The Stovebolt Site Forums The Shop Area Engine Shop Broken main cap bolts

Well I picked up a 283 from a 66 Chevelle with a 2 speed power-glide for next to nothing. Not much info on the engines history except it had been sitting in a gurage for 2-3 years. My plan is to use the engine for my 49 1 Ton. So I pulled it apart and it was apparent it had been rebuilt at some point. TRW .30 pistons, power pack heads were clean inside and no lip on the cylinders with very little carbon on the pistons and valves so I would say within the last 10-20 years but who knows.

Anyway... when I was disassembling it I found the front main and rear main both had a broken bolt. The bolts were still there but I removed both by hand, 1 broke off and I had to extract the lower piece and the other was stretched and attached by a thread.

My first question is what would cause this? Maybe over torqued or a shady mechanic? Could there be any adverse effects on main caps?

Next question, when I send it off to the machinist should I have them align hone the mains?

Main caps are pretty sturdy items- - - -not likely to get distorted by overtorqued bolts. I'd be more concerned about the threads in the block. Get a complete new set of main cap bolts- - - -no telling how badly the ones that didn't break have been stretched. Then lube them with 30 weight oil and torque them down to specifications. If there's any sort of "mushy" feel before the torque wrench clicks, have the machine shop Heli-Coil the threads in the block.

If the line bore is good, the crank should spin freely in the mains with the bolts torqued down, as long as the rear oil seal isn't installed.

Jerry

Brings up another can of worms ..........dry torque or threads lube.........can be a huge difference in the clamping force. Which can be the reason the original bolts were stretched or broken.

http://forums.mtbr.com/tooltime/torque-dry-vs-lubed-716358.html

http://www.intermotive.net/Tech%20Tip/Tech%20Tip%20-%20Jan%2008.pdf
Dry vs. Wet torque
• When applying torque to a dry bolt more friction is created than applying torque to a wet bolt with oil or
other automotive fluids on the threads. With less friction (wet threads), the bolt will stretch more before a torque wench will click. Because friction is such a big factor in bolt torque, it is important to know the difference between applying torque to a dry bolt and a wet bolt. Using oil, anti-seize or other types of thread lubricant is a common practice, but an understanding that wet threads require less torque than dry threads because of friction is very important. Since every type of lubricant has a different loss of friction coefficient, it is recommended that every technician own a chart showing how much to reduce the torque when using different bolts and lubricants. This type of chart can accompany a bolt torque table, a drill index chart for drilling and tapping, a conversion chart and a basic calculator. The “Pocket Ref” by Thomas J. Glover is a great all-in-one book with just about everything.

I did say it was CAN OF WORMS

Not at all for anyone who can read, and has more than two brain cells to rub together. Every factory service manual I've ever read specifies that torqued fasteners need to be lubricated with motor oil. If a high-shear lube like Never-Seez or hypoid lube is used, reduce the torque spec. 10%.
Jerry

Hmmmmm, Interesting, think I will have them chk the alignment just encase the main cap got out of wack. I would say this is why it was not still in a vehicle. probably has some knocking going on. Thank you all!

I go with clean/dry threads and I usually stop torquing when my elbow starts to hurt. If I use the wet method at 10% less, how will I know when to stop torquing?
Carl

"Tighten it until it strips, then back off 1/4 turn!" One of my competitors on the round track called me one night and asked if I could install some heli-coils in the race engine he was building. It turns out he had stripped about 1/3 of the head bolts on a 350 Chevy while assembling it, after consuming about a case and a half of beer. He had about a 20% error in the calibration of the torque wrench he had borrowed. I asked him why he didn't stop when the first one stripped instead of continuing to bolt the heads on. "I was in a hurry to get the job done!"

I ended up installing heli-coils in EVERY head bolt hole on both sides of the block! The car never made it to the race track- - - -he was road-testing it on the dead end street where he lived, lost his brakes, and ended up crashing into his neighbor's bedroom! He had to sell the car to pay for the repairs to the house!
Jerry

If the main bolts were damaged, I would be looking at the rod bolts and head bolts too. I wonder how it ran with the main bolts damaged.

For 52Carl.....

Proper Torque Spec = 1/4 turn before you ring it off.

Which is why we were never allowed to assemble our round track engines.

Our engine builder from back then reminds me of Jerry (HRL). There are PROPER specs for assembly that somebody came up with, and it wasn't speculation.

I would think, if the broken bolt unscrewed by hand, sounds as though the threads in the block are suspect. If the bolt had to be extracted, sounds like it was over torqued.

Mike

One other thought.
I did not work in the automotive field, but mechanical maintenance in the nuke world.
We were required to not only lube the threads, but also the bolt head or nut face to get a proper torque.
Friction there also.

Don

I did find one rod bolt stripped so I think they just over torqued everything. No idea how it ran, then again maybe it ran too good and torqued the engine too much racing? Would that explane why just the two ends had a broken bolt and not any of the center caps?

It sounds like you're seeing the results of a mechanic with a vast amount of self-confidence, and half-vast methods! You need to change all the rod bolts, all the main bearing bolts, and plastigauge everything on reassembly.

I'd also do a complete teardown on the heads and see if they're as messed up as the rest of the engine.
Jerry

Back in the late 80's a lot of GM 4 cylinder's were blowing head gaskets.

Factory fix was to torque the bolts to a low initial torque rate then follow the bolt tightening sequence, three more times, turning each head bolt so many degrees, about a 1/4 turn.

And to make sure the threads in the block were clean and to clean the head bolt threads then lubricate them with oil. Dirt would add friction and yield false torque readings. Jeffrey

That's known as a "torque-to-yield" bolt, and it can be used only once. The bolt actually stretches a given amount to apply the proper clamping force to the head. The head gasket problems were most common with dissimilar metals- - - -cast iron block/aluminum head, etc. The early Cadillac Northstar had massive head gasket problems until they cast the aluminum block material over the iron cylinder liners. The early ones had the iron liner exposed to the head, and it was almost impossible to keep the head gasket sealed.
Jerry

Torque to yield brought back memories as well as that head bolts were not to be reused. Jeffrey

I received the new ARP main bolts today, they came with lube. I also have the line hone checked. It was within tolerance. So no major issues there. Thanks all