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Engine Mount alignment - 66' vette???

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Old 03-06-2010, 06:26 PM   #1
JOHN383
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Default Engine Mount alignment - 66' vette???

I have a 66' roadster that was involved in a front end accident. Just
finishing a frame off restoration when I installed the engine and
realized that the tailshaft is offset to the right (about 3/4"-1") making
the tranny mount impossible to bolt together. Any suggestions
for dimensions that I can check, or where I should take my measurements. The motor mounts do not appear bent.

I would assume that the drive train should be straight inline from
the engine damper to the differential yoke. Are there tolerances
for misalignment?
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Old 03-06-2010, 06:59 PM   #2
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Who straightened the frame???????????
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Old 03-06-2010, 07:02 PM   #3
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Quote:
Originally Posted by JOHN383 View Post
I have a 66' roadster that was involved in a front end accident. Just
finishing a frame off restoration when I installed the engine and
realized that the tailshaft is offset to the right (about 3/4"-1") making
the tranny mount impossible to bolt together. Any suggestions
for dimensions that I can check, or where I should take my measurements. The motor mounts do not appear bent.

I would assume that the drive train should be straight inline from
the engine damper to the differential yoke. Are there tolerances
for misalignment?
the mill is not centered on the frame and is of by about that much anyway, towards the passenger side.....however, the centerline of the crank and tranny should be centered on the pinion shaft too..good luck...
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Old 03-06-2010, 07:05 PM   #4
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Quote:
Originally Posted by JOHN383 View Post
I have a 66' roadster that was involved in a front end accident. Just
finishing a frame off restoration when I installed the engine and
realized that the tailshaft is offset to the right (about 3/4"-1") making
the tranny mount impossible to bolt together. Any suggestions
for dimensions that I can check, or where I should take my measurements. The motor mounts do not appear bent.

I would assume that the drive train should be straight inline from
the engine damper to the differential yoke. Are there tolerances
for misalignment?
The engine is offset to the passenger side (also transmission) but not angled. I forget exactly how much but not more than a couple of inches. The differential is centered in the car - so the driveshaft is angled slightly from the transmission to the differential.

Last edited by DansYellow66; 03-06-2010 at 07:07 PM.
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Old 03-06-2010, 07:08 PM   #5
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Originally Posted by DansYellow66 View Post
The engine is offset to the passenger side (also transmission) but not angled. I forget exactly how much but it's about a couple of inches. The differential is centered in the car - so the driveshaft is angled slightly from the transmission to the differential.
bad info...the drive shaft is straight and the offset compensates for the offset in the pinion, so both half shafts are of equal length for the independent rear susp.....
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Old 03-06-2010, 08:18 PM   #6
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Your frame is still bent

jack
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Old 03-06-2010, 09:07 PM   #7
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Quote:
Originally Posted by JOHN383 View Post
I would assume that the drive train should be straight inline from
the engine damper to the differential yoke. Are there tolerances
for misalignment?
There is some small tolerance. If you can't get the bolts through the frame and into the mounts, you're out of spec.

You could cheat and pry the transmission around and get the bolts in or start elongating holes. Both are poor choices and will probably result in other problems.

It doesn't take much of a bump on the front frame horn to drive the frame back, just in front of the side rail.

I'd take the thing to a frame rack if you're sure you're not overlooking something obvious. That frame could also be off, up/down and you won't know it until you get all the weight back on the suspension.
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Old 03-06-2010, 09:17 PM   #8
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The front end accident was minor... basic fiberglass damage...frame is OK but one engine mount was torn (old rubber) away. I figured that
several hundred pound of force at a quick stop could impact the steel
motor mount a bit too. I would figure that both motor mounts should be
equal in distance from the transmission cross member. I don't want to
bend the wrong one with my portapower! Engine fits nice otherwise.
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Old 03-07-2010, 01:50 AM   #9
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Take a old bare block, hang a bellhousing and transmission off of it, and take the car back to the frame shop. The reason for the old block and all is so if it gets damaged, you don't lose anything.

When the frame bends in the accident, the engine bends the engine mount horns because it isn't letting it move. So when the frame is "straightened", the horns now move as there is nothing holding them. Having an engine in there will let the shop see how much they have to bend the horns around to get it aligned. They will bend with the engine in place and that could break the block, so it is best to have a junk block in there for this.
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Old 03-07-2010, 08:16 AM   #10
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I have the car assembled now....everything went together fine, new body mounts, bumper
alignment etc. I don't believe the frame is bent however I would like to check diagonal tolerances on the frame to verify. Any suggestions on key point-to-point frame measurements that I should take? I have the car on a lift that will allow plumb bobs and measurements easy for this evaluation. The car body sites perfect right now and there
is no stress on any the fiberglass body parts.

I
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Old 03-07-2010, 08:35 AM   #11
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bad info...the drive shaft is straight and the offset compensates for the offset in the pinion, so both half shafts are of equal length for the independent rear susp.....
Yep - I forgot about the offset pinion. After carrying that heavy thing around in my garage several times you wouldn't think I would make that mistake.
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Old 03-07-2010, 05:57 PM   #12
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Here are some diagrams that might help. The first is the bottom view which will help if you have the body on. Unfortunately there are no measurements for the engine and trans mounts.





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Old 03-07-2010, 06:51 PM   #13
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Thanks for the diagram & dimensions. Measurement C (59 3/8") should
be adequate. The frame rails on my car site perfectly in alignment as
a matter of fact I got the tranny bolted in today. I took several rough
measrements today and they looked OK, got the tranny bolted in too
without much effort (slid over with a 18" prybar. I suspect the problem that I had was the brand new motor mounts from Corvette America. These are so tight that you would swear they didn't use rubber between the plates...I guess I expected the tailshaft to position itself right down the middle of the tunnel and forgot that the manufacturing tolerances are not what they are today.

I would guess that the rubber motor mounts will stabilize after some mileage and use.

Want to thank everybody for their help! My new GM ZZ383 is looking
pretty sweet right now.
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Old 03-08-2010, 12:24 AM   #14
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Quote:
Originally Posted by JOHN383 View Post
...
The frame rails on my car site perfectly in alignment as
a matter of fact I got the tranny bolted in today. ...

got the tranny bolted in too
without much effort (slid over with a 18" prybar. I suspect the problem that I had was the brand new motor mounts from Corvette America. ...

forgot that the manufacturing tolerances are not what they are today.

I would guess that the rubber motor mounts will stabilize after some mileage and use.
...



Frame rails can be perfect and the engine mount horns can be bend way out. That what I was saying above. that is because in a wreck, the side rails move from impact, but the engine block holds the mount horns in place, so they are bent in relationship to the side rails.

When the side rails are pulled back in place on the frame rack, the engine mount horns are now out of alignment.

1 inch is NOT manufacturing tolerance. 1/8 inch is more likely.

All you have done is place the engine rubber mounts and the transmission rubber mount in stress. They will eventually rip and then you will have an engine that jumps with you jump on it or a transmission that wants to bash on the tunnel.


You asked for advice, and you have been given it from someone who has already been where you are.
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Old 03-08-2010, 01:34 AM   #15
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I'm gonna hazard another guess as to what's happened. Since the impact tore the mount up, I'm gonna guess there was stress on it, like a lot of stress and the impact made it pop, so previous damage is what I'm thinking, to the frame and it's overall alignment . Someone pushed the trans over to get it bolted up, knowing the u-joints would make up for the angle, and now another jolt made it worse. In any case you need the best lazer frame machine and operator to make sure you get it perfect, and going down the road in a straight line. That's what I would do.
~~~~~~~~~~~~~~~~~~~~~~

Last edited by Kerrmudgeon; 03-08-2010 at 01:38 AM.
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