Not an engineering expert on Solid Motor Mounts, but I do have the Moroso.

This is strictly a guess because I do not know where the cut-off point on maximum torque for rubber mounts.
Again, just guessing, I would say under 300 horse with matching torque, it's safe to use rubber.
Numbers of 300 - 400 horse & matching torque would be questionable with todays quality of rubber or old mounts or poly.
When you start getting north of 400 horse, its time for steel.

Like others mentioned, rear tire width, rear axle ratio and type of transmission will determine stress on all three mounts.

 

I offer no advise but merely an observation. In the early 70's GM issued a recall on pretty much all their vehicles from the 60's up to say 1972 to correct engine twist due to a broken motor mount. Main issue being the throttle linkage would bind and not allow carb to return to idle, causing accidents. The fix was a bracket bolted to the drivers side exhaust manifold with a steel cable attached at both ends and ran around inside of upper A arm cross shaft limiting engine movement should a mount break. In 73 they used a redesigned mount and throttle cable instead of a steel rod. Mounts were always rubber isolated with interlock after that and still use rubber to this day I suppose. Customers want vibration free cars. Solid mounting an engine to make the car more rigid is not something I would do but that is just me. I also like vibration free cars. I would merely ask the OP, if you have worry about damaging an expensive engine using solid mounts and don't plan to hammer on the car, why not use rubber mounts? Plus, don't most airplane engines use rubber isolated mounts of some kind?
I still have some of those retrofit kits around that have pulled off cars I've parted out over the many years. There was several variations though, to fit the various chassis and engine combinations.

 

Someone asked about height. They are designed by height for specific models. If you buy any of the 3 discussed for a Corvette, they will be the same height.

If your motor is creating alot of vibrations you have other issues.

Most vibration issues everyone here is concerned with is shifter vibration. You want a rubber, poly mount on the tranmissiin as thise vibrations are usually coming from the driveshaft and rear.

Ive got a $2000 dollar 69 427 block. Just over $2000 in machine work and balancing.
$2000for the rotaing assembly and cam. About $2500 for heads .. $8500k. I need bearings, gaskets, pumps, fluids, ARP bolts. This build will be over $10k plus my labor to build it. ...not one bit concerned about motor mounts, never will be....but will I blow a head gasket, lose a lobe, spin a bearing, hang a valve, crack the block all of which can happen during break in.....ya of course. I all ways do and i usually build or rebuild a motor every winter. Motor mount giving the motor a booboo. Not even a possibility or a worry

 

In a related question.....the solid mounts are narrower.....so the original engine mount bolt will be quite long, so do most of you just find a good Grade 8 bolt the correct length?. The width of the Moroso mounts is right at 3 inches. The other interesting thing is the diameters. Original engine mount bolts, and the ARP replacements have a small 1/2 diameter shoulder on the hex end of the bolt....then taper down to 7/16 inch diameter. The replacement bolts I used on my 77 were 1/2 diameter the whole way. Seems to me, given the bolts are in a shear load, they should be 1/2 diameter the whole way....AND the shank of the bolt should continue through both side of the engine mount....so in the case of Moroso solid mounts,.....seems like the correct bolt would be a Grade 8 bolt, 1/2 diameter....with a minimum 3 inch shank.....and then of course, enough threads for the nut. IN reality, with a washer under the head, and washer under the nut, the shank may need to be 3-1/4 inch, and then if necessary a few washers under the nut.

I suspect the smaller diameter is to accomodate the wide manufacturing tolerances.....which include the fact you have to line up the transmission, etc.

I just measured the height of the Corvette Central rubber mounts and the Moroso.....same exact height.

 

Well since you brought it up - those bolts are not loaded in shear. When properly tightened/torqued, they are loaded in tension. The tension load provides a clamping force between the motor mount and the chassis mount. That force (friction) resists movement between the motor mount and the chassis mount on the frame. If the bolt was loose then it would be loaded in shear with the mount pulling against the bolt itself.
This is similar to wheel studs. They are loaded in tension and the clamping force between the wheel and the rotor keeps the two surfaces from moving

 

Hmmmm.....just realized another difference in these mounts. With factory style rubber mounts, the weight of the engine is sitting on the frame horn pads and all the bolts do is keep it from clocking from torque. Proof of that is that I can push the bolts in and out of the mounts with the engine sitting in the frame. With these Moroso mounts.....the weight of the engine is sitting on the bolts as there is no built in pad. So....to me, and I am not an engineer, that means the bolts have a shear load especially on these solid mounts. The entire weight of the engine is held up by the bolt shank....PLUS the clocking or torquing stresses from the engine trying to spin in the car. BUT....there is NO WAY in hell any torque on any SBC or BBC engine is going to shear a 7/16 bolt.....not going to happen.

.In any case it obviously works for the thousands and thousands of people who have used them.....WHATEVER the loads are. I know from airplane stuff I have done, bolts are capable of way more than they appear to be capable of. For me....if I use these solid mounts, I will find a full shank, 1/2 diameter bolts that runs the 1/2 diamater through BOTH lugs of the engine mount. That is the way we were trained in aircraft maintenance to select bolts, and how most are spec'd...if they are in shear. Having only one of the lugs supported by the threaded portion of a bolt is not good enough for me.

The other thing is true that the holes in both the mount and the frame horn are much larger than the bolt diameters......so at some point something has to come up hard against something to prevent rotation of the engine. I find it hard to believe tension on the bolt would produce enough friction to prevent that on a big torque motor.....but its probably a combination of both.....in any case no big deal.

This was not meant to start another bickering session.....I just wanted to know what bolts you guys use.....since you are saying that hundreds of thousands of people have used these mounts for years. But I will probably do it my way anyways.......thats the way I roll.

I am not an engineer....but I have stayed at Holiday Inn.

 

My car came with a failed driver side OEM mount. I used the stock bolts. They had enough thread to work with the Moroso mount. If that hadn't worked, I would have gone to whatever hardware store was closer, and bought a grade 8 bolt of the correct length. If you add a few washers to get the threaded section out of the shear zone, that would certainly be best practice, but I doubt it will matter.

Don't overthink it. You spent $18K on an engine that you'll run at half-throttle. Mount the engine, and go drive the car another 8000 miles.

 

So using rubber or poly mounts will not shift the motor up, forward, wherever? I'm going to be running a SBC stroker with Air Gap intake and standard 3" tall filter where clearance is already going to be an issue, which is why I'm using a L71 big block hood (not L88-style). I cant afford to lose even a 1/4" of height.

I would prefer to use poly to minimize vibration, understanding that excessive vibration is another issue (which I won't/don't have) - plus, I already bought them. Just saying I'd like to minimize it wherever possible and enhance/improve original designs, where appropriate.

 

I used 1/2 inch I believe grade 8 bolts all the way though and just enough shank to cover the mount so when it was torqued the mounts bottomed on each other and not the threads. The shank pretty much filled the hole and I would want sheer strength as much as tensile strength incase they loosened up.

 

I went against the majority of articles/advice from google, my gear head friends, and a few here and went with Solids based on the numerous posters here on the C3 that use them, and prefer them, and not a negative post about them here that I read or any new vibrations. I would rather not work on my car, and if I can eliminate having to change these as often as I hear they fail/melt. I'm in even if there was a small vibration. I've only drove my car a couple hundred miles since installing new motor/solid mounts, but can't tell a difference at all.

 

I already bought my poly mounts from Summit months ago, so I would like to use them so long as I'm getting some benefit. Moreover, thinking about it - my headers will either be fiberglass wrapped or ceramic coated, so these things are not going to melt. My primary concern is longevity, and even more importantly, height. From what I've read in this thread, there is no change in engine height whether using solid, rubber, or poly. If that isn't true, then someone who has switched from solid to poly and noticed the engine sits higher, please comment. I cannot sacrifice hood clearance in my build.

Also, I'll be at around 425HP/450TQ, so can they take that?

If solid mounts were/are the best, why even offer them in poly? Is it a gimmick or something? I bought them because reducing vibration wherever possible is generally a good thing, as vibration (even normal) will eventually take its toll somewhere. If folks here who have experience with this are saying poly mounts do more harm than good, offer no advantages, and are essentially a solution looking for a problem, I'd like to hear it.

So in summary:

• Longevity under NORMAL driving conditions (maybe a track day or two), how long should I expect these poly mounts to last, and does 450 ft lbs tq shorten that life span?
• What are the advantages of poly over solid, and what are the tradeoffs?
• Engine height - from what I hear, poly mounts will not change engine height. Just hoping that is true.

 

Just as there's no panel of experts approving posts on this Forum, companies sell poly bushings because they can sell them for more than it takes to make them. Why do they still make and sell high-arch composite springs? To give folks on this Forum something else to complain about.

Energy Suspension makes great bushings. I've never had a problem with them. But I've never tried poly engine mounts, nor would I ever bother, based on anectdotes from this Forum.

As for changing the height, the dimensions of the stock mount were already posted. Moroso solids come in at least two sizes. Perhaps the poly mounts do as well. It's certainly worth checking.

Summit has a great return policy, BTW. Perhaps it isn't too late to change your mind.

 

I hesitate to jump in. But here is some food for thought perhaps.

If a rubber mount breaks, what are the potential consequences?

maybe just a little damage to an air filter assembly and a fan hitting the shroud, if you still have that.

Worse is it rising enough to destroy the hood? Maybe adding throttle or jamming it open by pulling on the cable creating a condition in which you can’t react quickly enough to shut it down? If it’s at the wrong time or place into a ditch, car or tree is a possibility I suppose. The remote possibility of damage to an engine block from a solid mount at that point will be mostly irrelevant.

I prefer to eliminate that ,albeit remote, possible scenario with the use of a solid at least on the drivers side.

its your vehicle, choose your poison.

 

I just stuck my finger in the water jacket between the block and the cylinders through the freeze plug today....they dont connect to the outer block except at the top and bottom. Anything that is going to happen to the wall of the block will not touch the cylinder so your worries are solved with that issue

 

Ahhh.....not really. IF the outer wall cracks, I have a coolant leak and a messed up block to replace. Not saying its going to happen.....but its not just about the cylinder wall.

Despite what seems to be a majority here on the forum, voting for solid mounts.....my google searches on many other car sites are not as positive. Here are severeal important factors in this discussion, or in my decisions:

- For those that have ripped rubber mounts, were they the cheap local car parts rubber mounts......maybe that is the problem...the mounts are junk to start with. If you look at the Corvette Central heavy duty mount, its spec'd to be same metal thickness and design as the original mounts. So that leaves the quality of the rubber used undefined. They do say that THEY, Corvette Central manufacturers these mounts....but not sure if that means in USA or ?? Many companies proclaim they make things....when all they really do is arrange it with China. I have over 18,000 miles and 8 years on these mounts...and they are still in perfect condition.

- Second......how many miles a year are you guys with solid mounts putting on your car? I put 5000-6000 miles A YEAR on my Corvette. Point is,....if you put 500 miles a year on a car ....not the same as 5000 a year over ten years. The durability, longevity and reliability for me has to be measured in the long run. An experiment has to be proven over time...not measured in the short run. Almost everything works if its never used. Now...you may say this reinforces the idea of solid mounts....versus rubber, but not necessarily. The real and actual twisting of the frame,....inducing stresses now into the self proclaimed "structural member",...the engine....has to be considered. Every time I drive out of, and into my driveway, the car is twisted from the approach angles I have to make. I can feel it in the C3 AND the C6 Corvettte...you can hear it creaking. So the frame IS twisting. One side rail is going up, i.e. one engine mount is going up....the other is not. We know that. Might not hurt anything at 500 miles a year....but different when done repeatedly....which is how stress finally gets to metal.

- I will NEVER do 5000 RPM clutch drops. I am not drag racing. I am normally shifting at 4000 at the highest, normally far less. When you have torque,...you don't need RPM. What I see is that alot of guys who vote for solid mounts are either drag racing at the track,.....or do equivalent full throttle drops on the street. I will NEVER do that. I accelerate,....but I do it smoothly. I have never spun a tire, and never will. l expect I will never use the 560 ft lbs torque Mark built into my engine. SO that does make a difference. I will use the torque developed at 3000-4000 to have fun with.

In the end....I am still very unsure what to do. Its not causing me to have high blood pressure or a heart attack.....its part of the fun. I like to be confident in my decisions, based on rational thought. This one may have to be decided with a flip of the coin!!! Both versions have pros and cons.....I just need to find the one that best suits my use of the car.

 

C’mon dude. It’s a motor mount.
The more I read through your posts and the interactions you have from the other members that come as a result of it, this emoji comes to mind.
If ever there was a more tragic waste of money, spending 18k on a 500+ ft lb rated engine that you never intend to realize, this is it.
And I feel for you, seriously )
You should use a stock rubber mount.
You should also remove the engine a second time, and sell it ( or keep it on a stand in your garage, as it’ll serve you in exactly the same way out of the car as it would in the car….as a talking point). Put a stock engine back in its place.
That way, you’ll never have instance to worry about stressing the block when you baby the thing up to 4000 rpm, and 60 MPH.
( Those are your stated shift points, and targeted cruise speeds, correct?)
I have a used, stock, aluminum, carbureted, engine with exhaust manifolds, with a tiny *** cam that should make 350-375 hp, and a similar amount of torque, AND use steel mounts. Yet,
I intend to drive the thing like I stole it occasionally. Probably shifting it at 5500 rpm when I choose, but the build intent is to capitalize on handling.
That’s why I changed out the suspension, and stiffened the frame, and that’s why there’s steel mounts. I understand the structural benefit to using that big chunk of aluminum between the frame rails as a solid block, with the perceived benefit being to help reduce chassis flex.The rest of this cars’ intended usage will be to putz around, on my way to some weekend cruise event, or to work, or to dinner, driving a total of 3-5000 miles annually as I have done with every other street car I have built over the last 20 years.
I could’ve used steel, or spent more money on a set of poly mounts to achieve the same end goal. ( seemed kinda stupid to spend more for the same end goal).

 

[QUOTE=MLM7447;1605931910]C’mon dude. It’s a motor mount.
The more I read through your posts and the interactions you have from the other members that come as a result of it, this emoji comes to mind.
If ever there was a more tragic waste of money, spending 18k on a 500+ ft lb rated engine that you never intend to realize, this is it.
And I feel for you, seriously )
You should use a stock rubber mount.
You should also remove the engine a second time, and sell it ( or keep it on a stand in your garage, as it’ll serve you in exactly the same way out of the car as it would in the car….as a talking point). Put a stock engine back in its place.
That way, you’ll never have instance to worry about stressing the block when you baby the thing up to 4000 rpm, and 60 MPH.
( Those are your stated shift points, and targeted cruise speeds, correct?)
I have a used, stock, aluminum, carbureted, engine with exhaust manifolds, with a tiny *** cam that should make 350-375 hp, and a similar amount of torque, AND use steel mounts. Yet,
I intend to drive the thing like I stole it occasionally. Probably shifting it at 5500 rpm when I choose, but the build intent is to capitalize on handling.
That’s why I changed out the suspension, and stiffened the frame, and that’s why there’s steel mounts. I understand the structural benefit to using that big chunk of aluminum between the frame rails as a solid block, with the perceived benefit being to help reduce chassis flex.The rest of this cars’ intended usage will be to putz around, on my way to some weekend cruise event, or to work, or to dinner, driving a total of 3-5000 miles annually as I have done with every other street car I have built over the last 20 years.
I could’ve used steel, or spent more money on a set of poly mounts to achieve the same end goal. ( seemed kinda stupid to spend more for the same end goal).[/QUOTE

I am a broken man now and feel so useless that I have let you down. I might sell all this stuff and buy a Ford.

 

Agonizing endlessly over such inconsequential decisions is not efficient use of time or effort. Move on. There simply are no guarantees.

You're engines may injest a couple of ducks and you end up in a non -survivable forced landing too. It's the chances we take.
Keep it all in perspective.

 

[QUOTE=CorvettePassion;1605932119] Naaah….It’ll be one more thing that you’ll agonize over….they use motor mounts too.

 

cant wait until its time to break it in and it'll be a dilemma of break in oils, rpm, and ZDDP percentages.