Motor mounts

Tiny
MONTANA40691
  • MEMBER
  • 1990 CHEVROLET ASTRO
  • 4.3L
  • V6
  • 4WD
  • MANUAL
  • 160,000 MILES
I have a 1990 Chevy Cheyenne 1500 heavy half ton with a 4.3l v6 in it and a granny four speed transmission. I pulled the engine and I am wanting to drop a 1970's 350 into it. The transmission bolts up to the engine just fine and I have everything ready to go except I need different motor mounts. Is there a conversion motor mount I can buy or is this a fabrication job?
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Saturday, April 22nd, 2017 AT 7:42 PM

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Tiny
CARADIODOC
  • EXPERT
The 1990 model was available with with a 350, so look up the mounts for that application.

Be aware the 350 is a lot heavier, so you are going to need stronger front springs, and the weight distribution and weight transfer during braking will be different, so you will need a different brake system combination valve. Modifications that affect braking and handling, especially those that involve lifted trucks and lowered cars, can easily land you in court. Lawyers and insurance investigators will find these things and use it to convince a jury that you were partly at fault for the crash when their client ran the red light, because you were less able to avoid it, and they will be right. What you have going for you is all the parts you need to build the truck the way the manufacturer would have done it are available and were part of the package.

The way I would approach this is to find a donor vehicle in a salvage yard, that came built the way you want it. Combination valves are a real low-failure item, so every truck you find will have its original one. If you are still able to buy one through the dealer, there is such a huge list of variables you need to answer to get the right one. It is easier to just get a good used one. For the springs, I would go with new ones. They get weak with age anyway. Why go through the work of pulling old weak ones off a truck when it means doing the job twice?

Besides reduced handling and steering control, the problem with weak springs is they allow the suspension geometry to change more than normal as the truck bounces up and down over bumps in the road. That results in miserable tire wear, even when the alignment computer says everything is perfect. The alignment computer only sees the wheels while they are standing still, on the hoist.

Do not forget the larger radiator.
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Saturday, April 22nd, 2017 AT 8:56 PM
Tiny
MONTANA40691
  • MEMBER
This truck has 3/4 ton suspension and differentials under a half ton body. So the stock 350 motor mounts for a 1990 Cheyenne will bolt to a 1970's 350 engine? I am going all new with this, gaskets, ball joints, tie rods the whole nine yards. This is my hunting project anyways because I can only drive a max of 50 with the transmission it has in it. I have thought of using stock 350 motor mounts but I really did not want to get everything ready to drop and the mounts not work out.
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Saturday, April 22nd, 2017 AT 9:08 PM
Tiny
CARADIODOC
  • EXPERT
GM was famous for making parts just different enough between models, engine sizes, and years, that parts did not interchange and you had to run to the dealer to buy the right stuff. The 350, on the other hand, stayed the same for many years. Thati s why I suspect the mounting points on the block are the same, and you just need the right engine mounts to match the cross member. My concern has to do with switching from a V-6 to a V-8. The rear of both engines should fall in the same place so the transfer case and mounts will be the same, and no changes are needed to the drive shafts. The new engine should only be longer in the front.

Along those lines, I do know that if this was a Dodge truck, the mounting points on the V-6 and the V-8 would indeed position both engines so the rears where in the same place. I suspect yours is the same. That means when they built the truck, it got a cross member as it came down the assembly line. It did not get one of multiple cross members based on the intended engine.

So, if the mounting points are the same on the 350's for both years, and the cross member is the same for both engine sizes, I would expect this to be a drop-in install with the engine mounts made for that engine size for that year truck. I would not be offended if you stop in at a GM dealership and verify this with an older mechanic. The people in the parts department can tell you if there is a difference in the frames between the two engines, and the mechanic should be able to tell you if the 350's from both years are the same.

If you look on the Rock Auto web site, you can look at pictures of the parts. The mount numbers are different between the two 1990's engines and a 1974 350, but I cannot tell if they will fit the same applications and were just redesigned. You might be able to spot a difference in number or location and spacing of the mounting holes.

Also, consider asking at some local salvage yards. They usually have a few people who have done all kinds of swaps at home and know what is different.
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Saturday, April 22nd, 2017 AT 10:26 PM

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