Need assistance with factory engine specs.

[Warren F.]

One of my '71 LS6 Corvettes has a camshaft with a couple of rounded lobes. This was probably caused by the previous owner using an oil with little if any of the ZDDP additive (he was using Pennzoil 10-30 wt). I have since acquired 3 NOS GM 3904362 camshafts and a couple sets of NOS GM 5232695 lifters. When the original camshaft was removed, it was sent along with one of my NOS cams to Competition Cams to have both evaluated. To my surprise the camshaft from the car did not measure near the specifications of a LS6 profile. The NOS unit came close to most specs that has been written various publications.

Now I am wondering if my valve springs are correct. Can anyone provide the spring rates for those? The parts book seems to show '66 to '71 427, 454 together, with only the HD 427 with a different part number. I have a set of NOS GM valvesprings part# 3970627, that I would use if they are needed, however I am trying to save what NOS parts I have in case they are needed for my other LS6.

[Lyndon S.]

Re: Need assistance with factory engine specs.

I have a chart showing valve-spring specifications as installed height. For 71 LS-6 the height is 1.88 Load (Lb) 69-81 Min Load 66

[Duke W.]

Re: Need assistance with factory engine specs.

Check last year's archives for a thorough discussion of BB valve springs along with photos of the various production and HD springs. Use 3970627 as the key word. It is the best and consists of a dual spring with built in oil seal/retainer that was first used in '71 on the LS-6, and I recommend them for all BB rebuilds going back to '65.

They are still available from GM and the GMPP catalog lists a seat force of 105 pounds at 1.88" installed height and a rate of 267 lbs/in, but I believe this is an error and the actual rate is more on the order of 325 lbs/in. It would be easy to take one to an engine shop with a valve spring tester and have them checked at 1.88" and 1.38" and calculate the rate.

Sealed Power and Clevite offer 627 clones that you can probably buy at lower cost and are most likey manufacture the part sold by GM. The prior production spring was single with a damper and prone to failure.

Duke

[Duke W.]

Re: Need assistance with factory engine specs.

That sounds like just one spring of the dual spring assemblies used on LS-6. It's less than the 76-84 pound seat load of the production 3911068 SB spring, which clearly could not be the case given the BB's larger, heavier valves and greater lift.

Duke

[Clem Z.]

Re: Need assistance with factory engine specs.

3970627 springs are the correct springs for the BBC engines with stock camshaft. the installed height is 105#@ 1.880" these springs are 450# per inch

[Joe L.]

Re: Need assistance with factory engine specs.

Warren------

The GM #3970627 springs are the springs originally installed for your application. Sort of, anyway. The deal is that GM #3970627 is not a valve spring; it's a valve spring KIT. It includes a dual valve spring + a GM #3964264 retainer with seal. Kits are not used in PRODUCTION, so I'm sure the valve spring that was supplied in the SERVICE-only GM #3970627 KIT had a different part number.

In any event, this is an academic point. The valve springs that were originally installed on the 1971 LS-6 were the same ones that are supplied in the 3970627 kit. Plus, with the 3970627 kit you also get the same retainer and seal as was originally installed on your engine.

[Duke W.]

Re: Need assistance with factory engine specs.

450 lb/in!!! Where is that listed? The highest rate listed for the various BB springs in the GMPP catalog is 406 lb/in, and the notes call this a "competition spring".

I can't imagine such a stiff spring is required for the production cams as they have fairly mild dynamics. I don't believe the quoted 267 lb/in in the GMPP catalog for the 627. I could believe 367, but my hunch is they are closer to 325.

Duke

[Clem Z.]

Re: Need assistance with factory engine specs.

chevrolet power manual 7th edition. 2/1994. that would be 330# over the nose at .500 lift with 105# at 1.880

[Warren F.]

Re: Need assistance with factory engine specs.

Joe,

That is exactly what I have the valve spring kit. However there is a Note 1, that says "When using 3970627 it is necessary to discard the valve spring cap and valve stem oil seal which were used with 1st design spring", in the P&A 30B book.

The GM boxes have 1823M4 stamped in bold print above the blue Genuine General Motors Parts GM stripe. Is this some sort of important code? Just below the bold 1#3970627 is a very small 1217.

[Warren F.]

Re: Need assistance with factory engine specs.

Duke,

Thank you for information. I will check the archives. My car is at Precision Motive in Hayward, CA. There is a machinist there that I will give this information to. I have the valve spring kit that Joe alludes to further down in the post, I just don't know whether these NOS kits contain the valve spring cap and valve stem oil seal that should be discarded.

[Duke W.]

Re: Need assistance with factory engine specs.

The note refers to using the 627 "kit" on a pre-'71 engine with the (weak) first design spring, retainer, and seal. Since the 627 kit includes a specific retainer/oil seal, the first design parts are not used and should be "discarded". On a LS-6 that was originally built with the same parts as included in the 627 kit, use all the new parts from the kit.

There's not big deal about saving these "NOS" parts since the exact same kit is still available from GM and the aftermarket.

Ask the shop to test spring force of at least one on a spring tester at 1.38" and 1.88" to verify the rate that Clem mentioned. It seems awfully high.

I don't know what the other codes on the packaging mean.

Duke

[Duke W.]

Re: Need assistance with factory engine specs.

Okay, (330-105)/0.5 = 450. Have you ever tested one to verify that the specs are in the ballpark? It still seems like an awfully high rate for the OE cams.

Duke

[Warren F.]

Re: Need assistance with factory engine specs.

Duke,

Thanks for that clarification. I was somewhat concerned by that notation. I did search the archives with your suggestion of that part #, it was extremely helpful and answered a lot. I will have the machinist check on of those springs.

Since I have the set of 16 NOS spring kits I will just have those installed. I want this engine to be as close to factory assembly as is reasonably possible. Thank you, again.

[Duke W.]

Re: Need assistance with factory engine specs.

You can always buy a spare set of 627s or aftermarket equivalents, but AFAIK this spring setup is bulletproof with the OE SHP cam and you will likely never need to replace any over the engine's useful life.

Such was not the case with the first design spring/damper. Broken valve springs were not uncommon on SHP BBs that were run hard. It suprises me that GM took six years to come up with a more reliable design, and that design is still good.

Duke

[Joe L.]

Re: Need assistance with factory engine specs.

Warren------

I agree with Duke regarding the reason for the "note 1"; that applies to 65-69 big blocks originally using the GM #3859911 spring. The 3970627 is SERVICE for those applications. In fact, that's why it was probably released as a kit since the replacement for 65-69 also requires replacement of the retainer. For 70-74 replacement of the retainer is not required, but as long as you have them, replace them. Primarily, I recommend that because the new retainers also include the new seal.

I believe that the other series of numbers on the box refers to a source ID code, date, and, possibly, other GMSPO inventory information. I've never come up with a means of decoding it, though.