So I thought I would share this with you everyone. I talked to this guy at CC Electric and he wants $595 for a starter to fit this engine, which is a complete rip off! I have a lot of experience with 3406B Cat engines and got to looking and measuring and the flange on a 3406B and a D315 are the same. I was kind of nervous about the rotation though. Last year, I converted my starting system on my K100E cabover with a 3406B from 24 to to 12 volt. The starter that Kenworth supplied me is a Mitsubishi gear reduction unit, part number PC1479. I pulled that starter off my cabover and it fit perfectly on this D315! When I bought this starter last year, it was $300. Soooo, please see the attached photo. If your engine has the flange, and if you have the room, this starter will most likely fit your engines. I have spun it over and the tooth clearance sounds great and also rotates the correct way! This starter is available at any Kenworth dealership or probably Cat as well at higher price. If anyone has any questions, please feel free to PM me. Now I can see if I can get this dude running without having the pressure of getting the pony motor rebuilt first. Cheers everyone!

That starter has a straight tooth drive. D315's unless the ring gear has been changed are normally helical gear. That's the challenge of conversion.
All the flywheel openings are SAE-3 standard. It's the starter drive end housings that need to be indexed for starter/solenoid clearance.

i bet the reason it was helical was it is rotating as it meshes also helical is stronger as the contact patch is bigger its also quieter.

Yes you are right. I didn't even realize that until you said something. I took cover off of the right side of the engine and looked at the teeth. I will try to call the starter manufacturer today and see if they make a helical gear for it. If not, I will use it very short term so I can at least hear the diesel run and evaluate what else this crane needs. Can't really see what works and doesn't work without engine power. I will still get the pony on my list to rebuild. Thank you for pointing this out.

I think your running a huge risk trying to start the engine with mismatching teeth. If it doesn't release when the engine starts then the engine will destroy the starter and maybe other things will it.

There is no helical drive gear available for those gear reduction starters.

Ok so I got this old thing started up and running. I have a pretty evident loud popping noise coming from the exhaust. What might that be? A bad valve or injector?

Good job on getting it up and running! Hard to say exactly what might be causing the popping noise. I wonder if a valve is stuck open slightly maybe from the recent use of the decompression release?

The popping is most likely a stuck exhaust valve like Seyser said but it may also be rusted exhaust valve and seat.
Been there done that. Turned out on my 315 (that came out of a 15B) it was the rusted valve and seat.
I had to pull the head and counter bore the exhaust valve seat on two valves and install new valve seat inserts and new valves. While I was in there I put new guides in because they were almost out of spec. Not that bad of a job. Valves and guides are readily available. I was able to grind the seats on the other exhaust and intake seats.