I thought I'd encapuslate a few minor observations about putting an older style (2-piece rear seal) block in place of the original one-piece rear main engine. In case anyone cares to reference it later.

Yes, they're supposed to be interchangable, and they basically are but you'll have a few heart attacks along the way. Swap in question was yanking an L-98 350 out of a 1987 GTA and dropping in a 1975 vintage 400. However, all this applies to very common older 2-piece rear main 350s as well.

I won't talk about the heads and intake because they're so easy to swap around mix-n-match style. Ditto the exhaust. THere's a ZILLION different ways you can go and I can't possibly cover them here. If you care, this engine had a Miniram on top of the original L-98 350 as well as a set of Hooker 1 3/4" headers and 2 1/2" y-pipe. Both intake and exhaust were swapped over in tact to the new 400. Headers are pretty common, the miniram is not so common but it's a bolt-on in place of the TPI so it's not really a factor here.

Starting from the bottom and moving up the engine:

Oil Pan. I was STUNNED that the original 1975 350/400 truck pan fit both the chassis and the 2 1/2" cross-under exhaust pipe like it was built for it! I'm pretty sure I could have swapped the pan off the one-piece rear main 350 engine onto the older 400. You would think it would be impossible for some reason but it looked like a bolt-on to me and I compared the 2 side-by-side for at least 20 minutes.

Oil dipstick. On the L-98 it's on the pass. side and goes down to the upper lip of the oil pan. On the older block it presses in to the deck surface on the driver's side of the engine. If you use headers the older style dipstock will clear everything jsut fine. If you use stock manifolds, however, it defintiely will NOT clear no matter what you do. GM went from the block deck mounted driver's side dipstick to other configurations in about 1977, BTW.

Starter. The starter from the 350 would not bolt to the 400 but that's becuase the 400 uses ONLY the "staggered" bolt pattern. But this is a quirk of the 400- 350s would not have this problem. All (or almost all) 350s come tapped for dual starter bolt patterns. So the original L-98 350 starter would bolt to just about any old 350 you're likely to try to use it on. We used a mini starter with appropriate bolt pattern on the 400.

Flexplate. The one piece rear main L-98s use a unique 153 tooth flexplate (this probably applies to the 305 as well). Don't try to swap it onto an older block EVER. There are 2 reasons for this: Almost all old style V8s use a larger 168 tooth flexplate. Bolt a 153 tooth flexplate on them and the starter won't engage! Second reason, the newer style 153 tooth flexplates are part of the EXTERNAL balancing of the engine. I know everyone told you that only 400s were externally balanced from the factory but it's not true. Once piece rear main engines appear to have external balancing back on the flexplate. (I can email pictures if you don't beleive me). Don't reuse the one pieece rear main flexplate on an older engine of ANY description. It might bolt on, but it's not right.

Stock Torque converter. It uses the smaller diameter bolt pattern that goes with the smaller diameter 153 tooth flexplate on the one piece rear main engine. Most aftermarket flexplates (either 153 or 168 tooth) are drilled with dual bolt patterns so this should not be a problem but you DAMNED WELL BETTER CHECK THE FLEXPLATE YOU INTEND TO USE AGAINST THE TORQUE CONVERTER WHILE YOU HAVE THE ENGINE OUT! Better safe that sorry. BTW- 168 tooth flexplates fit in the bell housing of the trans just fine without interference.

Odd quirk worth noting: There may be quite a bit of distance between the flexplate and torque converter when you go to bolt the two together- like 3/8"!!! I was freaking but several people told me to calm down and just bolt 'em together and forget it. They were right. The converter pops forward that distance as you tighten the bolts and no problems to report at all.

Harmonic balancer: The L-98 uses a small diameter 6 3/4" balancer that appears to be neutral balanced. The L-98s EXternal balancing seems to be limited to the afore mentioned weirdo flexplate on the back of the motor. THe L-98 balancer also has the EXACT same bolt pattern for crank pulleys as older balancers do. So putting the original pulleys on an older 350/400 with their respective unique balacers is no problem at all. Or yanking the original balacer from your L-98 and putting it on an older 350 should pose no problems either.

The 400 I dropped in had a larger 8" balacer on it and caused no clearance problems with anything on the front of the engine. We even ran the stock hard oil cooler lines under the balancer (stock routing) with more than 1/4" clearance with no modification.

Knock Sensor. Took it out of the L-98 and screwded it into the lower water jacket drain plug on the 400. Same thread, same location, spun right in like it was made for it. Not a problem.

Oil Pressure sender. The one down by the oil filter, I mean. THe older block had a plug in that set of threads so we unscrewed the plug and swapped the sender over. Not a problem. Ditto the top oil pressure sender that screws in the top of the block up by the distributor, BTW.

Odds-n-ends. There were several bolt holes that were drilled and tapped on the L-98 block on the front, driver's side, down low. THe older 400 had none of these bolt holes dirlled or tapped. 2 were for a power steering pump bracket that can easily be discarded and not used since the power steering pump is held in place by 3 other locating points. And one is for the negative battery cable ground. You'll have to find another bolt somewhere to run the cable to since this isn't tapped out on the older blocks often times.

Everything else bolted right on and fit beautiful- water pump, all the front end accessories (even the smog pump!) and almost all the odds and ends, give or take a wiring loom clamp or two.

So nothing to fear but fear itself if you're considering a transplant in the future!



[This message has been edited by Damon (edited August 24, 2001).]

 

Thanks for the excellent info Damon.

 

You're the man!

 

i have a question about the knock sensor? when changing from a 305 to a 350, you are supposed to change the knock sensor because the bore of the engines are different, and the fregquency that they detonate is different also. what about the big bore of the 400? will that make any difference?

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Wide Open Till You See God...Then Brake

 

I for one dont believe the knock sensors know the difference. If you follow the myth trail, people will tell you that a gear drive will trigger false knock due to the harmonics created by the drive, but the same drive will set off both a 305 and 350 knock sensor according to the same people. Im still trying to figure that one out.

I have a car with a 'quiet' gear drive and I dont get false knock so I think sometimes some of the stuff you hear comes from people a little off their rocker.

 

OFFICIALLY, you're supposed to have the knock sensor CIRCUITRY recalibrated when switching from 305 to 350 (or whatever). They look for a certain "frequency" of vibration that corresponds to spark knock. Sorta like when you strike a bell it rings at a certian frequency. The knock sensor circuit looks for only that frequency. So, a siamesed-bore 400 block probably "rings" at a different frequency than a 305 or a 350. The difference is not in the sensor so much as in the programming of the ECM or knock module. 3rd Gens have the programming built into the ECM. 4th Gen LT-1 motors have a separate knock module.

However, I agree with Madmax to a point- the differences are apparently very minor. The stock L-98 knock sensor and circuitry appears to work jsut fine with the very different 400 block in this application. We even have roller rockers on the motor which are supposed to trigger false knock pretty bad, but we have seen no indication of this so far.

 

a 1 piece rear seal oil pan WILL NOT fit an older 2 piece rear seal block. the difference is where the oil pan meets the rear main cap. there is almost a 1/2 inch difference. jegs once sent me the wrong oil pan and when i went to put it on there was a 1/2 inch gap at the rear oil pan seal. and all flywheels are balanced like a tire so the weight you are seeing may be what they used to balance the flywheel. that doesnt mean you have an externaly balanced engine.

 

i used my 153 tooth flywheel on my mid 70's block. runs nice and smooth.

i had that gap with the TC and flywheel also. i just didnt tighten one bolt at a time though. i went ahead and tightened them all in incriments to avoid damaging the splines or whatever the TC slides onto (dont know much about trannys ) may not have been necessary but i didnt want to chance it either way.

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former 305 crossfire fuel injection
350 cubes
hooker headers
750 vac secondary
700r4 with shift kit
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"no man with a good car needs to be justified."

 

B92Z- I stand corrected. I never actually tried to put the one-piece pan on the early block since the pan that was on that motor fit perfectly in the engine bay.

83 Crossfire- the late model 153 tooth flywheel may have run smooth but believe me, if you didn't get it balaced as part of the rotating assy. your bottom end WAS out of balance. It won't be by a lot. Certainly the imbalace is not anywhere near as severe as with a flexplate used for a 383 stroker or 400 small block, but it's very very real. I have the flexplate and it's definitely NOT a balanced unit like earlier 153 tooth flexplates were. I'd say it off by about 3 oz. It's obvious where they put the weight as it looks like a set of bird wings tack welded round one edge of the flexplate.