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6.5 Starter Geometry


Noob, but quickly learning!
Woodbridge, CT
Thread starter #1
The purpose of this thread is to collect as much information as possible about the starter geometry on the 6.5L GM diesel engine. Proper starter alignment will ensure longevity of the starter and ring gear. I just got finished changing a ring gear and hope none of you have to do it.

I have only a small amount of data to contribute at this point. When I had the truck apart, I thought I was in a unique situation. Judging from some of the responses I received on my frustration post in this thread, I took Will's suggestion to spin this off as a separate thread and gather as much info from the community as possible, for the benefit of the community, so that we can keep people from chewing up ring gears unnecessarily. Screwing around with a starter is one thing, but replacing a ring gear/flex plate on your back in the cold is something that will not easily fade from your memory. Not the worst thing out there, but sure is far from fun.

I found that the best way to test the starter engagement without cranking is to disconnect the batteries (BOTH OF EM!!), remove the wires from the starter and then hook up a jumper to the small solenoid screw of the starter. On my automatic truck, I removed the crossover pipe and the inspection cover from the transmission to get a look around. Not sure how visible things are on a manual transmission truck. Hook up a ground on one of the batteries, get another jumper from the positive of that battery and join the two jumpers to active the starter solenoid, advancing the pinion into the teeth of the ring gear. Rotating the engine backwards will drive the bendix out all the way and give you a real idea as to where it is actually riding on your ring gear when it's in operation.

Four things are important here.

1. If your front starter bracket is not installed, buy one and install it. GM part number 23502557. I found one on Amazon for under $20. This IS NOT OPTIONAL. While you're shopping, your starter bolts may be bent if you didn't have this bracket (and if you got really lucky and didn't break the ear off your block). Part number 15544950, also cheap. Just do it.

2. Pinion clearance. I measured this by taking a piece of 63/37 rosin core electronics solder and running it through the gear teeth. That stuff is very soft and with the rosin core it has someplace to crush to. Try it a few times. Measure with calipers to see how much space was between the root of one tooth and its corresponding crest. Plastigage may be better for this but solder works well for me. I have 2 GM starters - one measured 0.065 and the other 0.060. Does anyone know what the spec is from GM? The manual I have doesn't give specs for starter gear clearance on a 6.5.

3. Engagement. Not sure of the correct term but this describes it for me. I'm using this term for how far the pinion gear meshes with the ring gear in the axial direction. My ring gear teeth measured 0.500" wide. Both of my 28MT starters engaged across the entire face of the ring gear and passed through the other side.

4. Clearance. This is how far it is between the ring gear and pinion gear with the pinion retracted. I'd say a miss is as good as a mile, but I'm sure there's a spec somewhere.

Now the next trick is going to be for the guys who have a bare block kicking around, and those who have spare starters. We should have some means of measuring things to see what variances we can observe across the community in the hole locations on the block. Next time I'm under there I'll see if I can figure out some repeatable things that the hobbyist can do while under the truck to measure things up, but that's going to be tricky. Any advice from those a little better seasoned than me are welcome here.

Thanks to any and all who can provide some input.

GM Guy

Manual Trans. 2WD Enthusiast
NW Kansas and SC Idaho
Cool thread!

One little tidbit that I think might be overlooked is people not putting the heatshield back on, and the front mount of the shield sandwiches between the bolt and the bracket, so that thickness needs to be taken into account.