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Discussion Starter · #1 ·
Looking at many different Bell Housings, Bell Housing to Adapter or Bell Housing to Block (gas'er), Some have connecting bolts around the entire Bell house edge. Others just run bolts around the upper half of the BH. I am just wondering, how much strength is lost on the BH's that do not run full circle bolts, such as GM. In most applications the
BH/Transmission is the rear engine mount.

I know there is little that can be done as there is little material in the unused area, so really its just a question. Is it done to save money or are the lower bolts really not needed ?

Paul (wondering
 

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Well I guess you would want to see if the stress from the weight of the trans/engine would cause it to be concave up or down.

Your stress will be from either tension on the bolts or the shear on them from slight side to side vibes. I'd say neither really get in the range to destroy 6 of those bellhousing bolts so it's a fairly safe with the ones on the upper half.
 

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Discussion Starter · #3 ·
Its just one area of design I have always wondered WHY not bolts all the way around. Look at the transmission to BH, 4 huge bolts, yet on the GM/Cummins only a few 3/8" bolts on the top of the BH, yet they see the same torque as the huge Tranny to BH bolts.

Oh Well, I guess I can still sleep not to many engine.BH connections are snapping on the road ha ha

Paul

Paul
 

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The trans to BH bolts will see larger loads since they are closer to the centerline of the crank. Think of torquing a bolt to 150 ftlbs. A six foot wrench will make the job much easier than a 1 foot wrench. Same principle applies here. The BH to engine bolts are much further away from the centerline of the crank, and therefore have greater leverage over the torque produced by the engine.
 

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You are also forgetting the dowel pins in the bellhousing to block area. They will take the torque that the 3/8" bolts don't have to see.
 

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Agreed, the bolt pattern is larger compared to the BH/gearbox pattern. Torque loads on the larger are reduced (Torque results a shear load on the fasteners/pins). The compressive loads (on the fasteners) are not relavent.

The fastener tensile loads are due to fore/aft movement, and bending between the motor/tranny, and dynamic loads like vibrations. My guess, without numeric analysis, is the dynamic loads (although small amplitude) are the highest. We often simulated a dynamic load with a single static load. (So if a hard vibration/shock resulted in 20g (as with landing a "jump" with the truck) then we could simulate a single spike load on the Motor/BH interface).

From there it is a matter to find the worst location for a fastener. From the side profile, the bolt patterns of Motor/BH and BH/trans are almost the same height for say, the GM 700R4 (Which only has the "upper half" BH). The loads through the entire assembly are largely dependedent upon the location of the mounts to the frame in relation to the CG's. CG of the entire bolted assembly and also the CG of components for fastener/interface loads can then render the proper load path.

I noticed that GM did add some struts going from the underside of the "missing" BH (there's just a cover there) going up to the engine mounts.
They did this on the 700R/4L60 on HD applications. There were no rubber contacts with these struts (metal-to-metal) so as all the other elastic mounts shifted under load, and the struts would load up first.

I had a couple of friends who worked at GM but have since lost contact, they might know which fasteners might be close to failure or if the BH has a weak spot.

I have a TH400 with a broken BH (looks ripped apart) but the BH/Trans is one piece. It failed due to hasty disassembly on a lift with one fastener still connected while the motor was allowed to fall to the ground...

Sorry for the rant...:emb: (didn't realize I wrote so much)
 

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The mounting of the engine and gearbox affect it a lot. As Mook and others have said that changes which direction the engine and gearbox try to bend in.

The torque between an engine and bellhousing is quite small. Even if you get 500 Ft/lbs from an engine, that's only 100lb on the end of a 5ft long lever. It doesn't take many bolts to resist that.
 
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