren't used in something else.

need original bolt pics--Stan

stan martin said:

I need a few 1/2 in. rod bolts to finish a engine rebuild. The original bolts are so old and have been torqued to death. The thread is actually distorted to the point where the nut slips over the first few threads. If I get grade bolts and mill off 1/2 of the head so the bolt will fit the rod flats, will this bolt still have the strength is needs to hold the rod cap? If I could but new ones I would. Thanks for any help Stan

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1-please furnish views of the original bolt pics--top, side, right side, underhead.

2-in past life, your's truly supervised eng. dept's. in fastener companies. Including OEM for Allis-Chalmers, GM, Ford, Chrysler, CAT, all aerospace/nuclear, DOD, and 100's others-world wide.

3-The steels and metallurgy in the late 30's, was not what it is today. The SAE gr. 8 designation, should work fine.

4-However, the fit of the bearing surface of the head (which you're talking about milling), and in particular the grip diameter and length to full thread is critical.

If the old bolts use a close fitting grip diameter to maintain alignment of the the rod cap halves---you will have to duplicate this.
The standard gr. 8 sae hex bolts have cold-formed taper on the grip diameter--since they are not used as alignment bolts.

That's a BIG rod bolt!

Rod bolts are designed to STRETCH by a certain ammount when they're torqued. You're actually not supposed to re-use them after they've hit a certain length, at least on most automotive, and motor cycle engines. I dunno if I've EVER seen a 1/2-20 rod bolt, let alone one that long. I used to have an ARP complete product line catalog, they didn't have anything CLOSE to that in there, I don't think anybody is going to make that part, but you MIGHT be suprised. I'd try to find actual replacement rod bolts first. 55 ft/lbs isn't much torque for a 1/2" grade 8 bolt, I don't know if the originals were that strong, I doubt it would have been enough to screw them up no matter how many times it was rebuilt! Now if they were overtorqued, they SHOULD be replaced, because it would have stretched them too much, and they'd be weakened. A 1/2" grade 8 bolt should be extreme overkill on a tractor engine connecting rod! However 55 ft/lbs might not be enough to stretch a grade 8 bolt enough to keep the nut on it after a few heating and cooling cycles. Does the original bolt narrow in the middle to a smaller diameter??? That would explain where the stretch comes from. Stock rod bolts are about as strong as grade 8 in car engines. I'd go ahead and use grade 8 bolts IF your search doesn't trun up new bolts somewhere. Just put loctite on the threads so even if they didn't stretch enough, they won't back off. I'm thinking the originals might not have been designed for stretch. Should be good enough for a tractor, I've seen things like this done in the past and it held up.

I would think 1/2" would be standard fare in a diesel.

1-At this juncture, we haven't gotten past the form and fit phase, without more definitive info from Stan Martin.
To repeat myself, what establishes the alignment of the rod cap?

2-Joint pre-load--
All bolts are designed to stretch to pre-load the joint. Proper pre-load, in itself goes a long ways towards not having the nut come loose.
If machined substitutes are used, or even if original bolts are used (with NEW gr.8 nuts) one method for accurate pre-load is measure the overall length before and after assembly. This eliminates huge pre-load variables induced by variations in bearing surface friction, thread lubrication, etc.
Shoot for .001 measured, overall stretch.
Will require some sanding/grinding of the hex head top and the point flat.

New bolts

If these bolts are 3" long & we assume the working length is 2.5" then 001" extension would stress the bolt to 5.35tons/square inch. (please check this someone better qualified) If I am right surely you need more than this?

Stan Martin,
I deal with oddball rod bolts on a daily basis. The rod bolts on my Minneapolis-Moline engines are also half twenty. If you mill off one flat to allow them to fit up to the shoulder, it is no problem. The original bolts were around what we call grade five today. Grade eight is actually overkill for these old dino power machines. Most Molines actually use four bolts to keep the rod cap secure.

Just make sure you mill off parallel with one of the flats and do not mill to the point where you cut deeper than the shank of the bolt. I would stop when the head needs a little tap to drop alongside of the rod shank shoulder. Your original rod bolts probably had castle nuts for wiring too. Make sure you get new nuts and I would personally go with Locktite instead of trusting wire.

Best of luck and remember to have fun.
Charlie Biler
www.molineparts.com

a couple of comments

the bolt torque is critical to keeping the bore of the rod end "round"
i doubt that the original builders used anything other than empirical evidence to come to that torque spec. but,, that was what the rod bolts were torqued to before the bore was sized/honed. Should you decide on a bolt stretch method you have no assurance that the rod end bore will be round when you are done.

you are not going to have any issues with grade 8 bolts provided they are american and not of some unknown quality/conterfeit crap.

also one needs to bear in mind that there is little stretch forces imparted on an old tractor diesel because the vast majority of the force on the rod is from compression and ignition and very little force actually pulling the piston down which would pull on the bolts.
tractors usually don't rely on the engine for brakeing coming down a mountain pass like an automotive application would. also the rpms envolved are but a fraction of what is seen in automotive applications.

detroit diesel 53,71,92 and 149 series engines used rod bolts that were so closely spaced to the rod bore that a good portion of the side of the bolts where cut away in the boring process. the 71/92 series used 7/16 bolts that were torqued to 75ft lbs and i don't think i heard of any of them failing. it is worth noting those engines were primarily 2450rpm no load engines and had some heavy engine brakeing on steep mountain passes as well.

your primary concern is keeping the nuts from backing off, the use of loctite is ok i guess, although i would never use it myself in this application, however i would get some of those half height nuts to provide the needed locking ontop of the standard grade 8 nuts.

bob g

I was thinking the same thing about the stretch forces.

mobile_bob said:

a couple of comments

the bolt torque is critical to keeping the bore of the rod end "round"
i doubt that the original builders used anything other than empirical evidence to come to that torque spec. but,, that was what the rod bolts were torqued to before the bore was sized/honed. Should you decide on a bolt stretch method you have no assurance that the rod end bore will be round when you are done.

you are not going to have any issues with grade 8 bolts provided they are american and not of some unknown quality/conterfeit crap.

also one needs to bear in mind that there is little stretch forces imparted on an old tractor diesel because the vast majority of the force on the rod is from compression and ignition and very little force actually pulling the piston down which would pull on the bolts.
tractors usually don't rely on the engine for brakeing coming down a mountain pass like an automotive application would. also the rpms envolved are but a fraction of what is seen in automotive applications.

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Most of the time when a rod cap lets loose, it's @ high rpm from the piston being slung out. Braking forces don't generate as much force as high RPMs especially in a diesel because it's not fighting the throttle blades making vacuum. With an exhaust brake the pressure is always pushing on the connecting rod in diesels. Seen as how diesels are low RPM, and don't generate a lot of vacuum, rod bolts aren't stressed as much as they are in a gasoline engine. A castle nut would be an even better way to do the rod bolts than loc tite, but I've never had a bolt back out that was done properly with it. First prep the threads with brake clean to make sure there's NO oil in them. Then the lot tite will adhere with NO problems. Without lubricant, the torque won't impart as much stretch force on the bolt, hence the only problem I see with using loc tite. Castle nuts are a good idea, lock nuts migh still back off. I think the loc tite would be adequate to lube the threads enough to get enough stretch force, but I dunno for sure. Castle nuts are pretty much fool proof!

Have you thought about trying socket head cap screws? Might help with your clearance issues, and you can get a quality brand like Allen or Cardinal (I think Cardinal makes down to 1/2) that you don't have to worry about.

Luckily you have a low stress application.

But I can tell you there is a very very high chance ARP will have a bolt that matches your need.