The Stovebolt Site Forums The Shop Area Engine Shop 1951, 216 main bearing cap bolts, size & torque

I’m posting to both share what I just learned (that I suspect many already know) and to get input if I’m wrong. On another post I've shared the story of changing the leaking rope oil seal out for a nos asbestos rope seal. Due to not thinking through the entire process I did create a bit of extra work, but the point of this post is the 1/2 bolt that snapped while torquing the 4 bolts in the #4 main bearing cap.

If you check the 1948 - 1951 Chevrolet Truck Shop Manual, under Engine Assembly 6-17, paragraph 11 HERE, you'll see it calls for all crankshaft bearing cap bolts to be tightened to 100-110 ft. lbs.

I broke one on the number 4 cap (one of 4 bolts on that cap) at very near 105 ft. lbs.

Most queries online agree with the manual and most online responses say all the bolts are 1/2" and the thread count is all over the board.

The images below show what the '51 216 in my original 57k mile truck has in it. Back 4 bolts in #4 cap are 1/2" 13 tpi bolts. The other 6 bolts in the front 3 caps are 9/16th 12 tpi bolts.

I found a very few sources that support the above, and they show the #4 bolts with torque specs generally at 70 to 75 ft.lbs. Some sources spec max of 60 ft lbs.

I now think I know why I broke a 1/2" grade 8 bolt. Installed wet it is not safe at 105 ft. lbs.

RonR

Yes, the outboard bolts on the rear main cap are a smaller thread. The threads should be clean, and lubed with 30 weight motor oil, also. If a high pressure lubricant like hypoid gear oil or Never-Seez is used, reduce those torque specs by 10%. I like to use a drop of lube under the head of the bolts, as well as on the threads. "Torque" is simply a controlled stretching of the bolts to prevent them from backing off due to vibration. There is also a different torque spec for grades 2, 5, and 8 bolts with the same thread.
Jerry

I did not know the main bearing bolts were different diameters, but I've always kept them with their cap.

However, the torque spec for main bearings in the service manual is the same for all bolts, 100-110 FT-LB. No differentiation between front and intermediate caps and the rear main cap with the smaller bolts.

Jerry, how tight do you go on the 1/2" bolts?

I asked the AI: "This question is about the main bearing cap bolts on a 1951 3600 truck with the original 216 engine. 3 caps each have two of the 9/16" bolts described above. The 4th cap at the flywheel end of the crank has 4 of the .5" bolts described above. On The 9/16" bolts the FSM calls for 100 to 110 ft lbs wet, I'm trying to match that on the 4th cap with a very close clamping load. What should the .5" bolts be torqued to wet with 30w oil."

The AI said: "If your goal is to match the structural clamping force across all four main caps on your 216 engine, you have an advantage here: the factory

engineers already accounted for this calculation when they designed the 4th cap.The Factory Service Manual (FSM) specifies 100 to 110 ft-lbs for the main

bolts, but the correct factory specification for the four smaller bolts on that rear flywheel cap is 65 to 75 ft-lbs

(wet with engine oil). Do not try to force a single bolt to match the tension of a single bolt. The load distribution

across the caps explains why you should stick to the factory specs. Total Clamp Load vs. Individual Bolt Load on Caps (Caps 1, 2,

and 3): These use two bolts per cap. Torqued to 105 ft-lbs wet, each bolt creates roughly 15,000 lbs of tension. Combined, each of these caps exerts

30,000 lbs of total downward force onto the crankshaft journals.The Rear Cap (Cap 4) Because this cap has to deal with the heavy

leverage of the spinning flywheel, GM designers used four bolts instead of two to distribute the stresses more evenly across the block casting. The Math

Behind the 4-Bolt Cap. If you torque the four rear cap bolts to a safe, factory-approved 70 ft-lbs (wet with 30-weight oil), the physics

balance out: One bolt at 70 ft-lbs wet creates 11,200 lbs of clamping tension.Because you have four bolts holding that single cap, you

multiply that force, 11,200 lbs times 4 bolts = 44,800 lbs of total clamp load. By dropping the torque to the correct 65–75 ft-lbs

range for the hardware, the 4th cap safely secures the rear journal with more structural clamping power than the 2-bolt caps (44,800

lbs vs. 30,000 lbs)."

Well, take it or leave it. I'll let the engineers make the call. I'm likely going with 70 (or was it 80) ft. lbs. wet.

RonR

I'm wondering what basis the AI uses to state "the correct factory specification"?
I do see that bolt manufacturer websites call for grade 8 1/2"-13 bolts to be torqued at 80 ft-lb but doesn't say wet or dry.

I've seen AI state that the correct specification for 5/16 bolts as 20-5000 ft-lb. That's like saying "torque it down till it breaks and back off 1/2 turn.

My experience with AI’s is you really need to either know a bit about the subject to spot crazy, or always check with another AI, same question. Often there is a different answer. You have to wrangle the correct info out. Claude is my goto but there’s a rolling limit on free daily use.

RonR