BBC motor mount perches have slotted holes. Positioning?

[71CHEVYSHORTBED402]

Rebuilding a 71 BBC 400TH. The top of the engine perch and the engine cross member have slotted holes, giving 3/8" play front to back. If I knew better I'd have marked them, but I didn't.

Anyone know how they're supposed to be positioned, or ideas where I should start? I figure right in the center, but can't say for sure. Never done this before, but I suppose the worst case scenario is I leave everything loose, and adjust them square in position for the transmission mount. Thank you.

[71CHEVYSHORTBED402]

Probably a dumb question. I think that's exactly what you do in this case, line up the transmission and torque everything down.

[geezer#99]

Now comes the question, where do you line up the transmission?

[HO455]

The transmission will be fine, as the yoke will take up any thing the slots will allow. Remember these were designed to allow assembly quickly without a bunch of alignment steps by people with hangovers. I always take time to check that the tail shaft is fairly centered between the frame rails.
Most of the time the front slots will be in different spots due to the loose tolerances in the motor mounts. You will have to have the bolts all loose when you set the engine (so you can get the bolts that come up through the stands started. Best to start the transmission bolts and float the engine while you get all the rest of the bolts started. ) then check the tail shaft of the transmission is centered (+/-1/8") Of a bigger concern is checking the drive line angles, especially if you have altered the vehicle's height. I had to put a spacer under my transmission mount to get things in line.
It you have a K model you may find that putting the mounts as far forward as possible to start may give you bit more clearance behind the distributor when you're done.

[Mike C]

That's what I would do. Bolt it all together loosely then push it around until it all seems to be lined up then torque it all down.

[geezer#99]

Tailshaft centered!
Is it supposed to be centered?

[HO455]

The transmission mounts usually have 3/8" or larger slots in them. By centering the the tail shaft you help eliminate one point of vibration. Driveshaft alignment needs to be done not only vertically but horizontally for smooth ride. If the engine and transmission are in the frame at an excessive angle (4 degrees) it may give you a vibration that will be difficult to locate later. A simple check with a tape measure is all that is needed.

[71CHEVYSHORTBED402]

Originally Posted by geezer#99

Now comes the question, where do you line up the transmission?

Strikes me there's little to no slop for mounting the transmission. Unlike the motor mount perches, the holes are round for 7/16" bolts.

Originally Posted by HO455

The transmission will be fine, as the yoke will take up any thing the slots will allow. Remember these were designed to allow assembly quickly without a bunch of alignment steps by people with hangovers. I always take time to check that the tail shaft is fairly centered between the frame rails.
Most of the time the front slots will be in different spots due to the loose tolerances in the motor mounts. You will have to have the bolts all loose when you set the engine (so you can get the bolts that come up through the stands started. Best to start the transmission bolts and float the engine while you get all the rest of the bolts started. ) then check the tail shaft of the transmission is centered (+/-1/8") Of a bigger concern is checking the drive line angles, especially if you have altered the vehicle's height. I had to put a spacer under my transmission mount to get things in line.
It you have a K model you may find that putting the mounts as far forward as possible to start may give you bit more clearance behind the distributor when you're done.

Good info.....Why would a K model differ for your point about the distributor? I am dropping this 4 /2.5... I planned to install the transmission with the two spacers it had from factory. Perhaps I shouldn't? I also plan for a new one-piece drive shaft.

For what it's worth, picture of the transmission spacers.

Originally Posted by HO455

The transmission mounts usually have 3/8" or larger slots in them. By centering the the tail shaft you help eliminate one point of vibration. Driveshaft alignment needs to be done not only vertically but horizontally for smooth ride. If the engine and transmission are in the frame at an excessive angle (4 degrees) it may give you a vibration that will be difficult to locate later. A simple check with a tape measure is all that is needed.

Good point. I'll be sure things are square.

[geezer#99]

Originally Posted by HO455

The transmission mounts usually have 3/8" or larger slots in them. By centering the the tail shaft you help eliminate one point of vibration. Driveshaft alignment needs to be done not only vertically but horizontally for smooth ride. If the engine and transmission are in the frame at an excessive angle (4 degrees) it may give you a vibration that will be difficult to locate later. A simple check with a tape measure is all that is needed.

Do you centre or measure the front of the motor too or just assume it’s good and the motor centreline is parallel to the frame?

[HO455]

Originally Posted by geezer#99

Do you centre or measure the front of the motor too or just assume it’s good and the motor centreline is parallel to the frame?

For the most part the slop in the front mounts is fore and aft with not a lot of play side to side. But for peace of mind it is another simple check with a tape measure that may reveal a hidden problem.

[HO455]

Originally Posted by 71CHEVYSHORTBED402

Strikes me there's little to no slop for mounting the transmission. Unlike the motor mount perches, the holes are round for 7/16" bolts.

The play is in the bolt holes which are slotted not to mention any slop in 45+ year old cross member mounting holes. And with my Monday/Friday built truck I didn't even assume the cross member was perfectly centered between frame rails.

[HO455]

Originally Posted by 71CHEVYSHORTBED402

Good info.....Why would a K model differ for your point about the distributor? I am dropping this 4 /2.5... I planned to install the transmission with the two spacers it had from factory. Perhaps I shouldn't? I also plan for a new one-piece drive shaft.

For what it's worth, picture of the transmission spacers.
.

K models have the engine set back farther in the chassis than a 2wd and it gets tight at the firewall if you are running an HEI.

Starting with the spacers is fine. You have to start some where. Although my guess is with a one piece drive shaft you won't need them to match the pinion angle. But that really can't be determined until the truck is fully assembled and at ride height.

[geezer#99]

What I’m trying to get at is everybody assumes the tailshaft needs to be centered.
That’s not always correct. Usually the centreline of the motor is parallel to the frame. The motor could be offset an inch or more to the passenger side.
The offset is usually there to compensate for the pinion offset in the rearend.
When people assume the tailshaft should be centered they can induce some strange hard to find vibrations.

[HO455]

Originally Posted by geezer#99

What I’m trying to get at is everybody assumes the tailshaft needs to be centered.
That’s not always correct. Usually the centreline of the motor is parallel to the frame. The motor could be offset an inch or more to the passenger side.
The offset is usually there to compensate for the pinion offset in the rearend.
When people assume the tailshaft should be centered they can induce some strange hard to find vibrations.

You are correct. I was thinking only along the lines of my Burban with a centered carrier bearing and a trailing arm suspension. And then I assumed everyone knew that. Where as a short bed with a one piece drive shaft may be a very different situation. I apologize for making a blanket statement that was inaccurate. I should have stated what you did is that having the crankshaft parallel the frame rails is a better goal. Although this does bring up the discussion if your going to a one piece shaft with a dropped trailing arm rear end, what is the best way to make sure that the pinion and the crank shaft angles are correct in the horizontal plane?
There certainly are some trucks that have this combination and have vibration issues where others don't. Where the axle is located side to side may be heart of the matter. Some sort laser alignment may be the solution.

[jocko]

Great discussion with geez and HO! Yes, "parallel" is the key word (and NOT in line). "A" is bad, "B" is good. The engine offset isn't necessarily a compensation for the pinion offset, it's by design to provide proper phasing of the driveline. Also, as HO455 was pointing out, when a center bearing is used, note the phasing takes place on the rear driveshaft.

[jocko]

Tony here's a good article on driveline phasing, most of which has been covered and am sure you're already aware. Pertains more to a custom build rather than a resto, but good info nonetheless (and the source of the 1st picture above).
https://jniolon.classicpickup.com/dr...nephasing.html

[71CHEVYSHORTBED402]

Originally Posted by HO455

The play is in the bolt holes which are slotted not to mention any slop in 45+ year old cross member mounting holes. And with my Monday/Friday built truck I didn't even assume the cross member was perfectly centered between frame rails.

I was just thinking about that. Don't have the parts in front of me, but I recalled the spacers having oval holes as well for side to side.

I found the holes in the transmission cross member with perhaps 1/16" play, but they're perfectly round. I also measured 14.3/8" from the top front corners of the carrier cross member (still has the rivets) to the top rear corners of the transmission cross member. On a side, they missed 3-4 rivets on this truck, smoking their lunch and all. Two of those were for transmission cross member pieces. Surprisingly, it had just shifted a pinch.

Almost everything I measure now ends in 3/8", it's starting to freak me out....Including my E-brake cables. A well known distributor sent me the wrong ones, and need the originals to copy because "we've never seen anything like your application" and "we've never seen a leaf spring truck with E-brakes that snap into axle plates".

My truck is an odd duck 71 SWB, 2WD, with rear leaf springs. Strikes me as a Chevy with 2wd GMC suspension, what's the big deal? Anyway, it throws everyone off, lots of screw-ups. Their front cable is an inch short. Probably based on bad info..........BBC with 400 TH, where's the mystery?

[71CHEVYSHORTBED402]

Originally Posted by HO455

K models have the engine set back farther in the chassis than a 2wd and it gets tight at the firewall if you are running an HEI.

Starting with the spacers is fine. You have to start some where. Although my guess is with a one piece drive shaft you won't need them to match the pinion angle. But that really can't be determined until the truck is fully assembled and at ride height.

Great to know on that drive shaft. I may start with one spacer. The front will be down approx. 2.5", and the rear down 4".

I am running HEI. I think I make-out an indent in the frame where one of the engine perches was mounted. The HEI had nice clearance before I tore her down.

[71CHEVYSHORTBED402]

Originally Posted by jocko

Great discussion with geez and HO! Yes, "parallel" is the key word (and NOT in line). "A" is bad, "B" is good. The engine offset isn't necessarily a compensation for the pinion offset, it's by design to provide proper phasing of the driveline. Also, as HO455 was pointing out, when a center bearing is used, note the phasing takes place on the rear driveshaft.

No kidding, great info., and thanks for the link and graph Jocko.

[71CHEVYSHORTBED402]

Turns out I have a picture, since relabeled. An engine perch is clearly shown on zoom. The perches were set forward approx. 1/8" an inch, and the frame has very small indentations to support it

Doesn't mean it's factory. Buddy and I R&R'd the rebuilt engine in 1993. I don't know if we removed or adjusted the perches. I do know it worked or close. There was no apparent vibration and the HEI had nice clearance.

That said, still doesn't mean the angle was perfect. Got to thinking about it, haven't viewed your link yet Jocko. It would be miserable to measure from the frame to the engine, because the frame rails aren't a right angle. The patience required is probably beyond me

But then I thought, if I measure the rear corners and center of the engine/trans vertically to say the axle, a square cross member and/or a random horizontal square line, that should confirm parallel.

I suppose you could even run a string line down the center of the frame from the front cross member to the rear. If the center of the engine and trans are the same distance from the string line front to back etc., strikes me the engine and trans are positioned parallel the frame. I may do that as well.

Thanks a bunch. Now I have to figure out the drive-shaft and trans. shims, if any............fun Edit: I mentioned everything I measure of late seems to end in 3/8". Guess how thick the transmission shims are..........I need a vacation