[grandam1979]

Originally Posted by Mr Anonymous

this works.

That’s what I had on my G/A worked great.

[15bhardwick]

Also need to say this is the 2 bolt style motor mount. Would the turnbuckle be an easy way to go for a little more support to take some of the stress off motor mount. Would I want to set it up with some slack or tight. The engine is already in the car with the solid mounts.

[VCho455]

Originally Posted by Sirrotica

Some good information about stock blocks and using solid motor mounts. There are at least 2 people that posted that attributed their #2-3 main failures to block distortion, and using stock style mounts.

http://forums.maxperformanceinc.com/...t=motor+mounts

Some don't realize that Pontiacs original 2 bolt mounts (59-69) were putting tension on only the #3 main web. Using that location is going to stress one portion of the block with that mount system. Remember that the original Stratosteak V8 displaced only 287 cubic inches, the displacement leap frogged up to 455 cubic inches in 1970, along with a much longer stroke that produced much more torque than the 1955 engine did.

In 1970 Pontiac re-deigned the mount placement to straddle the #2 and #3 main bulkhead plus added stiffening ribs on the outside of the block. All the 455 applications used the later style mount system, shown that the A bodies had one set of mounts for the 400 and less cars, and a separate mount system with the 3 hole setup on the A bodies with 455 engines through 72, the G bodies got a new 3 mount system in 1970 that they used in all engines.

Pontiac evidently was worried that the early system was putting too much stress on the #3 main with the longer stroke 455 engine. Afterall they had a 2 bolt system for 15 years, and completely redesigned it to use 2 main bulkheads too disperse the stress over a wider area. If you look at the bottom of a 5 bolt 70-76 block it's easy to see the change, and why it was done, just remember if you're using the early 2 bolt mounts, the stress is being concentrated on only 1 bulkhead. Using the later style system (3 bolts) is probably why some have gotten away with using a left hand solid mount with a standard right mount. The attachment points of the later mount are a much better design to spread out the loading of the stock block.


Pontiac engineers saw the long stroke 455 engines were going to put too much stress on a small portion of the block, and cause distortion. A strong running 455 will destroy the left stock type mount in a short time if not restrained with some other means. Even the captured mounts, the rubber gets worn out quickly. One poster in the thread referenced, tore a mity mount in two on his first pass on a drag strip.

I personally keep both stock mounts, and restrain the engine with a turnbuckle on the left side. The further away from the crankshaft centerline the restraint system is, the more leverage it has to stop unwanted movement. The stress is further spread and uses the water crossover as well as he intake manifold to tie to the right bank. It worked flawlessly in my race cars, using the 2 hole system with new mounts they would rip apart in 2-3 weeks. After affixing a turnbuckle I used the mounts for 3 years, and when I removed the engine, the left mount looked like new.

Great information. Another item proving GM didn't do anything without a reason.

[Jim Moshier]

I'm wondering if this was why people started filling their block to combat the block flexing and stressing the main bearing? I have been using the 3 bolt system on my GP. Good information for sure.

[Sirrotica]

Originally Posted by 15bhardwick

Also need to say this is the 2 bolt style motor mount. Would the turnbuckle be an easy way to go for a little more support to take some of the stress off motor mount. Would I want to set it up with some slack or tight. The engine is already in the car with the solid mounts.

If I were you, I'd remove the left solid mount, and put a conventional mount at least in the left side. I ran my dirt track car with 2 conventional mounts and 2 turnbuckles, but with a dirt track car, you're also dealing with deceleration entering a corner, which you never have with a drag car, that was the reason for a turnbuckle placed on the right side head also.

I set mine with 1/8 inch of free play with a flexible mount, that gives some flexibility between the frame and engine, but limits it as soon as you twist the engine 1/8th of an inch.

With solid mounts you already have all the stress on the block as soon as the engine accelerates, putting a turnbuckle on it with a solid mount is really going to serve no purpose. There needs to be some flexibility in the mount system so the force can be applied to some other portion of the engine other than just above the oil pan rail.

The 2 bolt early style is what the Pontiac engineers were concerned about when they re designed the mount system, because it puts all the twisting force on 1 main bulkhead, #3. By putting a turnbuckle from the cylinder head area you're redistributing the stress forces to other parts of the engine, instead of applying all the force to the left center side of the block.

If you ever compared Pontiacs mounting points to other brands, you'll notice most manufacturers used at a minimum of 3 mounting points, many use 4 points such as the LS engines. Pontiac used 2 mounting points for more than a decade, they were due for an update.

If you're trying to use the 2 bolt system which was designed in 1959, and you're making 2022 power, you need some help to stop block distortion, 5-600 ft lbs is a lot of force to concentrate on 2 bolts, and one main bulkhead.

I did a lot of reverse engineering, and studying when Pontiac changed their second design mount system, because frankly, I couldn't understand why the change happened. There is no record that I'm aware of that explains why the engineers changed their system. So i just started studying the changes from one system to the other to realize, how and why they changed it. The explanations I posted are my observations, and nothing more. I just studied the changes, and drew my own conclusions. I also studied the engines that have been posted on the net that could be attributed to tension being concentrated on the #3 bulkhead, and resultant damage incurred. There are more than a few engines that had failures in the #3 main area.

Pontiac engineers did their homework, and succeeded with their improvements. We just need to follow the cue, and be aware of the damage that the early mount system can do when stressed to it's limits.

[Tom Vaught]

Originally Posted by Sirrotica

If you're trying to use the 2 bolt system which was designed in 1959, and you're making 2022 power, you need some help to stop block distortion, 5-600 ft lbs is a lot of force to concentrate on 2 bolts, and one main bulkhead.

I did a lot of reverse engineering, and studying when Pontiac changed their second design mount system, because frankly, I couldn't understand why the change happened. There is no record that I'm aware of that explains why the engineers changed their system. So i just started studying the changes from one system to the other to realize, how and why they changed it. The explanations I posted are my observations, and nothing more. I just studied the changes, and drew my own conclusions. I also studied the engines that have been posted on the net that could be attributed to tension being concentrated on the #3 bulkhead, and resultant damage incurred. There are more than a few engines that had failures in the #3 main area.

Pontiac engineers did their homework, and succeeded with their improvements. We just need to follow the cue, and be aware of the damage that the early mount system can do when stressed to it's limits.

Going back a bit farther in history to the even older 1956, 1957, 1958 front engine mount, it is apparent that the front/sides of the engine saw very little stress because the mount was in a direct line with the crankshaft centerline.

Unfortunately it allowed a lot of engine movement (counter-clockwise viewed from the front of the engine) on acceleration. It also was a complex part for assemble on the line.

The two bolt mounts clean that up a lot and if you use the simple retention methods provided in previous posts you are good to go. No stress applied to the block and a simple fix using a strap or turn-buckle.

Tom V.

[w72]

What would be the best course for a 2nd gen bird using a '72 block at the 575-650 lb-ft range? Won't be drag racing, but will do some autocross on modern street tires.

The car currently has oem style mounts that are new, but we've discussed using a chain or turnbuckle to the cyl. head. Block is not filled.

[Sirrotica]

You use the same mounts as the 69 GP in my signature pics. That car was my first use of turnbuckles on my race car. Both sides I used stock style mounts and because I was running it hard into the corners and lifting, then braking, I used 2 turnbuckles (one on left head as well as one on the right head) as there is some engine twist upon deceleration, and braking. I ran that car for 3 race seasons with the turnbuckles, no damage to engine, or mounts. I still have that engine in storage from the 1981 season, just as it came out of the racecar.

Your engine is already assembled, so nothing can be done about hard blocking, I've never used it in any of my engines throughout the years, because in the 70s and 80s no one was talking about it in Pontiacs at that time. I don't think it's a bad idea, just that I've never done it personally.

It's used primarily to stiffen cylinder walls that may be thin due to core shift, or a large overbore. My own engines were all stock bores, as they were low mile engines. I always tried to run low mileage blocks that hadn't been bored, not realistic at this point in time because of the age of the castings. Back then I had a service manager at the local Pontiac dealership that used to keep me informed if he had any good blocks available from warranty, etc.

[KGTO]

Guess I was lucky...I ran solid motor mounts with a poly trans mount on a Jerico 4 speed/400/ 150 shot for several years. Ran a two step set anywhere from 4500-5500, block was (is) bottom filled. Made 600 RWHP and 700 TQ. I clicked on this since I have been thinking of going to poly mounts just to reduce/eliminate any harmonics/vibrations from the solid mounts since this is now just a street car

[Ken MacNicol]

I have stock rubber mounts on both sides, with a solid rod going from head to frame on left side. It has been that way since 89' and trans brake the whole time

[Probird]

I run a front motor plate and stock rubber mounts as a limiter