I've done this swap in a 66 Malibu before using the existing motor mounts off of the 283 and didn't change the frame mounts. The install went fine and the headers fit as well. Even bolted it up to the existing Powerglide without changing the trans mount.
Many folks say you must change both the frame brackets and the BB motor mounts. What is the difference using the BB motor mounts?

the BB frame mounts have a "rain gutter" looking relief stamped into them, I think its to clear part of the larger BB mount but I honestly cant remember. I have never compared the assembled sets next to each other, just the frame mount pieces. I also put a 454 in my 66 vert using the original 283 mounts, fit fine. People who say you have to change have never put a wrench to one and done it themselves.

I don't know what the "raingutter" is for (never heard it called that, but makes sense).

The big difference is the BB frame mount has a relief for the rubber block mounts. The BB rubber block mounts have a "T"-shaped interlocking device so that in the event the rubber mount separates, the motor will still be restrained. Just a safety feature to keep the engine from torquing up and out of the engine compartment if the mounts break...

If you look at the small block mount, the portion where the long through-bolt goes has three distinct "sides" about 1-inch tall, with the fourth side sloping gradually toward the lower part of the mount... The BB frame mount will have all four sides, therefore eliminating that sloped portion (this is where the space is required for the interlocking device...).

So you can use small block rubber mounts on either small block or BB frame mounts, but you must use BB frame mounts with BB rubber mounts... Solid mounts will work with either.

Otherwise, they are mostly interchangable...


I think alot of people get confused with 1969 and up mounts, where the BB and 350 sit at a different level compared to 307s, using different mounts...

Yes, the 307's are different, as well as the 55-57's.
Every article or post I've read says you need BB motor mounts. Nice to hear from somebody else you don't!

There is a lot of confusion on the frame mounts. I did a lot of investagating and measuring and the two are identical except on the big block frame mount there is a piece welded in for strength. In fact some resto houses use the same part number for the frame mounts. I have ALWAYS used the small block frame mounts with the big block rubber motor mounts and haven't had a problem yet.
Mark

I have ALWAYS used the small block frame mounts with the big block rubber motor mounts and haven't had a
problem yet.Marc, Are you sure these were 1966 part (year/application). The factory 1966 Chevelle BB rubber mounts (or standard replacements for BB application) will not work on factory SB frame mounts (or correct repros)... The T-shaped interlocking device on the Rubber mount hits the frame mount before the bolt holes line up... Small block rubber mounts will work when bolted to a BB (Maybe this is the same part numbers you were referring too)


the two are identical except on the
big block frame mount there is a piece welded in for strength.Oh, must be the repro pieces... The factory frame mounts are definitely different looking, SB to BB...

The replies you have received are correct, I just wanted to and this picture for reference. The Big block frame mounts are in the middle.
http://bellsouthpwp.net/k/s/kss396/Frmmnts.jpg

Ken, great pictures...

That last picture (bottom/right) with the BB rubber on SB frame mount... Do you have another view showing the "through-bolt" hole not lining up, etc?

This place never ceases to amaze me! Thanks Ken! Pictures are worth a thousand words. Now I know what rain gutters look like on BBC frame mounts ;)
So the 'positioning' looks the same between both small block and big block. The BB's are beefier and made for torquey 5,000 rpm launches? :D

Originally posted by 1966_L78:
Do you have another view showing the "through-bolt" hole not lining up, etc? No... but thats a good idea. I'll try to get one from that angle. I hope I can remember where those frame mounts are. smile.gif