Restoring '72 LT-1 Engine

[Scott S.]

[Dick W.]

Re: Restoring '72 LT-1 Engine

Scott, I believe that the cam and lifters are still avaiable from Generous Motors. I would definitely at least look at the bearings as the metal from the cam had to go somewhere. Best case would be to tear the engine down and have it cleaned so as to remove all the metal residue. If you are going back to 350" you will have to change the crank, rods, and pistons which should be no problem to obtain. Just remember to tell your machine shop not to deck the block so as to preserve the original stampings on the pad.

[Joe L.]

Re: Restoring '72 LT-1 Engine

Dick and Scott-----

The cam and lifters are GM-discontinued. However, they can be obtained from aftermarket sources.

As Dick mentions, you will need to change crank, rods, and pistons. The crank is GM-discontinued but available in the aftermarket. The original "pink rods", GM #3973386 are also discontinued but replaced by GM #10108688. The latter are MUCH better rods than the originals and also MUCH less expensive. You can buy a complete set under GM #12495071 for a GREAT price. These rods are more than 3 times stronger than the original rods. Pistons are available from Federal-Mogul or Keith Black-Silvolite.

You will also need original heads of GM casting #3973487X or #3998916. These will need to be purchased used and re-conditioned. You will also need intake manifold GM casting #3959594 if the car does not have its original on it now. Also, you'll need exhaust manifolds GM #3932461 with AIR fittings if the car has headers installed. All of this will need to be purchased used, too.

After all is said-and-done this is going to be an expensive proposition. However, it can be done.

[Joe C.]

[Joe R.]

Re: Restoring '72 LT-1 Engine

Hi Scott:

I suggest that you consider an option part way between restoring it to original and rebuilding the present configuration. How about restoring it to *look* original on the outside, but keep some upgrades inside.

For example, I see no reason to change it back to a 350 if it is presently a 383. The cam could be replaced with an aftermarket copy of the stock flat tappet LT1 cam, or, if you prefer, a mechanical roller cam with similar lift, duration, and overlap characteristics. Roller cams are less susceptible to getting wiped due to modern low-zinc oils, but they are more expensive.

Then, have your original heads rebuilt and put them back on the car with the original intake manifold. If I understand your description, the only remaining (detectable) major component that would not be original is the carb. That is easy, but possibly expensive, to change later if you or a future owner decide to make the car fully NCRS correct.

The goal for this option would be to take advantage of the fact that the engine will be apart and restore its *external* appearance to NCRS standards while you have a convenient opportunity to do so. The externally visible items that would remain unchanged are far easier to address at some later date if desired.

Remember that NCRS judges can not look inside the engine and tell that it is a 383, and unless you deviate markedly from the characteristics of the original cam, they won't be able to detect that change either. The judges base their assessment on what they can see, hear, and feel, so don't go to extremes to make something that isn't judged be "original."

[Dick W.]

Re: Restoring '72 LT-1 Engine

Good point. Leave the stroker kit in the engine. I know of a couple of '69's that are .030 over bored and have a 454 crank in the for 468". As long as the judge can't see it and it sounds right, who cares??

[Lyndon S.]

Re: Restoring '72 LT-1 Engine

The NCRS should consider getting a cubic inch tester like NSCAR uses to make sure the car is the right cubic inch. I would bet a whole lot of cars would fail that test!

Kelly

[Joe L.]

Re: Restoring '72 LT-1 Engine

Joe-----

Theoretically, it would be possible to externally discern a 383 from a 350. The 383 uses a counterweighted balancer and, if someone were to inspect the rear of the balancer carefully, that fact could be discerned. Even with the balancer mounted on the engine, this can be seen, especially if the counterweighted area were in the right position. Of course, one could get around this by neutral balancing the crankshaft with mallory metal and, then, using a 350 balancer.

Nevertheless, returning the engine to externally original configuration would be a fine way to go and the above-referenced "nuance" of non-original configuration would not deter me. The only "downside" to all of this is losing those AFR heads. The AFR heads are probably the best small block heads out there.

[Joe R.]

Re: Restoring '72 LT-1 Engine

Hi Dick:

I subscribe to the view that if it isn't judged or it can't be detected during judging, I am free to use non-original parts and configurations.

One tiny example is that the screw that holds my ballast resistor on the firewall is a machine screw that has the exact same pan head as the original self tapping screw that had stripped out the fiberglass. It goes into a nut plate that is covered by insulation on the inside of the firewall. There is no way to detect this change without removing the screw or the insulation, which of course is not allowed during judging. That's just fine with me.

Oh, and by the way, my camshaft and valve train are not "correct" either, but I was careful selecting the components and you can't tell they aren't original from listening to the engine. That doesn't bother me either.

Now, I know that there are others in this hobby who want EVERYTHING to be original, whether it is detectable or not. I even recall seeing a posting on this board where a member opined that he would never bore an engine .030 because that would destroy its originality. So, I guess each of us has to draw the line where we are comfortable.

The main thing that I think some people who are new to the hobby may not understand is how the judging process is based only on what the judges can see, feel, and hear. I think it would be a shame for Scott to change his engine back to a 350 just for "NCRS judging." I'd prefer the 383 configuration any day.

[Joe R.]

Re: Restoring '72 LT-1 Engine

Hi Joe:

I had the impression that the aftermarket 383 kits for converting a 350 were fully internally balanced. I've seen several kits that made no mention of having to use a special balancer.

Is it possible that GM 383 crate motors use the special balancer but that some (or all) of the aftermarket kits don't?

I'm interested in this topic because I'm thinking about building a 350 or 383 that *looks* exactly like an L79. A small detail like the balancer problem you mention would probably bother me.

[Joe L.]

Re: Restoring '72 LT-1 Engine

Joe-----

The GM crate 383 requires a counterweighted balancer. This balancer is specifically applicable to use with the the GM 383 crankshaft which has a 1 piece rear seal configuration. Other 383's using a GM 400 cid crank with 2 piece seal require a different counterweighted balancer.

Some aftermarket 383 cranks may very well be set-up for use with a 350 balancer (i.e. non-counterweighted). However, those will not be the lower priced ones. I'll bet about 99.9% of the folks who want to build a 383 would not be concerned, at all, about using a counterweighted balancer and would not pay extra for a crank just so they could use a non-counterweighted balancer.

[Joe R.]

Re: Restoring '72 LT-1 Engine

Thanks Joe. This gives me an additional consideration to research and evaluate. Right now the project is just in the planning stages, although I have accumulated most of the externally visible L79 parts (fortunately, not the balancer, though). The conservative choice is to simply use a 350 displacement, but it's tempting to go for the 383.

[Scott S.]

Re: Restoring '72 LT-1 Engine

Hi Guys,

First of all, thank you for all your responses.

A bit more detail about the engine:

Intake is original
I have the factory heads in the basement.
Carb is a 780 Holley 4150, not original, but close enough for now.
Cam is an Edelbrock 7102 Hydraulic, flat-tappet

When I said it's more engine than I need I just mean that it makes a lot more power than I need. (It makes 475hp at the crank) Don't get me wrong, it's really, really nice...and fun too! but I'd be willing to sacrifice a bit of power to make it at least look more correct. To the untrained eye only the headers and the heads would give it away. The TI is installed so nicely that unless you really knew your stuff you'd think it was factory.

I wish I knew why the cam went bad. The AFR's have dual valve springs and the Edelbrock website specifically says to not use dual valve springs with this particular cam. Not sure if that was a contributing factor or not since the motor has been together for a few years now without incident.

What I want out of the car:

I bought this car because it was a reasonably well documented 2-owner LT-1 that the 2nd owner had since '79. It had been driven a lot, however he had a book detailing the complete restoration of the frame, suspension, powertrain, interior, etc. Basically everything but the paint was mint. The fact that he had built the engine to be an absolute beast was not relevant to me. What I want is a nice, driveable, cruiser. I have no racing aspirations, nor do I feel the need to be the fastest car on the street. My intention is that anytime I have to do anything to the car it will be done with the intention of maintaining or improving upon it's originality.

Leaving the 383 internals and replacing the externally identifiable parts is interesting and something I hadn't considered. Particularly since I am told by people that know the car that the PO put a lot of high-quality parts into it. Could I put the original heads back on without swapping out the crank, pistons, etc and not suck the life out of the engine. I assume they wouldn't breathe as well as the AFR's but like I said I am ok with some loss of power. Swapping the carb, ignition and exhaust manifolds in later should be relatively simple if I decide to get really serious about originality. For that matter, would it really be a big deal to change the heads later?

Again, thanks for the advice guys!

[Joe C.]

[Duke W.]

Re: Restoring '72 LT-1 Engine

My first advice is "always follow the mfgs recommendation. Likely the valve springs killed the lobe because they are way too stiff. Too stiff valve springs will kill lobes on road engines that spend a lot of time idling and at low revs because the low relative velocity at the lifter-lobe interface will not generate enough hydrodynamic film pressure to support the load at low revs.

If you want original appearance back, have the OE heads pocket ported/port matched with multiangle valve seats, and buy a Speed Pro repro LT-1 (CS-1145R)cam with their VS677 springs that are equivalent to the OE 3911068 and install the cam with 4-6 degrees retard from the design indexing.

You won't be giving up much top end power relative to the AFR heads, but it will probably have more low end torque and make usable power to about 6500.

Or just keep the AFR heads. You didn't say what model or provide any physical or flow data, but assuming they have not been massaged, they likely won't work much better than massaged OE heads. You might loose a little compression because the AFR heads may have smaller chambers than the OE 76 cc chambers. With the LT-1 cam retarded as above this configuration will have no problem running 10.5 CR on premium unleaded. If you swap the heads take all the arequisite measurements so you can calculate and manage the final compression ratio via head gasket selection. (Do some research in the archives.)

Keep the bottom end as is. As was stated, no one will know it's a stroker, but the original heads and original cam will get you back to OE appearance and personality with OE SB valve train reliability, which means bulletproof! The LT-1 cam provides good low end torque for the specific power output, and with a long stroke configuration and more compression, bottom end torque will be quite strong relative to the original OE configuration.

Duke