[yahooskidoo]

Ok so I've been researching motor mount options for installing the new 455 into my 74 Firebird. Since my original mounts with 73,552 miles look pretty good I've decided to upgrade them by adding the $100 bolt. My dilemma is this, should I snug the bolt or leave a bit of a gap?

[steve25]

You want it to compress the cushion just a bit to compensate for how it will soften up a bit with heat, especially if your running Header tubed near by!

[leeklm]

One thing to watch out for is that the nut on the engine side does not interfere with the block. Had that issue on the 66 mounts I worked with

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[78w72]

what i did on my home made mity mounts that have lived behind a 500+hp stroker for over 5 years & been drag raced numerous times at low 11sec is to tap the bottom plate for the bolt & snug it down, then use a thin nut for clearance to the block as leeklm mentioned... then a couple tack welds to secure the nut & some lock tight.

the modified mounts work good & should hold under decent power... but i also use a limiter chain to take stress off the mount & ensure that if/when the mount breaks there is something there to stop the motor from damaging anything.

[yahooskidoo]

Originally Posted by 78w72

what i did on my home made mity mounts that have lived behind a 500+hp stroker for over 5 years & been drag raced numerous times at low 11sec is to tap the bottom plate for the bolt & snug it down, then use a thin nut for clearance to the block as leeklm mentioned... then a couple tack welds to secure the nut & some lock tight.

the modified mounts work good & should hold under decent power... but i also use a limiter chain to take stress off the mount & ensure that if/when the mount breaks there is something there to stop the motor from damaging anything.

I did tap the back plate, then a washer and jam nut. I was thinking of putting a tack weld onto the bolt to the jam nut.

[78w72]

Originally Posted by yahooskidoo

I did tap the back plate, then a washer and jam nut. I was thinking of putting a tack weld onto the bolt to the jam nut.

consider adding a limiter chain of some form if the engine makes any kind of decent power & you plan on any track time or use stick tires.

[Sirrotica]

If you look at the bottom end of a Pontiac with the 5 motor mount holes for both styles of mounts, (1970-1976) notice that the rearward (older 2 bolt design) straddles only 1 bulkhead, #3. If you look at the newer style 3 bolt mounts forward mount, you'll notice with the advent of the longer stroke 455 engine the design was changed to straddle #2 and #3 bulkheads.

Pontiac engineers made the switch to distribute the left side load better along the area of the left bank, and load 2 bulkheads, as opposed the only 1 on the earlier 2 bolt design. IMO loading the block with conventional mounts only, especially if you're using the earlier 2 bolt style is putting a tremendous load where there is a deficiency. Making that mount solid under stress with the $100 bolt is asking for problems. That isn't just my opinion, Pontiac engineers spent a lot of money to relocate the mounts and address this potential weak spot, putting stiffeners into the castings of the later blocks to further head off broken blocks internally.

Since the late 70s I have used a standard OEM style mount, and tied the left cylinder head to the frame in my own cars. That includes my race cars, as well as street cars. This takes most all the stress off of the motor mount area of the block. There have been plenty of pictures posted on PY of the #3 bulkhead failing in race engines to make me wary of using the OEM mounting points to hold the entire load of torque in a concentrated area of the block.

I have used a turnbuckle with about 1/16 inch of slack before it physically stops the engine from the torque reaction. The turn buckle is tied to the left front cylinder head area, and the other end is tied to the left frame rail. It has worked flawlessly for me since 1977. This arrangement allows the motor mount to be flexible as it was designed to do, without a physical limiting device under tension, stressing the left side of the block in a small, concentrated area.

Just my own thoughts, observations, and experiences, YMMV......