cracked/broken crank case mounting lug.

I noticed a crack in the top rear engine g/box mounting lug r/h case. when I removed the engine half the lug broke off. After reading some old threads I P.M.'ed Jim Comstock and got some advise about this, which I passed don to the guy doing the welding repair.
Jims advice. Don't get the case too hot [ max 100 degrees if pos ] and don't put too much weld on. He also recommended reaming out the 5/16 through bolt hole and replacing the bolt with 3/8 bolt.

I noticed the through bolts were all too short in the blank shank so my first priority was to obtain longer bolts, even if it meant adding a couple of washers to the outside ends. It jut happened that the nearest bolts I could get had shanks that were just under 4 inches, which should be ok. When I went to pick up the crank halves I checked all the holes. They were all far too large, so after much deliberation I decided that I would make up some aluminium sleeves which I could make a tightish fit in the cases and Loctite them in place in all case mounting holes. Next was to make up hat bushes for the steel g/box frame that could be loctited into drilled out holes. [ these holes also were too big and evidence of threads wearing into them from the bolts threads which had ground into them ] It was obvious that too much stress was put on the lug that broke because of the loose holes. In my mind I have the feeling that the holes were made loose to help alignment. What a great compromise. Oh well. I guess the shits going to hit the fan once I started making theses mods. Reminds me of working on the bloody Mini's years ago.

It is a pretty weak idea how the g/box frame is bolted to the engine but this is another mod which to me needs to be done to stop the lugs breaking. Just think how heavy that crank is and how light the cases are. THAT CRANK WANTS TO FLY TO THE MOON AND BACK.
Personally I would have designed extra lugs onto the cases so as to bolt the g/box cradle to the engine in a similar way to how Ludwig bolted his swing arm pin to his alloy g/box cradle.

Dereck

Derek, the design is not really a weak point as there are not too many instances of the damage you describe, this would typically happen if the mounting bolts come loose. I would recommend using brass rather than alloy for the sleeves. Some of the racers do this as the brass is more robust can be reamed accurately for a more stable crankcase under extreme duress where it is known to flex a bit.

Those mounting bolts were dead tight. Even the broken on was tight but I think not dead tight because of the fracture. I have had trouble reaming brass as well. A Conundrum??

850's tend to fracture out the lower cradle bolt hole. Repair by adding contoured plate on outside and fitting longer bolts. I'd go with 3/8" or even 7/16" like Peel 750. Aircraft spruce has scale of size and shaft lengths in grade 8 golden. I'd take a strong light and magnifier or even dye test for microscopic cracks in all the rear engine bolt flats so may want to nip those in the bud too.

Kerin, they may have been loose at some earlier period, if run like this they will make the holes in the cases oval and then prone to fracture even if subsequently kept tight.

hobot said:

850's tend to fracture out the lower cradle bolt hole. Repair by adding contoured plate on outside and fitting longer bolts. I'd go with 3/8" or even 7/16" like Peel 750. Aircraft spruce has scale of size and shaft lengths in grade 8 golden. I'd take a strong light and magnifier or even dye test for microscopic cracks in all the rear engine bolt flats so may want to nip those in the bud too.

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Hi Steve I read your earlier thread where you put 7/16 bolts through the mounting holes. I thought about that as well and decided the lug areas were not thick enough for my liking so came up with idea of sleeving.

Peel had thicker alloy cradle so followed old racers advice after DynoDave found extremely fine cracks on surface and welder found others in other areas under my grinding radiuii as he melted melted deeper deeper into my tender baby following them to their ends then back filling layer upon layer upon layer til a proud bead to suffer back down to fit again.

I also think that 1st owner or two didn't do retorque routine down there so holes wallowed and cases distorted so next fellow tried just tightening to stabilize so you could find and later fix it for your self, our entertaiment and the next fella to discover then nod in appreciation.

dave M said:

Kerin, they may have been loose at some earlier period, if run like this they will make the holes in the cases oval and then prone to fracture even if subsequently kept tight.

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Hi Dave, I hadn't checked to see if the holes are oval yet, but that was not obvious. What was obvious was the size of the holes.
Just checked now. The new bolts are almost perfectly 3/8 and they rock in the holes 40 thou with dial indicator. Might be too thin to make bushes, may just experiment with 15thou shim and see if that feels ok.

Norman White has been providing a service for at least a couple decades now, where he installs hollow steel dowel pins through cradle and into cases, reamed to fit everything tightly. I think he used 5/16" through bolts with them, but I'm not sure of that. I always thought that looked like a clever idea, but never took the time to try doing it myself. The stock arrangement does allow the cases to mover around a bit in the cradle, no matter how tight you make the fasteners.

Ken