Discussion Forum - Northstar Chevelle Club

Yes, adding a shim to the rear trans mount will should bring the two working angle closer. That is if the numbers are correct. So to simplify things the Nos. are as fallows:

Engine/trans is  -3.9*

Driveline            -1.2*

Pinion                +.7

Because the slop of both eng/trans and the D/L are the same (both down towards the rear) the foreword operating angle is calculated by subtracting the smaller no. from the larger no.

3.9 - 1.2 = 2.7* Foreword working angle

With the pinion slopping down towards the front(opposite the D/L) the rear working angle is calculated by adding the numbers of the Pinion and the D/L.

So 1.2 + .7 =1.9 Rear working angle

This gives a difference of 0.8* not bad, very driveable and should not cause any problems.

Now, back to my original statement That is if the numbers are correct. All measurements should always be taken from machined surface, ether parallel or perpendicular to the crankshaft center line. On a BBC I have found the best place to measure the slop of the eng/trans is on the pan rail just ahead of the left engine mount. Jim you said that you are measuring at the cyl heads, If you are on the machined vertical surface at the front of the engine, that would be acceptable, just subtract 90* from the reading. As for the diff/pinion measurement, disconnect the D/L from the yoke, the bolt surface of the yoke is 90* to the pinion center line, measure and subtract 90*.

With the car up on blocks or jack stands (under the axle)in the rear and car at ride height, I would get a more accurate measurement at the pinion, than reassemble the D/L to the yoke. Once you have the pinion measurement you don't have to remeasure it again when calculating your working angles after shimming the rear of the trans. Hopefully by adding a shim to the trans mount both working angle would be reduced and the difference between then would be minimal. Always much better to have lower working angles.

Your last statement about the pinion not being centered causes what is called a compound angle. You have to know your "Trig" to figure and add the offset into the vertical angle (not me). Because the offset to the side is so minor, I never get concerned with it. Again if the working angles are very low, adding in the offset would still be well within acceptable tolerances. And with the pinion offset to the side you can never get a zero working angle which can prematurely ware Ujoints.

The gauge needs to be position upright to get an accurate reading, so by placing directly on the gasket surface or tipping the gauge upright in the edge I would think leaves a lot of room for error. When I say "pan rail" I mean the gasket surface just ahead of the left engine mount, it is a wide spot on the block that is machined. Do not use the pan or pan bolts to measure. Just looked at my Impala (396),there is a narrow spot, not as big as the chevelle 454. Always recheck to see if your reading repeats. If the bell housing is machined out far enough past were the trans bolts up, take a vertical reading. If nether spot will work remove the alt and alt bracket and measure there.

If you are measuring both vertically and horizontally you should check your gauge for accuracy. If I calibrate my gauge in the horizontal position than vertical is off, if I cal for vertical the horizontal is off. Some times placing a square on the vertical surface with the gauge placed on the horizontal part of the square make readings easier.

As long as you don,t move the car the diff shouldn't change, but raising or lowering the rear of the car will alter the readings. Also the front tires should be on block not jack stands on the frame as that will unweight the rear of the car. The car should be up close to level to distribute the weight and to gain access.

Is the the car some what level? 3.9/4.2* seems abet steep for eng/trans, usually see them around 3 give or take a little. Not concerned if the car is not level of the car for measuring angles, just wondering what the eng/trans is at with car level/normal ride height

Probably raised the engine with the engine mounts, were they aftermarket or std replacement from the auto store?

When I installed my engine, I couldn't find one or both mounts.  I bought one new, found one or two on the shelf. I ended up with different height mounts, used the thick one on the left to get header clearance and thin mount on the right for a/c box clearance. Eng/trans cam in low 3*s. After I installed the Gear Vendors I raised the rear(didn't like the low rider look) which rotated the diff and reduced the slope of the D/L ,adjusted the diff a bit to end up with very good numbers.

I also measured axle to frame and then frame to ground per the assembly manual and service manual, after accounting for the larger tires, when the frame to axle was correct the frame to ground was low. So, I through the books away and used my eye ball and protractor the determine the best ride height.

The quick way to check the garage floor slope is to find a place on the car that you easily measure, turn the car around and remeasure the car. If you find a difference of say .4*, the garage floor is sloped .2*, than add or subtract that from the eng/trans* (with car on the ground) reading.

Well I got +0.60 when I measured diagonally across the front of the pinion flange both ways.  The surface is flat; but not machined flat.  This is the new position with the rear dropped just a bit, it was +0.70

This would make the rear working pinion angle at 2.20.

So the forward working transmission angle would be 2.60 and the rear working pinion angle would be 2.20 or differ by 0.40. This is all stock non-adjustable CA's.

I don't think the pinion measurement is very accurate because the flange not being machined.