Broken Piston Pin Clamp Bolt

[Tom 1948]

Hi

I have an interesting situation that is apparently fairly rare.

Several years ago I overhauled my 48 Roadmaster 320 engine and put in '49 connecting rods with the insert bearings and new pistons. After about 300 miles a knock developed in the engine. Becoming disgusted with my first overhaul I let a professional rebuild the engine. They found that the clamp bolt in #8 cylinder had broken in the middle of the tread section.

So this year I put about 600 miles on the engine. Same knock developed. This time I took the initiative and removed the head and pan etc ----- yup same thing, broken piston pin clamp bolt ( middle of thread) in #8 cylinder.

My old car mechanic friend and I inspected the #8 connecting rod. The end portion of the bolt is still in the rod. Also the piston pin rocks up and down in the rod bore. Obviously after 2 events it is apparently the connecting rod that is at fault. (We believe) So another rod will be installed after the bores are checked for dimensions and with new insert bearings.

Well has anyone come across this situation before? Are we missing something?

Any thoughts you have would be appreciated.

Thanks

Tom

[NTX5467]

Something sounds a little flaky . . .

is the bolt which breaks of the same hardness spec as the other similar bolts?

when assembled and tightened, I would suspect the piston pin should not have any movement in the rod end, nor should it have any slack in the piston, but it should have enough clearance to pivot as needed. I suspect it'll be a "press-fit" situation rather than a "full floating" situation which would need "end locks" (i.e. SpiroLocks) to keep the piston pin from touching the cylinder walls.

are the threads on the rod end "in line", between the two halves of the threads?

Please keep us posted on what y'all find!

Take care,

NTX5467

[Tom 1948]

NTX5467 Thanks for the reply

To clarify things a bit:

All clamp bolts were of the same grade - 5 radial lines on the head. These were installed with bevel washers.

The pin floats in the piston. There is a grove in the pin along the bottom circumference that fits over the clamp bolt. This prevents the pin from rotating and sliding back and forth.

The bolt treads appear to be in line. But that is a good thought and will be checked further. Could be the shoulder of the bolt did not fit snuggly.

The present theory is that the #8 insert type connecting rod is at fault --- twice. The pin bore is visually out of round. The original ‘48 rods gave no problem.

Plans now call for another (new) rod to be installed with #8 piston.

All clamp bolts will be replaced with grade 8 bolts, red Threadlocker and lock washers.

Should these solve the issue??

Any comments are appreciated.

Tom

Edited September 15, 2010 by Tom 1948 (see edit history)

[JohnD1956]

Any chance you could include some pictures of what's described here? This is among the things I know nothing about, but would love to learn.

[NTX5467]

Thanks for the update. I might recommend the "blue" Loctite rather than "red" (color of liquid rather than color of bottle) as the "blue" will allow for later adjustments and the red does not. This might also affect the capability to remove the bolt/rod/pin to re-use your existing piston on another rod, if necessary.

With Loctite, I would not suspect that additional "lock washers" would be needed, unless that's the way that Buick did it back then.

Please keep us updated.

Thanks,

NTX5467

[Tom 1948]

John thanks for the reply. I will take some pictures and include them in a later reply.

NTX5467 thanks also. Good advice about the Blue Loctite.

I am not sure if Buick originally used a lock washer. I thought they did. But then if you think about it the rod should provide some spring tension by design when clamped.

I did get a replacement rod from a Buick member, but the bearing incert locator tabs were on the oposite side if the oil hole. Shucks! So getting one from Terrill Machine.

Thanks

Tom

Edited September 17, 2010 by Tom 1948 (see edit history)

[Tom 1948]

Update

John a photo is attached showing two connection rods.

The rod on the left was going to be a replacement but the bearing clam shell locator tabs are opposite the oil hole. Terrill Machine tells me that these rods were produced by GM for the Flexible Bus Co (if the spelling is correct). The bearing shells were made of a harder material by GM thus the tabs were located differently.

The original rod on the right is the culprit in that the pin bore hole is out of round allowing the pin to rock.

The brocken bolt is show below the replacement grade 8 bolt.

The pin with the grove that fits over the clamp bolt is at the top.

And the two bearing shell haves are at the bottom.

All clamp bolts are being replaced to maintain weight balance as the new ones are slighty longer and a grade 8.

To reiterate, it is my belief and others that the clamp bolt was not at falut but rathers the poor fit of the pin in the rod bore.

Goes the show that all engine componets should be checked for fit. Even my professional engine rebuider missed this.

Thanks to all for the comments.

Tom

[JohnD1956]

Tom, Thanks for the photo. May I ask what the assembly prodcess would be for the piston pin and the clamp bolt? It would seem the pin should be inserted into the rod, and then the bolt and then the piston

[Tom 1948]

John thanks for the reply.

My procedure my be somewhat rudimentary compared to others. What I do is first place the piston over the rod. The pin fits snug in the piston but tight in the rod bore. Therefore the rod should be clamped in a vice and the pin carefully tapped through the piston and rod with a rubber mallet.

Care must be taken to insure that the grove in the pin faces toward the bottom end of the rod. Some use a wide blade screw driver in the slot of the rod below the bore to force the pine bore open more to allow the pin thru with less force.

The clamp bolt is then installed with a lock washer. Note that only one section of the clamp bolt bore is threaded. Factory manual specifications call for a torque of 25 - 30 ftlbs. It is difficult to get a torque wrench to fit into the limited room under the piston skirt and a socket with a cut off slanted socket section is necessary. If such a tool is not available the text of the 48-49 Buick shop manual does say to tighten the clamp bolt “securely” assuming with the use of a wrench.

Hope this is informative. And of course comments/techniques always welcome for a novice such as myself.

Thanks

Tom

[JohnD1956]

Interesting information. Not at all what I thought would have been the process.

Of course, I think striking anything, even with a rubber mallett, is less desireable than pressing the parts in some fashion. I helped a friend put new bushings in his lower control arms this summer and he did so by using a suitable sized "punch" ( in this case a cast iron pipe fitting on one side and two wooden blocks to create a hollow back on the other) and then squeezing them into place between the jaws of a sturdy bench vise. I don't know if something like that would be beneficial in a case like this?

Still, many thanks for your replies, information, and pictures, Tom. Maybe I'll get a chance to see this set up in the future.

[Bob Engle]

With the problems you have had, I would suggest that you test fit the pin in the rod and clamp it, The check that the pin is square to the rod. If it is not, all loads will be on one side of the piston only and would try to cock the piston in the bore. All things you do not want and could place abnormal loads on teh locking bolt.

Bob

[Tom 1948]

Bob

Thanks for the suggestion. Good point, Sorry to be so late in replying. We did in fact check that. The pin was square and fit properly. The engine is now back together again and runs a s it should.

Many thanks to all for your assistance!

Tom