T-Arm Bolts

[`Michael Southard]

Hi Guys,
I'm in the re-assembly portion of my resto and just received some T-Arm bolts in the mail.

Question is, are these bolts supposed to be grade 5 or 8? It seems to me they should be 8's but then again 8's may not do well with the shock loads of a T-Arm.

What I received was some standard japanese grade 5 garb..., uh, I mean stuff that I could have gotten at lowes. I think I will sendem back, but do want to know the proper grade too get.
Mike S.

[Joe L.]

Re: T-Arm Bolts

Mike-----

The original trailing arm bushing bolts, GM #3846867, and the GM replacement, GM #458984, are SAE grade 5 bolts (GM-280M). However, these bolts have a special "bullet nose" with a cotter pin hole. Both features are important from several standpoints. The GM #458984 are still available from GM.

[Michael H.]

Re: T-Arm Bolts

Michael,

If your car is a 63 that was built before roughly June or July of 63, it would use a bolt with a conventional threaded end and a conventional non slotted nut without cotter pin slots. The later replacement GM bolt with the pointed end and cotter pin hole didn't show up on the 63 cars until late in production.

I don't remember the exact bolt head markings.

[`Michael Southard]

Re: T-Arm Bolts

My born on date is June 1 1963.

I did replace these bolts 20 years ago and GM gave me the ones with the bullet on the end like Joe was speaking of.

The ones I just received from C.C. are marked "JH" and are flat on the end. I was primarily concered with safety in regard to my inquiry, but bolt style has me interested now.
Thanks for the responses,
Mike

[Michael H.]

Re: T-Arm Bolts

Mike,

If your car is a 1 June build, I would have to guess the chances are pretty good that you are supposed to have the original 1st design bolt/nut without the cotter pin. I know ser. #18669 and 19746 still had the 1st design and I suspect that design was used until at least July. Not sure what the date of change to the 3846867 was on the assy line but even if the date turns out to be prior to June, that's just the change on paper, not on the line.

The later design with the cotter pin is a nice touch but I wouldn't really worry about the first design being a bad system. It works and I've never seen one work loose if the bushing is properly assembled and the nut torqued to spec's.

[`Michael Southard]

Re: T-Arm Bolts

Mike,

My ser# is 16445 so I guess it's still early for a late 63.
I'm putting everything together with a dab of 242 threadlocker anyway.

Splitlock washers bother me, especially when they sit on flat washers which is the way these one are meant to be installed. I just don't understand the point of a flat washer under a split lock???

[Joe L.]

Re: T-Arm Bolts

Michael-----

The primary advantage of the bolts I described is the ability to get them installed. The bullet nose and the cotter pin hole immediately behind it allow the use of fishing leader line to pull the bolt through, since they are installed from the back-side out. I don't think that this was, at all, an intended feature of the bolt with the cotter pin hole, but it sure works out nice. Trying to get those bolts installed without the ability to pull them through with fishing line can be a nightmare.

The locking feature of the cotter pin is really no big deal since you can use Locktite to replace that. A lock washer is, basically, ineffective with or without an underlying flat washer.

[Michael H.]

Re: T-Arm Bolts

Michael,

Not 100% sure on this but I believe the 63 1st design bolt/nut without the hole/slots for the pin uses only a lock washer without a flat washer. The 2nd design drilled/slotted nut style would have used a flat washer only. I emailed Bob Jorjorian last night and asked if he would look at his car to verify. He said flat washer only, no lock washer.

I agree, the threadlock would probably be a good idea. Sure can't hurt. Never tried Joe's method of reinstalling the bolt using fish line but it sounds like it might work well with the drilled style bolt. Necessity really IS the mother of invention.

[Chuck S.]

Re: T-Arm Bolts

It's a lot easier to understand the function of the flat washer if you examine a flat washer closely after the nut/split washer has been torqued: you will find the surface of the flat washer heavily scored where the split washer end has "bit" into the flat washer.

The "corners" of the split washer ends physically dig into the nut and the flat washer to prevent the connection from loosening. It will also do this on the flat surface of the fastened part, but typically the holes in parts are made large enough for assembly that the locking effect is reduced or compromised. A flat washer fits the diameter of the bolt closer, providing a solid, flat surface for the lock washer to seat against.

Split lock washers have some "spring effect" built into the split offset, which enhances the locking effect. You will find split washers 30-40 years old that recover most of their offset after being disassembled. I have reused them all my life without any problems, but ideally, aged split lock washers should be discarded and not reused.

[Timothy B.]

Re: T-Arm Bolts

I installed the GM bolts on my S/W years ago and with a 3/4" total shim pack, flat washers on both sides of the bolt and no lock washer, the castle nut does not line with the hole in the bolt. I think with a lock washer over the flat washer on the outside it will work properly and I wonder if the bolts are made longer than originally used in production.

[John H.]

Re: T-Arm Bolts

Interesting to note that the '65 A.I.M. shows just a flat washer under the castle nut, and the '67 A.I.M. shows an added lockwasher between the flat washer and the castle nut; all other parts are the same (bolt, inboard flat washer, outboard flat washer, and castle nut) between the two sheets.

[`Michael Southard]

Re: T-Arm Bolts

Chuck,

I do see the principle behind the split lock. I work in the crane field and our engineers have determined that once you put a flat washer "under" a split lock all you have done is prevent the nut from separating from the flat washer. Their theory is that the the entire assembly (split, nut, and flat) can turn away from the object it was intended to hold, thus the nut now becomes loose from the lock and the split. I can understand what they are saying...but I'm not altogether convinced. I thought the split should dig into the object being clamped.

"They" say there is enough industry evidence to prove this, so we (inspectors) are required to ensure that mechanics do not use this combination of fasteners on critical load bearing gear. In fact they will not authorize split locks at any more. I personally have never experienced this type of failure after 28 years in the field.

C.C. did not send a castle nut, only a standard nut. I think that is why they sent split locks and flats.

Well it doesn't sound as though anybody has had a T-Arm pop out of the kick up recently, so I'll just get it secured!
Thanks for all the great responses!
Mike S.

[Chuck S.]

Re: T-Arm Bolts

"Their theory is that the the entire assembly (split, nut, and flat) can turn away from the object it was intended to hold, thus the nut now becomes loose from the lock and the split. I can understand what they are saying...but I'm not altogether convinced."

Michael, there is probably legitimate concern about this happening whenever relative movement can occur between the part and the fastener; e.g. when the bolt/washers/nut assembly is forced to rotate against a stationary part. Under those circumstances, friction between the part and the flat washer COULD cause the washers and nut to loosen; if the assembly loosens enough, the spring in the lock washer would eventually be unable to keep the nut locked, and you would have a loose cannon.

It would be like your trailing arm bolt assembly being tight in the trailing arm bushing, and the fastener assembly being forced to rotate against the frame as the trailing arm moved up and down. In the case of trailing arm, however, I believe the bolt is stationary relative to the frame and the trailing arm rotates around the bolt. This concern therefore doesn't exist for the trailing arm application under normal circumstances. It probably also explains how they were able to get by for years without a lock washer.

[`Michael Southard]

Re: T-Arm Bolts

Yes Chuck,
I would agree that it is hardly likely because the bolt is as you say...

...unless, in the unlikely event that an east coast car stalled while driving down a well salted winter road, the owner forgot about his car, then it sat outside for 20-30 years in a salt road environment, and somehow the bolt and the bushing sleeve rusted, then became bonded together, Then someone, thinking he had a barn find, happened accross the ill fated C-2 and started the car, drove off, all the while never knowing that the T-arm bolt is working the flat washer against the frame comprimising the integrity of the split lock washer, loosening it!

Naaa, the battery would never have lasted that long!
Good talkin with you
Mike S.

[John H.]

Re: T-Arm Bolts

Chuck -

The design intent for the trailing arm bushing attachment is the same as for the front upper and lower control arm bushings and the rear camber strut rod bushings; the inner sleeve is locked solid to the frame by the bolt, the outer sleeve is locked solid to the moving part by its press fit, and all relative motion between the two takes place via torsional deflection within the rubber, which is bonded to both the inner and outer sleeves. That's why all of those attachments are to be torqued with the suspension at design ride height so the bushing rubber is unstressed in that position; the bushing rubber is only supposed to be under torsional stress during suspension motion above and below design height.