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Aligning crankshaft to transmission input shaft

This months “hot rod to the rescue” article involves matching up a Chevy 350 replacement engine (replacing a 323 I think) to an existing Muncie manual transmission. All sorts of complications to deal with. One thing they talk about but I didn’t understand is how they make sure the crankshaft is properly aligned with the transmission input shaft. Presumably it was aligned ok with the 323, but now there’s a different engine and different bell-housing to deal with. It appears they install the bell-housing to the engine, then put the magnetic base of a dial indicator set-up on the flywheel, then turn the flywheel as they measure something near the big hole at the rear of the bell housing, near where the transmission will eventually bolt to. But what exactly are they measuring there? And how does that measurement help figure out the alignment? It seems like you’d have to measure two things, b/c the alignment between the crank and transmission input shaft has to both be (1) straight, and (2) not offset.

Can you link to the article? All Chevy engines used to be interchangable and all you needed was an alignment tool when you put the pressure plate on to center it. Then the transmission would just slip in.

Are you sure they put the bell housing to the engine before mounting the transmission? Usually the bell housing is attached to the transmission first. You sure it wasn’t the pressure plate you saw?

One point of my confusion is the photo subtitled:

“Before checking bellhousing runout, Smith advises you first check the parallelism of the block mounting surface “With a magnetic base on the flywheel, we checked the block bellhousing flange on the engine, and then mounted the bellhousing. The surfaces were 0.008 inch off!””

OK, imagine you have just mounted a rotor to a hub, but while you were putting it in place, a piece of debris got between the rotor and the hub on one side. Now if you were to check to see if the rotor was warped using a dial indicator attached to a magnetic mount stuck to the body of your car, you would get an indication that the rotor was warped, even if it was brand new and perfectly machined.

Now you remove the rotor, clean out the debris and reinstall the rotor and test again and it is perfect. That is sort of what happened here. The first thing they found was that there was a build up of paint on the top of the bell housing. With the transmission in place, it was not in perfect alignment with the engine but at a slight angle to it. Cleaning it up helped bring it into spec, but they continued on trying to achieve perfection.

To check the bell housing, the mounted it to the engine. They stuck the magnetic base to the flywheel and the dial indicator sensor to the transmission mating surface. Then they rotated the engine causing the dial indicator to rotate in a circle around the hole in the mounting surface of the bell housing. Any variation seen in the dial indicator would indicate a less than perfect alignment between the engine and the transmission.

Another issue he had was using a bell housing that didn’t match the input shaft bushing of the transmission. The hole in the bell housing was too large. That would be like using a wheel that has a larger hub opening than the hub it is being mated too. You can sometimes get away with that by using tapered seat lug nuts to center the wheel. Not a good idea in my opinion but it is done. The bell housing mounting bolts do not have tapered seats so you would never get the transmission perfectly centered to the flywheel. It would be like a two year old trying to put the round peg into the round hole, blindfolded.

BTW, the old engine was probably a 283 or 327. Most likely a 327 if it came with a Muncie but I can’t figure out how someone got a truck bell housing in there, unless the Muncie came out of a truck. Probably the first builder used a lot of junk yard parts without knowing what he was doing. I mean the builder this guy bought the El Camino from.


The article was pretty good until I see them measuring the new flywheel thickness with a caliper resting on a straightedge- but look at the depth side. He’s using a worn out wood bench as the stop for the caliper end and citing .001" measurements?

dad sent off 454 for rebuild and left bellhousing dowel pins in block. did not get them back. shop was 100+ miles away. yes having no dowel pins is an alignment issue. they sell offset pins too.

Good catch, I missed that. There was a lot of info to sort through in that article as they addressed a number of issues. I read through @George_San_Jose1 original post and the article again and George is right in that there were two things affecting the alignment and the article did a poor job of separating which one they were working on at any given time. The pictures they took were not the best angle either.

On issue was the that the mounting plane for the transmission was perpendicular to the axis of the crankshaft, or parallel to the plane of the flywheel. The way they were measuring was using the crankshaft axis as the reference.

The other is that the center of the input shaft bearing hole is exactly centered over the center of the axis of the crankshaft or the center of the pilot bushing. That changes how you mount the dial indicator which was clear in the last picture.

I do think that his biggest problem with the shifting was the centering of the input shaft bearing hole more than the plane of the mounting surface. If he used the right bell housing in the first place, he would have never had a shifting problem.

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Good explanation of separating the two alignment issues involved, and how to address each of them, thanks @keith :slight_smile:

I’m still a little unclear why the hole the input shaft goes through the bell-housing would expected to be precisely centered about the transmission input shaft. It seems like that reference would apply to something in/on the transmission case itself, not the bell-housing, as the bell-housing is manufactured independently of the transmission.

If the hole is the correct size, then the input shaft from the transmission is centered. Now the transmission with the bell housing attached is mated to the engine. This is where the issue is. The input shaft now has to hit the center of the pilot bearing. If the guide pins aren’t correctly located on the block, there is going to be a problem.

With a stock bell housing, this was all designed with a cad cam program and everything should just fall into place. But here, he used an after market bell housing so perfect alignment is not a given. So he had to mount the bell housing to the engine first and check the center of the transmission input shaft hole. If it is off, as it was a little, then he installed offset guide pins and tried again. Once it is right, then the bell housing is removed from the engine and reattached to the transmission.

Now with the clutch assembly in place, the input shaft should hit the pilot bearing perfectly as the bell housing slides over the guide pins sticking out from the block.

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I’ve heard of these offset pins before. Another thing I’m curious about, if offset pins are used to offset the bell-housing mounting point to the right/left & up/down so the input shaft hits the pilot bearing square on, wouldn’t that cause the bell-housing bolt holes to no longer line up with the threaded mounting holes in the block? Or is there enough play, the bolt hole diameter is big enough compared to the bolt diameter to allow some room for the bolts to shift w/respect to the housing?

There is some play in the bolt holes. That is why the truck bell housing didn’t work. Aligning it would be like as I said, a two year old putting a round peg into a round hole, blindfolded.

I expect he used the truck bell-housing b/c he had it. Not that he expected it would work w/out some complications. That’s part of the fun of working on old cars, just to see if something is possible or not. If it doesn’t work, try something else. Good excuse to get out of the house & into the shop. I do the same thing. Sort of like I’m trying to get an old discarded electric fan working again, in order to direct warm air from my wood stove further into the room. A new fan wouldn’t cost more than $30, and I’ve already spent 3 hours on the old fan, still not working, but I’m having fun … lol …