When Chrysler started to use the captive engine mounts (in ’73?) the mounts and the K-frames changed significantly between small and big blocks.
Captive engine mount
Block style mount.
Note: if rubber separates, then there is nothing to hold engine in place. Bust both engine mounts – then it is possible engine could rotate in engine compartment so oil pan might point up and carburetor might point down, when under a load. The captive mounts, you might have metal on metal inside of the engine mount, but engine is not going anywhere.
The older block style engine mount – for the most part, big block and small block K-frames were the same and not hard to swap one out for the other (except for Hemi’s).
Of the cars with captive engine mounts, there is A-body, B/R body, C-body, FMJ-body and pickup/vans (which continued to use a different style but a block style engine mounts well beyond when big blocks stopped production) and each has a different design K-frame.
Pickup/Van mount
The A and FMJ bodies did not have any big block versions made with captive mounts – but for some reason I think Direct Connection (Mopar Performance) did make one for over-the-counter sales – maybe (don’t quote me on that).
No R-bodies were made with block blocks, but it wouldn’t be hard to make one if you had access to a B-body K-frame.
This is a picture (stolen from someone on internet) of a ’73-74 B-Body K-frame. The ’75-81’s are similar. The blue circles show passenger side differences and red lines show height differences.
The drivers side mount/boss is mostly in the correct location front to rear and side to side – but is a couple inches (not sure of exact measurement) higher on small block, so a person would need to cut the K-frame down a bit (on both sides). The passenger side mount is about the same location side to side, but front/rear different as well as height is different.
The C-body K-frames are huge and the differences between small block and big block are like above picture for differences.
If you are considering installing a big block into an FMJ, I would highly encourage getting the mount set from Schumacher (part number A73B8 if small to big block, or A73B6 if /6 to big block)
'73 & UP A,B,F,M, J-Body Products (drop down a bit on the page).
I made a copy of the PDF file Schumacher had for installing their kit (for some reason, I couldn’t download it today) – but the 4-page PDF is 8MB large and is more than I can download onto our forum board (1MB limit). Here is a snip I took from that PDF:
This is a picture of what another member did to the passenger side mount. He moved the bushing over a bit then ran screws into the metal bushing to keep bushing from moving.
I have a bunch of brand-new big block engine mounts – so I’m going to do my K-frame modifications a bit differently.
The existing engine and transmission mounts will be replaced with new, first. Then perform several measurements – so I can build a K-frame jig. Measurements will be the 4-corners of K-frame, the center-line of front crankshaft (I might have to remove the crank pulley and insert a socket onto the 1¼” balancer bolt) and height of transmission mount boss to ground (car not raised any and measurements are to ground).
The K-frame measurements will be a bit different sense the existing rubber K-frame bushings are getting tossed in trash after removal – so can’t use the bottom of K-frame mounting bolt. My plan is to keep the crankshaft/transmission centerline in same location up/down and left/right and still use my new engine mounts. Modify the K-frame accordingly.
Record all measurements and repeat (for other car).
The small block engine mounts are not as tall as the big block mounts are. Another thing I want to play around with is if they could be swapped out or something, so less work is needed with K-frame.
One other thing is I do prefer to use rubber engine mounts. The Schumacher mounts don’t have much material and they advertise the material used is urethane (instead of rubber). I’m getting old and the more rubber – da better. My experience with urethane control arm bushings has left my kidneys in poor shape. I just don’t want the same for engine mounts.
Compare to top picture.
BudW