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Old 05-25-2023, 10:02 AM
tekuhn tekuhn is offline
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Default 400 vs 455 A-body Engine Mounts

I'm curious if anyone is familiar enough with the two different mount designs to comment on whether one is stronger than the other? There must be some reason why they chose to give the 455 a different mount when the 400 mount would bolt to the engine fine.

For my custom project, I'm using a dual pattern 5-bolt block, so I can use either mount and might use the 455 mount if it's indeed stronger.


Thanks in advance for any information!

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Old 05-25-2023, 10:20 AM
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That is a good question. My guess is that it’s not strength related but due to frame mount position? I don’t know.


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Old 05-25-2023, 10:37 AM
tekuhn tekuhn is offline
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In the '70-'72 A-bodies, you could get either engine. The frame is the same, but for some reason they used different engine mounts and frame brackets. The position of the engine is the same, and so is the weight, so the only logical difference is the amount of torque produced which makes me think the 455 mount might be stronger.

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Old 05-25-2023, 11:38 AM
SD455DJ SD455DJ is offline
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Here's a couple pictures of the different motor mounts. the blue ones are the std. 350/400 frame mounts, while the black ones are the 455 (repop from PY) mounts. The 455 mounts are wider to accommodate the greater torque of the 455 and the location is forward on the front crossmember. I'm guessing that this location was due to the fact that the block mount used 3 bolts and was longer than the 350/400 std. block mount.

Dennis
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Old 05-25-2023, 11:50 AM
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Ames makes really good 400 engine mounts that they had manufactured themselves. That's all I put on customer's cars now. Unfortunately, they don't make the 455 version, and the aftermarket 455 motor mounts from parts stores are total junk. If you're starting from scratch, I would far more trust the Ames 400 mount verses the aftermarket 455 one.

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Old 05-25-2023, 12:03 PM
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Quote:
Originally Posted by SD455DJ View Post
Here's a couple pictures of the different motor mounts. the blue ones are the std. 350/400 frame mounts, while the black ones are the 455 (repop from PY) mounts. The 455 mounts are wider to accommodate the greater torque of the 455 and the location is forward on the front crossmember. I'm guessing that this location was due to the fact that the block mount used 3 bolts and was longer than the 350/400 std. block mount.

Dennis
This was very helpful! This is the first time I have seen what the 455 frame brackets looks like and I did not realize the 455 mount was moved forward so much. I took measurements, and the 400 mount will also be much more centered on the Mustang II crossmember I've installed in my '63 Tempest. That along with Gary's comments above are enough to convince me the 400 mount is the way to go. Thank you all!

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Old 05-25-2023, 12:19 PM
SD455DJ SD455DJ is offline
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I totally agree with Gary's thoughts on the new PY 400 motor mounts being of superior quality over the foreign crap that we were forced to use. I bought the 455 "Mity Mounts" a few years ago for the 455HO going back into my GT-37...before the 'good' PY mounts came out. These are probably junk too...time will tell. Good luck with you project!

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Old 05-25-2023, 12:33 PM
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Just ordered a pair from Ames. Very reasonable price. I also have a pair of 400 Mity-Mounts in a '65 w/ 455. They seem fine, but are more expensive and the Ames mounts claim to be stronger.

I was ready to order mounts from Rock Auto - they have both 400 and 455 mounts for $6-$8 each which was shockingly low. Perhaps they would have been OK, but I feel better about going with a known product.

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Old 05-25-2023, 12:56 PM
'ol Pinion head 'ol Pinion head is offline
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Quote:
Originally Posted by tekuhn View Post
This was very helpful! This is the first time I have seen what the 455 frame brackets looks like and I did not realize the 455 mount was moved forward so much. I took measurements, and the 400 mount will also be much more centered on the Mustang II crossmember I've installed in my '63 Tempest. That along with Gary's comments above are enough to convince me the 400 mount is the way to go. Thank you all!
Have read the conjecture, that the only difference in the regular '70-72 A-body frames & the same year 455 A-body frames is the different stamped holes for the 455 frame stands. That's not true, as the top section of the cradle of the '70 & 71 455 A-body frames are also noticeably stamped differently than same year 112" wb 6cyl-400 A-body frames. Besides my '71's, also have a 72 455 HO GTO project sitting in the wings, & the top of it's frame crossmember is stamped different than the 71 455 frames. Need to take time & pull an engine & compare the standard '72 A-body frame cradle with the '72 455 frame.

As far as the long rubber & steel 455 motor mount goes, I'd like to see Ames have a superior repro rubber & steel mount manufactured. This style mount fits more than '70-72 455 A-body's & '70-74 V8 Birds. Fortunately, have several pair of nos 455 motor mounts (but not enough), & am not very trusting of the last 20+ years cheap motor mounts.

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Old 05-25-2023, 12:56 PM
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I used those $8 Korean motor mounts in my 455 LeMans ('70) and they totally collapsed after 2 years of minimal easy driving. The oil pan was actually resting on the crossmember and the center link was also rubbing the pan! The junk mounts were replaced with decent factory original mounts we fortunately had. You'll see that we didn't bother changing out the 350/400 motor mounts to the 455 style. I do see the factory holes for the 455 'frame stands' and would be curious to see what other physical differences there are as well.

Dennis
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Old 05-25-2023, 01:14 PM
'ol Pinion head 'ol Pinion head is offline
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Agree with you on the Korean mounts, have quite a few used ones & even some that have to pay to have them revulcanized.

Dennis, what car's frame is the left most picture of in post #4 above?

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Old 05-25-2023, 01:21 PM
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Quote:
Originally Posted by 'ol Pinion head View Post
Agree with you on the Korean mounts, have quite a few used ones & even some that have to pay to have them revulcanized.

Dennis, what car's frame is the left most picture of in post #4 above?
Roger, both pics are of the 455HO GT-37, although several years apart.

Dennis

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Old 05-25-2023, 01:45 PM
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IIRC…the “455” mounts were designed for the 2nd gen F body, since the engines were mounted further back relative to the x-member. I’m guessing GM was planning on the 455 going into them…quickly.

I’m also guessing it was/is, by it’s overall design, stronger than the 2 bolt mounts, thus it being used in the A body’s as opposed to the 2 bolt ones.

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Old 05-25-2023, 03:24 PM
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Originally Posted by rolling money pits View Post
IIRC…the “455” mounts were designed for the 2nd gen F body, since the engines were mounted further back relative to the x-member. I’m guessing GM was planning on the 455 going into them…quickly.

I’m also guessing it was/is, by it’s overall design, stronger than the 2 bolt mounts, thus it being used in the A body’s as opposed to the 2 bolt ones.
Definitely a stronger design, originally.

Also the long mounts were used on:
- '71-74 (?) Pontiac B series (until the stamped clamshell type brackets were introduced)
- '73-74 Pontiac V8 A body's, not sure if all of them.
- '73 Pontiac A-body, the 455 frame stands were a one or two year deal.

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Old 05-26-2023, 12:09 AM
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I converted my 72 LeMans GT to the 455 mounts when I built and installed the 455 D-port several years ago.

Being as it was a side by side comparison (before/after), I noticed something else with the more forward located 455 mounts & frame brackets. The stability of the entire engine/trans installation is much more stable (& stronger) than it is with the 400 style mounts.

I think of it this way. If you're holding the engine directly in the middle (400 style) vs the forward third (455 style), you end up with an effectively shorter 'mechanical arm' back to the trans mount. It feels like the engine/trans combo is being held less securely with the 400 mounts vs the 455 mounts. This would add stress to the 400 mounts due to the 'lever action,' but also the shorter rubber mount is less dimensionally rigid. Longer mounts and forward placement are both beneficial, especially with the extra torque of the 455.

I may not be describing this very well, but the entire installation feels/acts much stronger, like it more solidly attached to the car.

A bonus I would think is that I don't believe the issue Dennis had with the mounts collapsing and the pan resting on the X-member will ever be an issue with the length and location of the 455 mounts.

Just my thoughts here, but suffice it to say, I'm very pleased I went with the factory 455 design mount on my 72. I will be doing the same when I install the 455HO into my 71 GTO next season.

Hope that helps.

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Old 05-26-2023, 12:38 AM
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I found that having the mounts in the middle of the engine was superior due to not having to support the rear of the engine when puling the trans - it just kinda balanced there, especially if the exhaust was still attached.

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Old 05-26-2023, 01:34 AM
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Quote:
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I found that having the mounts in the middle of the engine was superior due to not having to support the rear of the engine when puling the trans - it just kinda balanced there, especially if the exhaust was still attached.
True enuf.

But I prefer to spend more time with the trans attached to my engines, so I'm willing to go to the effort to put the wooden block under the pan as needed.

Heh.

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Old 05-27-2023, 05:15 PM
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"I" would buy some Chevy (Camaro, so parts availability is guaranteed?) clamshell-style mounts that could be attached to the frame, and then fabricate a pair of brackets that bolt to the engine--potentially using modified steel brackets from a stock-style mount with the ruined stock rubber removed--so the bolt holes to the engine are already in place and correct. The clamshell mounts are enormously safer/stronger than the bonded-rubber style, readily available in OEM rubber, aftermarket junk rubber, or aftermarket Polyurethane (provided you choose the application wisely.)

The advantage to the clamshell style mounts being that even if the rubber totally disintegrates, the engine stays more-or-less in place under torque load and G-forces. It may clang as it rises up, and bang when it drops again...but safety is not sacrificed. (You'd never let your engine mounts get THAT bad.)

Anyone who can put a Mustang II crossmember into an A-body should be able to clamshell-mount the thing half asleep.


Last edited by Schurkey; 05-27-2023 at 05:23 PM.
  #19  
Old 05-28-2023, 12:51 AM
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If anyone cares to know why there are 2 motor mount systems in a 70-72 A bodies, here is what I've determined after doing a lot of studying. The potential of the 455 having enough torque to distort the #3 main web, and why Pontiac engineers moved the mounts forward, and stiffened the outside of the block.

I've studied the early 2 bolt motor mount design , as well as the late model motor 3 bolt motor mount re-design, and this is what I've found.

If you look at the double position motor mount blocks (70-76) stripped bare you'll notice that the old two hole mount area (59-69) only attaches, and straddles, the #3 main bulkhead. When Pontiac introduced the 455 in 1970 they also moved the motor mount position forward, and used 3 bolts, along with cast iron stiffening ribs. The longer motor mount with three attaching points straddles both the #2 and #3 bulkheads, halving the stress on the #3 bulkhead. The third upper mount bolt further spreads the load upwards towards the top of the block.

Even on the A bodies that used the 2 hole early design through 1972 on the 400, 350 engines. The 455 in A bodies in 1970-1972 used the newer late 3 bolt design. The 70-72 A bodies used the reinforced 3 bolt mount system only on the higher torque 455 engines.

When Pontiac redesigned their mount system in 1970, I always wondered what the advantage of the completely new system was. Years later I studied the double pattern blocks, and it finally sunk in why they redesigned the mount system to spread the load to 2 bulkheads and they also stiffened the new mount area with an additional ribs running top to bottom.

The anticipated extra torque of the longer stroke 455 was enough that Pontiac engineers thought it was better to spread the force over 2 bulkheads and have 3 attaching points on the higher torque engines. The positioning of the 3 hole motor mounts definitely is superior to the early design 2 bolt design.

There have been plenty of pictures posted over the years on PY that have shown the bulkheads cracking in high horsepower builds. I've seen images of both factory, and aftermarket blocks splitting. Many times it has been blamed on using the stock motor mount positioning in a high power build. Of course solid mounts exacerbates the problem of stressing the bulkheads

I'm far from a engineer, but common sense says that Pontiac re-designed the mount system in 1970 when they introduced the highest torque engine they had made to that point. Look at the engineering on a stripped block and I'm pretty sure most anyone can see the reasoning behind the changes.

I've been told that automotive engineers usually use a 25% safety factor above what the actual stress will be. Using that factor, the 1970 455 HO was rated 335 HP, and 480 ft. lbs. of torque, adding 25% to that, arrives at 600 ft. lbs. using the later design 3 bolt mount on a fresh piece of cast iron. Over that point is where Pontiac blocks seem to fail, especially when using the stock motor mount attachment points on 50 YO cast iron.

When using the early 2 bolt style mounts/blocks the limiting factors are going to be less than later 3 bolt style design.

Just trying to look at the engineering from the common sense vantage point. Maybe after looking at both the early 2 bolt design, and the later 3 bolt design everyone can see the advantages, and limitations of the later system. Hopefully looking at the engineering will save someone from over stressing a block using factory mounting points causing it to crack in the main web area.

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Old 05-28-2023, 07:36 AM
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Brad, That makes sense to me...very interesting deduction.

Dennis

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