[PontiacHO]

Wishful thinking here but thought I’d get set straight….
I can find plenty of 400 blocks. Could I swap my 350 crankshaft into a 400 block as well as using my 350 rods of course with 400 pistons? My 350 runs well with recent new top end rebuild (camshaft, lifters, valve springs (066), valves, seats, springs, and .020 resurfacing) but the cylinder walls looked glazed (some blowby) and of course rear main seal leaking. If this is possible I’d use the 350 rotating assembly, new cam-lifters, and swap my refreshed heads onto the 400. I realize these are small valve heads but for my street cruiser I’d never know the difference.

[steve25]

In terms of having to rebalance the crank I would have to look up the weight difference between 350 and 400 pistons as it may be close enough in grams difference to be wash.

What I mean is 350 pistons while being smaller in OD then 400 pistons have smaller valve notches so the difference in weight might be minimal enough to not need a balance job.

Some engine builders intentionally over balance or over balance some by 4 to 6 grams.

[johnta1]

What year is the 350 crank?

[John Milner]

I rebuilt a 400 engine back around 2003 and used a 1974 350 crank in a 400. Funds were tight at the time and I did not balance the engine. I simply honed the block with a 3 stone hone, cleaned everything up good, installed new rings and bearings on the factory pistons/rods, new cam/lifters and a 350 crank since my 400 crank was not in good shape. I don't know how many miles the engine has now but it is still running just fine. I couldn't tell you if it is out of balance but it runs smooth and has for 23 years so far. However, I would say that it would be recommended to have the rotating assembly balanced with the 350 crank.

[PontiacHO]

Originally Posted by johnta1

What year is the 350 crank?

1968

[PontiacHO]

Originally Posted by steve25

In terms of having to rebalance the crank I would have to look up the weight difference between 350 and 400 pistons as it may be close enough in grams difference to be wash.

What I mean is 350 pistons while being smaller in OD then 400 pistons have smaller valve notches so the difference in weight might be minimal enough to not need a balance job.

Some engine builders intentionally over balance or over balance some by 4 to 6 grams.

Thanks.
I’d even be good with a 389 block but being an older era engine there could be more differences than a 400.
It’s as much work to pull the 350 and have it machined with new pistons, rings, brgs, etc. as it would be to transfer the rotating pieces and heads on a fresh 389 or 400 block. Again, wishful thinking but not going to do the same work for possibly the same torque and fun.

[PontiacHO]

Originally Posted by John Milner

I rebuilt a 400 engine back around 2003 and used a 1974 350 crank in a 400. Funds were tight at the time and I did not balance the engine. I simply honed the block with a 3 stone hone, cleaned everything up good, installed new rings and bearings on the factory pistons/rods, new cam/lifters and a 350 crank since my 400 crank was not in good shape. I don't know how many miles the engine has now but it is still running just fine. I couldn't tell you if it is out of balance but it runs smooth and has for 23 years so far. However, I would say that it would be recommended to have the rotating assembly balanced with the 350 crank.

That’s a positive success story. Thanks.

[hurryinhoosier62]

Why not? ALL 350Ps and 400s have the same stroke, the same connecting rod c-to-c length; the only difference is the diameter and weight of the pistons.

[johnta1]

The later 350 like 400, used a hub instead of damper. (1976 and newer?)
Might have some balance with those?


68 would be a good one.

[PontiacHO]

Originally Posted by hurryinhoosier62

Why not? ALL 350Ps and 400s have the same stroke, the same connecting rod c-to-c length; the only difference is the diameter and weight of the pistons.

It’s possible that it’s that simple….but I’m wonder about the possibility of different counterweights on 350 vs 400 cranks?

Was hoping someone here would know.

[hurryinhoosier62]

Originally Posted by PontiacHO

It’s possible that it’s that simple….but I’m wonder about the possibility of different counterweights on 350 vs 400 cranks?

Was hoping someone here would know.

Earlier 350P cranks had counterweights comparable to the counterweights on 389 and 400 cranks. I think you are worrying needlessly. In order to fit a 428 crank into my ‘64 389 the ONLY counterweight to block interference problem I had was on the right front of the block.

[steve25]

For what it’s worth here’s what TRW showed for its forged piston line for both the 350 and the 400.

The 350 pistons are heavier than the 400 pistons yet they have a lighter wrist pin .

My take on this due to never having had a stock 350 piston in my hand ( can someone compare?) is that the 350 pistons are like all the earlier ones used the 326 389 421 in that they are big and heavy, NOT like the Cam ground pistons used in the 400s 428s and the 455.

Note that TRW states a + or - 10 gram possible difference from a factory piston.


Here’s other interesting info from the TRW catalog.
The standard pre 1975 replacement 400 piston came fitted with a 250 gram wrist pin.

The RAII and RA4 pistons which are the same forging as the std 400 piston came with a 194 gram pin.

The later 400 piston with the -10.5 CC chamfer came with the 250 pin.

All the 455 pistons had the 194 gram pin.

[PontiacHO]

Originally Posted by steve25

For what it’s worth here’s what TRW showed for its forged piston line for both the 350 and the 400.

The 350 pistons are heavier than the 400 pistons yet they have a lighter wrist pin .

My take on this due to never having had a stock 350 piston in my hand ( can someone compare?) is that the 350 pistons are like all the earlier ones used the 326 389 421 in that they are big and heavy, NOT like the Cam ground pistons used in the 400s 428s and the 455.

Note that TRW states a + or - 10 gram possible difference from a factory piston.


Here’s other interesting info from the TRW catalog.
The standard pre 1975 replacement 400 piston came fitted with a 250 gram wrist pin.

The RAII and RA4 pistons came with a 194 gram pin.

The later 400 piston with the -10.5 CC chamfer came with the 250 pin.

All the 455 pistons had the 194 gram pin.

Thanks Steve. I reckon it’s safe to assume that using a 350 crank in a 400 block is certainly possible and that there’s been years of rebuilt Pontiac engines getting different weight replacement pistons without rebalancing and most likely turned out fine for street use. I’ve always thought that very few busy machine shops took a lot of pains “balancing” a rotating assembly just for street use. Yes they charged for weighing the pistons but if they were all similar weight that’s all the “balancing” they did. I’ve torn down “rebuilt” engines back in the day that even had different brands of pistons in them. They ran fine but needed refreshed. Thanks again for posting this info.

[steve25]

Does anyone have a stock 350 piston out of a motor to just post a picture couple of for comparison to a 400?

Thanks in advance.

[J.C.you]

They made the balance difference in pin diameters between cids. That’s a old racers trick putting the lighter 455 pins in a smaller cubic inch motor.

[PontiacHO]

I think (May be wrong) the pin diameters are the same for both the 350 and 400. The rods are the same.

[steve25]

It’s the ID of the pins that are different.
All stock Pontiac V8 wrist pins have been the same OD since day one of the V8 in 1955 I am pretty sure.

Not sure about the 301 and that short lived 265 motor though.

[PontiacHO]

I think (May be wrong) the pin diameters are the same for both the 350 and 400. The rods are the same.

[Half-Inch Stud]

I think 68 350 pistons have steel in the skirts.

[J.C.you]

Originally Posted by PontiacHO

I think (May be wrong) the pin diameters are the same for both the 350 and 400. The rods are the same.

Sorry I meant inside diameter thickness