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Discussion Starter #1
trying to make sense of the SB / BB frame mount interchangability issues - its a little confusing with all the: a 350 will work on 307 mounts and 454 will work on 350 mounts but not a 454 on 307 mounts.
So I figure that I will post some pictures, hopefully someone can positively id the mounts and then we can leave the post up for other's to benifit in the future.
So this is from my '70; started life as a 307, had a 305 when I bought it and now I am swaping to a 454 - which mounts are these? are these the correct mounts for a 454?





 

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Below is my understanding of Chevelle motor mounts and frame stands.

All of the following information is for stock parts.

Note: The frame stand determines the motor mount regardless of engine displacement.

All Chevrolet V8 engine blocks from 1958 up have the same motor mount boss configuration regardless of displacement.

For 1964-1967 Chevelle

The frame stand for SB and BB are exactly the same in every dimension except the BB stand has an indentation for the motor mount safety lock feature.

The only difference between the SB and BB motor mount is the BB mount has the safety lock feature.

The BB frame stand will accommodate both BB and SB motor mounts. The SB frame stand will NOT accommodate BB motor mounts because of the safety lock feature.

For 1968-1972 Chevelle

The frame stands and motor mounts are interchangeable for all V8 engine displacements except the 307 cubic inch. There is no difference between a 350 or BB frame stand for these years.

307 frame stands and motor mounts are different than those for a 350 or BB. They can only be use together and not mixed with the 350 or BB style.


Examples:

When installing a 350 or BB engine in a 1968-1972 Chevelle that originally came with a 307 and still has the 307 frame stands use 307 motor mounts.

When installing a BB engine in a 1964-1967 with SB frame stands use SB motor mounts because SB frame stands do not have the indentation for the BB motor mount safety feature. If the car has BB frame stands you can use either SB or BB motor mounts.
 

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Pat is right on! He just solved my issue earlier today with this. 307 stands must be slightly wider. 307 mounts were the only ones that worked on my stands. They don't look as well built as the 350/BB mounts. Steel is a little thinner and I don't see an interlocking metal hook in case the motor mount breaks like I see on the 350/BB mounts. For me I think the 307 mounts will be fine since the 350 I put in it only has about 300 H.P. If I were putting in a BB I personally would switch the stands out so I could use the heavier looking motor mounts.
 

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2 3/8" width is 68-72 BB and 350
2 5/8" width is 68-72 307, 66-67 BB, and 64-67 all SB's
If you want interlocks on 307 brackets use the 66-67 BB engine mounts
or the current GM SB 307 68-72 engine mount (3990914) now has interlocks.
The 3990914 is also the current listed mount for all 64-67 SB's if you change to BB brackets on those years. I am using them on my 66 327.

Edit: 3980707 is part number for LH 307 bracket
 

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Below is my understanding of Chevelle motor mounts and frame stands.

All of the following information is for stock parts.

Note: The frame stand determines the motor mount regardless of engine displacement.

All Chevrolet V8 engine blocks from 1958 up have the same motor mount boss configuration regardless of displacement.

For 1964-1967 Chevelle

The frame stand for SB and BB are exactly the same in every dimension except the BB stand has an indentation for the motor mount safety lock feature.

The only difference between the SB and BB motor mount is the BB mount has the safety lock feature.

The BB frame stand will accommodate both BB and SB motor mounts. The SB frame stand will NOT accommodate BB motor mounts because of the safety lock feature.

For 1968-1972 Chevelle

The frame stands and motor mounts are interchangeable for all V8 engine displacements except the 307 cubic inch. There is no difference between a 350 or BB frame stand for these years.

307 frame stands and motor mounts are different than those for a 350 or BB. They can only be use together and not mixed with the 350 or BB style.


Examples:

When installing a 350 or BB engine in a 1968-1972 Chevelle that originally came with a 307 and still has the 307 frame stands use 307 motor mounts.

When installing a BB engine in a 1964-1967 with SB frame stands use SB motor mounts because SB frame stands do not have the indentation for the BB motor mount safety feature. If the car has BB frame stands you can use either SB or BB motor mounts.

that is right I found out the hard way with my 72 elcamino 307. lucky enough the engine was being installed on an open frame. had to go back and get the new engine mounts. thanks for the information. Ed
 

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Here is picture of the 2 types side by side with all of the part numbers.

Maybe I'm missing an earlier post but did anyone ever thank Mark for posting this? He posted Apr 26th, 06 3:59 PM.

Mark, a BIIIIGG :thumbsup: for this!!!!


the walrus, Thanks to you also.
 

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A lot of guys have header fitment problems and a lot of them can are due to the wrong frame pads. You can see that the 307 pad has the hole for the long bolt somewhat higher than the 327/396 pad. Sometimes the difference, If you happen to end up with the wrong pad, can be made up with the top section that bolts to the block.
As long as you know what is causing the fitment problems it isn't that hard to come up with a solution.
 

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Thanks Guys for all the answers on the motor mounts. I thought that lock tab ability might be the only difference. I ordered a set so I could use the locks.

Another note I put off getting the Ram Jet 502 for a few years since I didn't want to change from an SS hood to clear the plenum. Then found that Mark Cambell @ Street and Performance has a program where he will cut up to 1 1/8" off the bottom runners of the plenum (the little straight leg sections) so it the same height as a stock high rise with carb.

This is my first experience with chevells.com and is kind of cool! Actually about -10 degrees in Anchorage now. Later.
 

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Thanks again Mark.
that picture and parts number is what I've been needing. It's been printed and stored for many years of use! :cool:
 

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2 3/8" width is 68-72 BB and 350
2 5/8" width is 68-72 307, 66-67 BB, and 64-67 all SB's
If you want interlocks on 307 brackets use the 66-67 BB engine mounts
or the current GM SB 307 68-72 engine mount (3990914) now has interlocks.

The 3990914 is also the current listed mount for all 64-67 SB's if you change to BB brackets on those years. I am using them on my 66 327.

Edit: 3980707 is part number for LH 307 bracket

Why haven't I read that anywhere before... I'd much rather order motor mounts for the earlier BB car rather than buying both the motor mount and the frame mounts...

(I've got a '69 with a 307 (original), though someone put interlocking mounts in at some point...)
 

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Is the problem still the installed height of the motor?

I'd very much like to keep the 307 motor mounts on the car and not fight with the nut/bult install.
But if I got the fancy "locking style 307 motor mounts" for my big block motor, would the installed height of the motor cause problems?!

.
 

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I went from a 307 to a 350 in a 1969. I used the original frame stands with new motor mounts (energy suspension). There were no fitment problems whatsoever. If I am understanding you guys correctly when I install a 396 into my car I will need new frame stands designed for a big block?
 

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My 72 has 307 frame mounts.
I put a 396 BB into it, and the motor had GM standard Big block motor mounts.

Based on header fitment the fact that my 3" exhaust head pipe had to be "crimped" to fit under the cross member, I think the motor say up to high. However, the car has been wrecked sometime in its life, so I can't not say for sure that has something to do with it or now.

I guess I will just change the frame mounts and take the time to do the nut/bolt swap if I can't get a clear answer on wether the motor will sit higher or not using the "new locking 307 motor mounts"
 

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My understanding is that you could bolt the 396 to the 307 mount, but;
The engine would be higher which may cause header fitment issues, and may eliminate the space for your air filter
Would be much weaker, If you start jumping on the throttle you may break a driver side mount. If you do that with the interlocking mount you may pick up some vibration, if you don't have the interlocking mount you may have the engine laying up against the right fender

I went from a 307 to a 350 in a 1969. I used the original frame stands with new motor mounts (energy suspension). There were no fitment problems whatsoever. If I am understanding you guys correctly when I install a 396 into my car I will need new frame stands designed for a big block?
 

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hey guys, i have a 68 chevelle that i pulled a 307 out of and when installing a 327 the mounts wouldnt fit and as stated above mounts are 2-3/8" wide and pedestals 2-5/8" wide. is it true that by changing out the pedestals instead of the mounts i will gain air cleaner space? since the 307 mounts are taller? i can go either way just checking. thank, tim
 

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How does one figgure out which bolt holes to install the motor mounts frames to the frame crossmember. What is sad is that is one picture I did not take during dissemble. Many thanks in advance. This is last stage before body work kicks off.
 

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anybody got a pic? all I remember is that the frame stands hang over the front of the crossmember.
 

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My engine is freshly removed. I will try to remember to snap a few pics tomorrow.
 

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68 Chevelle, no other info on this pic.
must use 307 motor mounts with 307 frame brackets, just like you have to use 350-454 motor mounts with 350-454 frame brackets. engine should stay in same spot with both set ups. 327 will work with the 350-454 frame brackets and motor mounts or 307 motor mounts and frame brackets. I would use energy suspension 307 motor mounts. In fact I use ES 307 motor mounts and 307 frame brackets with my 454.

 
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