BBC swap in my k20

eseaton1993 · Jan 4, 2026

Hi guys in the middle of swapping my 86 k20 from a small block 383ci to a big block 540ci with a tremec 40/50, 5 speed. Having some issues with my motor mounts lining up wondering if anyone has had issues and what the solution is. Ordered brand new small block motor mounts at the frame because the 454 mounts are too long and the bolt holes do not lineup. But with the small block Mounts, the mounts don’t line up with the engine brackets. Ordered engine brackets from LMC was told they would work being that big block brackets are extremely hard to find, looking back at the part numbers they are for a small block. All the reviews say these drop right in but it’s not showing to be the case for me. Starting to think that the information I received was incorrect on the engine brackets. Hope somebody has seen this before thanks.

xm20k · Jan 4, 2026

There should be 2 sets of holes in the frame perches that the lower mounts bolt to yours are in the lower holes and covering the top set which is where yours need to be.

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Unless maybe the 80s trucks changed at some point and only have one set of holes. Then the perches will need to be drilled or swapped.

xm20k · Jan 4, 2026

Just realized K series so the perches are different you might have to bolt the mounts together on the engine and set it in the frame and square it up and drill new holes higher up.

Someone might be able to measure a bracket and give you the center to center for the new holes.

eseaton1993 · Jan 4, 2026

Ok hopefully someone has a measurement thanks

bucket · Jan 4, 2026

Where did the mounts and brackets go from your 383? Just use those, in the same frame holes that were in use before.

Right now it sounds like you're mixing small block stuff with big block stuff and it's all aftermarket too. That's a recipe for disaster.

xm20k · Jan 5, 2026

bucket said:
Where did the mounts and brackets go from your 383? Just use those, in the same frame holes that were in use before.

Right now it sounds like you're mixing small block stuff with big block stuff and it's all aftermarket too. That's a recipe for disaster.

He is using small block mounts and brackets, but the perches only have the lower holes for a small block and not the higher set.

eseaton1993 · Jan 5, 2026

These are replacement sbc engine brackets and sbc frame mounts. The brackets were sold with the 383 at the buyers request. I’m measured these engine brackets and they are a little over 3.5” we’re the bbc ones are 3” I believe the engine bracket lies my problem

xm20k · Jan 5, 2026

I've never ran into this but apparently the upper has 2 different sizes. I have had to move the mounts to the other set of holes on a 2-wheel drive but that may have been because of the solid mounts used vs the stock style I removed now that I think about it.

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bucket · Jan 5, 2026

xm20k said:
He is using small block mounts and brackets, but the perches only have the lower holes for a small block and not the higher set.

You do not need another set of holes for a big block. A big block literally bolts right in, using ALL the small block stuff.

bucket · Jan 5, 2026

eseaton1993 said:
These are replacement sbc engine brackets and sbc frame mounts. The brackets were sold with the 383 at the buyers request. I’m measured these engine brackets and they are a little over 3.5” we’re the bbc ones are 3” I believe the engine bracket lies my problem

The big block brackets don't matter. They are designed for different isolators and different mounting holes. The replacement brackets may or may not be correct for the application... aftermarket, remember.

bucket · Jan 5, 2026

xm20k said:
I've never ran into this but apparently the upper has 2 different sizes. I have had to move the mounts to the other set of holes on a 2-wheel drive but that may have been because of the solid mounts used vs the stock style I removed now that I think about it.

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What were those removed from? I know there were some cars that used side-specific brackets, but I don't recall that being the case with squarebody trucks.

bucket · Jan 5, 2026

Also, it's worth noting that it can sometimes be a real bear getting the engine down in place with new isolators. You might get one side in, then the other side is off by 1/2" or so. Even when it's the same engine and same brackets going back in.

Turbo4whl · Jan 5, 2026

There were some years GM did not offer BB in the lighter 4 wheel drives. Putting the the BB in my 2 wheel drive. I used the SB engine brackets and the BB mounts. The different holes for the mounts were there. In the 2 wheel drive, the BB needs to sit higher for the oil pan to clear the cross member.

Like David said, set it in, mark the holes. Pull it out and drill the holes. Yes more work. I believe you need the BB mounts. The BB is taller, they should end up with the crank and bellhousing at the correct height.

The SB and BB engine brackets are almost the same in a square. The BB engine mounts were stamped with thicker steel. The correct SB brackets (for a square) should be fine.

malibuman402 · Jan 5, 2026

i did a small block to big block swap and used the small block mounts, fits good. just bolted the small block mounts to the big block and put the motor in.

xm20k · Jan 5, 2026

bucket said:
What were those removed from? I know there were some cars that used side-specific brackets, but I don't recall that being the case with squarebody trucks.

Was a post on another forum the short one was from a G body and taller one was from a Camaro/Firebird he tried to drop the Camaro engine into the g body and ended up in the same position as the OP he then discovered that the ones on the original engine were shorter.

Personally, I've never ran into this issue, but my limited big block swaps were performance engines, and they got solid engine mounts, and when it comes to small block swaps it was either a 6 cylinder being swapped out or a blown-up small block swapped with another small block, and I never ran into the upper mount being different. Just got lucky, I guess.

As for squares I have heard of people having to move the lower mounts to the upper holes and on mine, I had to do this but was probably due to the solid mounts I installed. Never realized there are apparently 2 sizes of uppers.

BBC swap in my k20

eseaton1993

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