RD4 connecting rod bearing question

Hi Mike, I too have a RD4 pilot motor crankshaft ground to 40 thou undersize but have not found a source for the bearing yet so haven't proceeded, I was only able to find 30 thou under bearings, and I located 2 other crankshafts which I had ground to 30 or less under to clean them up so can't advise on the strength issue, but I did have a interesting talk to John Matta down in Riverside about this a month or 2 ago, he was a very experienced Cat Mechanic as you know, now retired in his 80's, we talked about cracked D2/D4 pilot motor crankshafts, and he said "Mike, yes you will often find cracked cranks in running pilot motors, but how often do you find a completely broken crank, rarely, rarely".

There was a gentleman on here recently talking about making undersize pilot motor bearings, what I discovered with those early RD4 cranks is there is no proper oil seal on the timing gear side, just a oil scroll in the bearing which in my motor was long gone and someone had rigged up a bodgy oil control tube, so I was leery of using that block as luckily I had a spare block from a later D4 7J / 2T I could use.
Regards
Mike

Unless I missed a detail or two in the initial post I have to ask, which crankshaft are you inquiring about in your RD4? Starting engine or diesel engine?

The smallest factory undersize bearings I've ever been able to find for those starting engines is .030" under, but one could go as far as .040" under on the main journals and manufacture the two main bearings to fit without having problems. The rod journals would have to stay at .030" unless suitable bearing inserts could be found.

If the .040" undersize bearings you're inquiring about are for the diesel engine, then by all means go for it. Unlike the hollow pony motor crankshafts, the solid diesel engine crankshafts on these old Cats were built plenty strong, and any undersize bearing that Cat offered for them will still be comfortably within the crankshaft's design limits. Considering your RD4's crankshaft journals are between 2-3" in diameter, at .040" undersize that diesel engine won't know anything even changed. Another thing to keep in mind, if your RD4 has the original babbitt lined bearing shells in the bottom end, you can have them re-babbitted to virtually any size you want, so if you don't need to go all the way to .040" under to get the journals cleaned back up you can have the shells re-lined to suit whatever size it ends up being.

Sorry for the confusion, I should have mentioned in the first post that I was talking about the main engine. The cat dealer sold me some .040 under rod bearings and said that he could get them all day long but not the standard rod bearings.

I still have the original rod bearing inserts, I had no idea they could be re-babbitted. I don't know much about babbitt bearings but I heard they need to be scraped prior to use? Do you know of anyone that does this babbitt work?

Sorry for the confusion, I should have mentioned in the first post that I was talking about the main engine. The cat dealer sold me some .040 under rod bearings and said that he could get them all day long but not the standard rod bearings.

I still have the original rod bearing inserts, I had no idea they could be re-babbitted. I don't know much about babbitt bearings but I heard they need to be scraped prior to use? Do you know of anyone that does this babbitt work?

If your crank doesn't need to be ground but you can't get standard bearings there is another possibility.

A machinist who is willing to fixture up the job can make standard bearings out of .040 under. I did it for my D4 6U. I had a bad rod journal (REALLY bad) on a .030-under crank. I bought a used standard sized crank and bored the bearings out to standard. It worked fine. I used that machine hard for a week or three per year for 10 years or so and always had good oil pressure and no problems.

The machining requires supporting the bearing well without springing it. Can't even remember all the details at this point, but it can be done. The halves have to be supported in position as if they were in the block and both bored at once.

Jim

[quote="Jim Davis"]If your crank doesn't need to be ground but you can't get standard bearings there is another possibility.

A machinist who is willing to fixture up the job can make standard bearings out of .040 under. I did it for my D4 6U. I had a bad rod journal (REALLY bad) on a .030-under crank. I bought a used standard sized crank and bored the bearings out to standard. It worked fine. I used that machine hard for a week or three per year for 10 years or so and always had good oil pressure and no problems.

The machining requires supporting the bearing well without springing it. Can't even remember all the details at this point, but it can be done. The halves have to be supported in position as if they were in the block and both bored at once.

Jim[/quote]

Jim, thanks for adding that - I should've remembered that option as well but it didn't register when I typed my earlier reply. I should've remembered it too because my dad is currently tooling up to take some .030" under pony main bearings out to .020" to fit a crank he just had ground. In that application he's going to install both bearings in the pony and once everything is squared away gonna line bore them so they make a true set. That's another workable option to keep in mind when parts supplies start to dry up.

I'm glad you boys clarified which RD4 engine Mike was talking about, obviously I jumped to the wrong conclusion straight away, sorry for that, I have 3 RD4 pilot motors apart right now with crankshafts and bearings ready for reassembly so I'm a bit one eyed about the little motors!:madgrin:
Regards
Mike