RD-6/D6 pony motor help

[Toby Nelson]

Does anyone have the standard crankshaft journal specs for the 3 5/8" bore pony motor that is on my 2H series D6? I need to determine if the crank journals are already undersize or not. I tried a search but couldn't come up with anything, if somebody could help me out I would really appreciate it.

[Old Magnet]

Rod Journals....2.000 - 1.999 in.
Main Journals....2.125 - 2.1243 in.

[Toby Nelson]

Thanks for the info, looks like it's still at standard spec.

[Frank Fox]

Toby;
how is the pony for your RD-6 going?
I am in the middle of a pony O H for my RD-6 2-H. Mine need a clutch rebuild and had a cracked block All the bearings are standard on mine, so will reuse them. I got some new piston rings from the parts place in Sacramento (cant remember there name) but it is close to Cal. Expo. I had the block welded, so hoping for the best. I was getting water in the pony motor crank case, none in the main engine. I am also gathering parts to build another pony for another RD-6 I got with no pony on it. Found all parts but a crank and flywheel, in no hurry, will shop for a cheap one.
Hope to have both running this coming summer, then I will have 3 running, drivable Rd-6 tractors
Frank in Wilton Ca.

[Toby Nelson]

Well, the whole thing started when I decided I needed to get into the pony clutch to do some work on it. I've only started this machine 3 or 4 times since I've had it and the clutch slipped worse every time I started the diesel, finally got to where I couldn't turn the diesel under full compression, and it wasn't an adjustment issue. So last weekend I pulled the pony motor off and got the clutch apart. I found lots of rust buildup between the pressure plates and clutch disc, so high in most spots that the clutch disc couldn't touch the surface of the plates. I was able to clean it all up and get the clutch disc trued back up, also made a needed pinion latch adjustment while all was accessible.

It was then that I noticed a lot of up and down movement of the pony flywheel, indicating some excessive crankshaft bearing wear. It makes sense now because when I first got the machine I noticed silvery streaks in the pony crankcase oil, I just hoped it was the result of old oil in a dirty crankcase. I changed the oil then but noticed the same thing when I drained it down again to remove it. So I took the bottom end of the pony apart first and found some really worn bearings. The rear main bearing is completely shot and both rod bearings scored, but the front main isn't too bad but still a bit loose. It almost looks like it ran low on oil at one time or something. The crank is still at original specs, but may need to be undersized for new bearings. I've going to check later this week to see what parts availability is like for these bearings and make a plan from there.

The whole top end is in very good shape, pistons, pins, rings, and valves are nice and tight with very little wear, but I'll probably do some work up there too while I'm into it. I did notice this block has sleeved cylinders, I didn't expect to see that. Unless someone did that at the last rebuild, I don't know. It's like every other Cat I've ever had, go in for one thing and end up doing a total rebuild before you're finished. Gotta love it, though.

[Toby Nelson]

I hope I'm not asking a stupid question, but while I was looking at my parts book for undersize rod bearings for this pony motor I noticed the following-

The standard size rod bearings I have now are 4B7463 and 4B7464.
The .020" undersize bearings are 5B4316 and 5B4317, but I also noticed different connecting rod numbers. It shows "4B7466 rod assembly std." and "5B4294 rod assembly .020" undersize".

The question is - I have the 4B7466 rods now, the .020" undersize bearings should fit in them, right? I'm assuming the different rod assy. part numbers just indicate what bearings would have come with them. I may be making this more complicated than it really is, but I don't know.

[Old Magnet]

I'm following along in my parts book and see what you mean.....it is a little confusing. I agree with your logic....a new assembly number would be required for the undersize identification. Being that the rods do not have separate numbers I would measure the big bore ends and compare the bearings O.D.'s just to be sure. Also check the casting numbers which most likely are the same for both assemblies.

Maybe some one who has recently done a overhaul using the undersize bearings
can advise.

[Frank Fox]

I think you will find that some of the rods have a bearing "insert' and some have "Poured" bearings, hence the need for different part number.
Frank

[Toby Nelson]

The poured bearing style rods are different part #'s than the ones I listed above. I almost would have preferred that style, there's more flexibility to be able to get the crank ground to only what you need to have the bearings custom poured to fit.

I called Cat for parts today, they still have all the .020" undersize bearings available except the front main bearing (Part #3B8276) so the search for that one continues. If anyone has one just sitting around, I'd be more than happy to take it off your hands. I can still get the standard size, but really need a matched set.

I was able to source a couple of used 5B4294 connecting rods and can get them if I need them, but I expect the undersize rod bearings I ordered today should fit the ones I have. I'll carefully check the bearing bores and shells before assembly, though. I'll post back again when I have some progress to report.

[Toby Nelson]

I have another question - I'm thinking about bringing the piston pins and bushings back to new clearance spec. I can get a couple of the .007" oversize #1C2235 piston pins that will tighten everything back up, but I don't know if it will cause problems by loosening the fit of the end plugs that hold the pin in the piston. The pin bore in the piston is the same size all the way through, so if the machine shop increases the size of the bore it will loosen the fit of the plugs, which fit very nicely right now.

So - how loose can the plugs be in the piston? Or could these pistons be machined to take retention rings, like modern ones? I forgot to add the pistons are cast iron, not aluminum.