1970 LT1 Engine rebuild question

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
The 70z I just got came with the numbers matching Lt1 out of the car. One of the previous owners put another built 350 in the car. I don't know any of the spec of that engine other than a claim it cost $7500. It has around 2k miles on it since it was installed and seems to pull nice and hard.

I want to rebuild the Numbers matching engine and someday put back in the car. However My engine rebuilding knowledge is lacking and I am not sure how much of the original engine components I should use in the rebuild. Does one try and get as much of the OEM parts that you can in it? I don't think I will every have the funds to do a rotisserie resto on the car so I am not sure how far I should go with regards to building the #match engine.

I have been going through all the parts that came with the engine and it's a mix of OEM and replacement parts, some are on the new engine.

What I have: Bold parts are the ones i believe to be correct and OEM.

Block - Date D280 3970010 Deck is a bad restamp but Vin on bellhousing is correct.
Heads- 3991492 - Date on one is from 77 and the other has a W243 date whatever that means.
Crank- 2690
Harmonic Balancer - 3942538
Water Pump- Installed on new engine- can only see 312 and a 0509 stamp.
Fuel Pump- Big AC -1O6U40987
Oil Pump- 43-43 8/96- metal tag 41/43 not sure what that means.
Cam? - Marked -TP178 1211 M1 DC 88
Pistons- TRW 2304 -.060
Rods - Appear to be the pink ones with 0 marks.
Distributor - 1112019 0C18
Intake - 3972110 (This is on the new engine)
Both exhaust Manifolds are correct and also on the new engine.
Clutch- Just has a white A a black W or M and a yellow C?
M22 is #match
12 bolt appears to also be correct.

IMG_4685 (1).jpg IMG_4686.jpg IMG_4699.JPG IMG_4755.JPG IMG_4823.JPG IMG_4701.JPG IMG_4699.JPG IMG_4795.JPG IMG_4797.JPG
 

ddc456

Veteran Member
Oct 11, 2010
502
Oregon
Here is what i would do if i was you, (speaking from experience on these 70 Z's since i've owned 3 of them) run the motor that is in it until you gather the remainder of the original parts, then rebuild the motor. Just the fact the you have the original LT-1 is a nice piece in it's self and the parts you have with it (nice). Seeing that you have 492 heads, they must of had a warranty replacement at some point as those are warranty replacement heads for that time period. The 70 Z i have now has one of the original 186 heads and one which is a 492, but then again i have a CE block (service replacement) original engine dropped a value and went bang. In those days they only replaced the parts that were having issues, when it would have probably been easier to just drop in a new motor.
 

70lt1z28

Veteran Member
Gold Member
Oct 3, 1999
4,726
Beavercreek, Ohio, USA
I am in the middle of a 70 LT-1 rebuild now, so I have some fresh knowledge of this.
The issue I can see for you is the 60 over pistons that are in it now. If any of the bores are scratched-scored or excessively worn you may be in trouble. There isn't anything past 60 over you can do. The exception is to go to sleeves. This isn't the end of the world but if you have to do more than one or two it gets real iffy. I have heard of folks doing all 8 to save a numbers matching block, but in my opinion that engine is more a show piece than a real viable engine. If you were building a trailer queen that only has to idle on and off the trailer, it may be OK.
If the bores are good, you may only need to scuff the bores on a hone and install new rings. Have the block sonic checked to tell you the cylinder wall thickness.
The rest of it depends on where you want to go with the engine. The block deck stamps are kind of minor issue as like you said the VIN is good. If you want the original 186 heads you may want to try and get those soon. Good ones (not cracked) are getting scarce and the 69 Z guys (they used them there too) are snatching them up even though the dates may not be perfect. They only made them in 69 and 70.

If you really want to get nutty on numbers (the pit that I fell into) than your fuel pump will need replaced (number is 40726 and MAYBE 40725 for an early car, which yours isn't and it is hard to find). Water pump is 3953692 with a date of C or D. How about the starter? Coil (should be a 270)? Carb (3972121 if 49 state, different for Cali). Lots and lots of little things that will really add up (price a throttle cable bracket sometime!).

I'm of the opinion (and many may disagree with me) that some of the engine internals are less important. Your cam I would not risk. Crane makes a very good blueprint cam for the '178/'182 for about 270.00. There is a NOS GM one eBay right now for 500. I think you may want to take a close look at the crank. My crank was cracked so I went with a SCAT forged one with their rods. Chevy hasn't made a 2 piece rear main seal forged crank in probably twenty years so either you are buying a questionable used one or paying real serious money for a NOS one. Even with an NOS one it could be bent and need ground. You just don't know. I would rather have piece of mind in the reciprocating assembly than the right numbers, if it comes down to it. Your rods would need magnafluxed to check for cracks but if that's OK than resize the big ends and install new bolts and you'll be good. You don't want it to go blam with the original block.
 

restore-z28

Veteran Member
Jul 4, 2010
468
Vancouver, BC
I would say leave the aftermarket engine in it for now, drive and enjoy the car. Start collecting the parts that are not correct. Appears you have the original dist, intake, block and exhaust manifolds. Being 60 over and without knowing if there are any other issues I would just take it to a local machine shop, get it checked for cracks, check the cylinders to see if you might need a sleeve, and if all good wrap it up and store it away. Agree the 186 heads are getting harder to find, the good news is each year you have the Portland Swap meet close to where you live and might get lucky. For me the internals are the last things I worry about in terms of being factory perfect....as long as the engine is built reliable and the outside components are OE then you are pretty much there.
 




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