I am installing a T56 in my 78 vette to my ZZ383. I have been reading about the bell housing alignment using a dial indicator.

I understand what people are trying to do ( center the transmission with the crank to .005") but I see a flaw in the method described.

(go down to the last posts the flaw is I believed that the T56 Mounts to the bell housing similar to a TKO or factory 4 speed .... wrong)


You have this nice round hole in the bell housing that you are measuring against, but what does this hole have to do with the actual bolt holes where the transmission mounts on. If the bolt holes where not drilled in the correct diameter to the center line how does measuring the opening help ? If the opening is not centered but the bolt holes are why measure the opening.

What am I missing.? (The T56 is not the same as TKO's or factory 4 speed)

The only way I see this working is making an assumption the opening is perfectly cut based on the bolt holes. Since no one is measuring this it seems to be a useless process.

Show me the light if there is some.


Answer is.....
For T56 Bell Housing alignment use the following.

From Quicktime Instructions

"Special note for bellhousings that are designed for T-56 transmissions:
In order to check register bore run out on these bellhousings, you must remove the front plate from the transmission and mount it to the bell.
This will allow you to check
for run out by following the instructions below.
1. Remove clutch assembly from flywheel and install bellhousing
on engine block. (It is easier when you leave the clutch assembly off the flywheel.)
2. Install dial indicator base on the flywheel and adjust plunger to
contact the register bore of the housing.

This is the only thing I found that makes sense.

 

The bellhousing center hole is an exact slip fit to the input shaft retainer. The input shaft is what needs to not be sideloaded. The bolts do not have much effect and the mounting holes in the trans have some sideplay with the bolts.

 

You don't have to do it. The effort is to align the center of the bell housing tranny hole with the center of the crankshaft pilot hole. All things equal your round tranny snout will fit tightly in the bell housing and align the input shaft with the crank. All this effort is to reduce misalignment in the drive train which wastes power as it moves toward the wheels.

 

So the input shaft (transmission) alignment to the pilot bearing (crank)

So how does measuring the opening in the bell housing help? You then must assume the bell housing opening is pefect ?

So does everyone use that assumption ?

 

The input shaft retainer is a snug slipfit In the bellhousing hole. Only a thousandth or two clearance. I assume the outer perimeter of the retainer is concentric and uniform with the centerline of the input shaft.

 

I think we are talking 2 different things.

The video I linked seems to make sense, and I think what you are talking about. Its the other methods people are using where they are measuring the bell housing opening with a dial indicator

such as this video. Its this measurment that doesn't make sense. Seems to be a waste since you are never measuring against anything on the actual input shaft.

 

The bell housing is aligned with the dowel pins on the block, not the bolt holes. You can buy offset dowels to adjust the centerline of the bell housing. The transmission is located by the OD of the input shaft bearing retainer on the front of the trans, not the bolt holes. The fit between the trans and bell housing is an important fit so manufacturers work to tighter tolerances on these dimensions. As been said, if the trans and crank are not on the same centerline this puts the input shaft in a bind and everything else down stream as well. If you want to ensure a smooth shifting trans, especially an older one with external linkage, then take the time to indicate the center bore of the bell housing and make sure it's concentric with the center line of the crank.

 

You're checking concentricity between the centerline of the crank and the center of that hole in the bell housing. That hole aligns the transmission with the crank via the input shaft bearing retainer which "should" be concentric with the bearing, and thus concentric with the input shaft.

 

This is the correct reason for aligning the bellhousing.

 

Just did this. Total bellhousing runout was .00425 top to bottom and .003375 side to side.
https://www.corvetteforum.com/forums...post1589191128

 

I guess when I was using the reference of the bolt holes I should have used the dowel holes / pins, since this determines the alignment.

Dowels in the block align the bell housing to engine and the dowels in the transmission align the transmission to the bell housing.


I understand why you need to do this, its the method to determine what adjustments / measurements are required.

Measuring the bell housing opening seems to have a big assumption that dowel holes on the bell housing are a perfect relationship / centered with the opening and the dowels on the transmission are perfectly centered to the input shaft.

So from the 2 video's. The first one I believe is the correct way to measure, and the second video you must assume things.

 

I saw that and thought "Wow. Cool. I haven't seen anyone even mention doing that in years..."...then I saw this thread. Go figure...

 

I see that. How do you know the transmission input shaft will be perfectly in the center of the hole ? I think you would have to assume the dowel holes / pins are perfectly placed ?

On my T56 transmission the bell housing hole doesn't come into play with the alignment other than 2 dowel holes in the bell housing with the 2 dowels in the T56 transmission. Does your transmission align with the bell housing hole for alignment. If so I then understand why that works. If not it would be a assumption to something.


Its this assumption I am having a hard time with. That is why I understand what the person did in the first video by mounting the front plate of the transmission to the bell housing and measuring to the race of the input shaft bearing from the transmission. The other method requires an assumption the bell housing dowel holes and the transmission dowel pins are perfectly positioned.

It would be interesting to see both methods used to see if they come up with the same results.

 

I think you're missing the point here. You can't assume any of the dowel pins are in the right place...that's why you have to check the location of the center hole. If the hole is not in proper alignment with the crank, you have to use offset dowel pins to correct it. It really doesn't matter if the dowel pins are correctly located or not...you're going to verify and correct as necessary. Older transmissions (like 63mako is using) don't use dowel pins. As has been said a couple times now, they use the front bearing retainer as a "dowel pin" to locate the transmission. If the hole in the bell housing is good, the trans will be good. IN the first vid, I'm assuming that is not the case with the T56 and it locates with dowel pins and not the center bearing retainer in which case indicating the bell housing would be unnecessary. Regardless, somehow the centerline of the trans needs to be verified with the centerline of the crank. That's all that really matters whether it uses dowel pins or not.

 

my T56 Magnum transmission doesn't use the bell housing hole for alignment. It uses 2 dowel holes in the bell housing mating to the 2 dowel pins in the transmission. Similar to the bell housing to the engine.

So if the measuring of the bell housing hole is used when the transmission uses this hole for alignment I understand. I need to see how other transmission mate with the bell housing.

My T56 seems to require a different method to measure. Like the first video I linked too.

 

"OD of the input shaft bearing retainer on the front of the trans" this is the part I could not see on my T56 because I dont think it mounts the same and the bell housing hole is not a round shape.

I am using a quicktime bell housing.

http://www.summitracing.com/int/part...6023/overview/

I think I got it now.


Looking at a TKO transmission the hole is used for alignment, my T56 doesn't use this for alignment. Uses dowel pins. only.

I would then believe the factory 4 speeds are similar in mounting where the bell housing hole is used for alignment ?

 

Now you got it!
Whew!


Old school Muncie trans. Look at the big flange around the input shaft. That is also the bearing retainer, and that is what locates the trans in the hole in the bell housing with a very precisie fit.

 

Thanks guys



It all makes sense now. But looks like I cannot use this method to setup my T56 since I need to use the first video to do it correctly.

 

the object is to align the center hole in the bellhousing with the center of the crankshaft

 

Taking the front plate off of the T56 is the only way to properly index with that trans. You also have to check for parallel alignment first because if that is off it will throw off your concentric measurement. Ive done a few t56 bellhousings and they are all way out of spec. My quicktime is/was so out that the input shaft dragged against the pilot bushing so much that it would turn the input shaft with the clutch disengaged causing the trans to grind going into gear.