Can any one tell me if a pony motor off of a D4 starting with s/n 7u will work on a 1937 RD4 s/n 4g4344? Thank,s for any info that can help me.

The basic block will mount but just about everything that bolts on is different. Fortunately it can all be interchanged if you have the 4G stuff to bolt on.

Thanks for the info OLD MAGNET. The pony motor off the D4 is suppose to be complete and running, where my pony motor on the RD4 is complete but the #1 cyl is cracked and I will have to rebuild the mag and the carb. I was thinking "If" it interchanged I might be money ahead to buy the D4 pony motor and install it instead of "rebuilding" my pony motor.Also the D4 poney motor has electric start. What would your advice be, as I know you to have much more knowledge in this matter than me? Thank you again for your help and advice.

If the cylinder is cracked on the bottom outside as so many are from freezing water, the hardest part is getting the pony off. I've welded several of 'em that had that problem without taking 'em apart. Drill the ends of the crack to keep it from spreading and then "vee" it out. I used Ni Rod 66. Just remember to weld NO more than half an inch and peen the weld immediately after welding. Don't let the adjacent areas get warmer than you can put your hand on between welds and there shouldn't be any problems.

Some might suggest JB weld but I'm not fond of the stuff in an application like that. I've even seen a chunk of doubled up inner tube wedged up against the crack to stop the leak. It even worked. . . .

If your pony needs rebuilding the 7U option might be a good one, depends on what you have to pay and what are you getting. Used pony motors top out at about $750 and rebuild could double that, depending on what all you can do yourself and what all you need. It's getting hard to even find shops that will touch these old motors. The bottom water jackets have always been a problem, they are thin to begin with and very vulnerable to freeze cracking. They have been commonly repaired with all kinds of techniques including welding, brazing, fiberglassing and epoxy patch.

The 7U will most likely have an American Bosch Mag where the 4G used Eisemen. They will interchange no problem.

The carb is different in that the early units had the vertical main jet adjuster and the 7U is horizontal to clear the later fuel tank. They will interchange to the 4G ok.

To make use of the pony electric start you will need to change the oil fill tower, the cross tube to the carb. and add the starter parts.
All in all they can be quite a jig saw puzzle of parts to assemble but can be done. Course if your bent on originality that's a different story.

Beware that used pony motors are often not what you expect them to be. Mounting flanges are often busted and needing repair, threads stripped, mismatched parts etc. If you can throughly inspect and know what to look for you can come out pretty good. Enthusiasm can be a hindrance here as its easy to visualize what you expect but it's not what you get. Got a few of them myself:D

The top cover on the engine will have to be used from the RD4 as the water passage buts up against the diesel with an "O" ring seal where the 7U has a pipe to the diesel head.

Ol" Grump, OLD MAGNET and SJ thank you for all your help and good advice.

Old Magnet I'am taking your advice and repairing my old pony motor. I took the motor apart and today took it to the weld shop to have the number one cylinder lower water jacket welded. I'am also having a "drain plug" added to the side of the water jacket so in the future I will be able to drain the water from the pony motor when I need too. The welder said he could do it with no problem, Will cost me about $150.00. I also had him check the block to see if I needed to have any machine work done to it while it was a part. (The welder is also a diesel machine shop repair/rebuilder) I got lucky, he said the block and pistons where ok, but told me to replace the piston rings while I had them out. Which brings me to the "question" Can anyone tell me where I can get piston rings for my pony motor? My caterpillar s/n is 4g4344.I have a part number on my piston's that is 2f3665. That part number does not match the part number in my parts book. Parts book shows 3b3913 for std and 3b5172 for "semi-finished" pistons up to .060. I didn't measure the cylinder diameter before I took the block to the weld shop. I can get it back in two weeks. Would you believe the gentlemen welding my block is 80+ years old and still going strong. He welded the lower water tank on my radiator that had also frozen and busted, did a fine job. Any help or info on where I can get the piston rings will be greatly appreciated.

Try Hastings:
What procedure did the welder use.....arc stick....wire....tig or gas?

I wont get it back for 2 weeks. I'am sure he will use gas and braze weld it and then stress relieve it by "peening". Thats how he welded my lower radiator tank when he repaired it for me.