70 Z28 heads 3991482

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
I've recently aquired a 1970 Z38 RS that came with the numbers matching drivetrain ( currently out of the car). It was pointed out that the 3991492 heads that came with that engine were over the counter replacement for the OEM "186" heads. All well and good, I figured when I went to rebuild the engine I would hunt down a pait of 186's.
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Well when I took a closer look I found the casting date showed W243 on the driver's side and F147 on passenger side. I get the F147 date = June 14, 1977 but I thought to myself what the heck does the W243 mean? So off to google I went and searched for 3991492 W243 and came up with 3 different hits. All of them were guys with 1970 Z/28's asking the same thing. I find it odd and one hell of a coincidence that multiple people are reporting this W243 marking on 3991492 heads when the common date matrix does not support W at all right?

Month: A = Jan, B = Feb, C = Mar, D = Apr, E = May, F = Jun, G = July, H = Aug, I = Sep, J = Oct, K = Nov, L = Dec

Then I find this http://www.mortec.com/castnum.htm

3991492....70......350..........LT1,Camel hump,accessory holes,64cc, straight plug
3991492....70-up................over the counter,Camel hump,64cc, straight or angle plug


i look at that to mean that the 3991492 Heads with the straight plug to be original to the 70 Z28 LT-1 in 1970. It appears that the heads I have are straight plug with accessory holes so I don't want to go tossing out heads that could be born with parts. I have only seen and been told that the heads for 1970 LT-1 are the 186 heads and 492's where replacement or over the counter.
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I guess I would like to know what's really going on here before I do something with these heads.
 

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COPO

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Sep 15, 1999
21,611
Ontario, Canada
"492 – 3991492. Another good performance head. They were found on 1970 LT-1 engines. They had 2.02/1.60" valves, 64CC combustion chambers, DO have accessory holes, intake port volume should be 161CC's, exhaust port volume should be 62CC's. These heads have double hump casting marks. This particular casting head was also available as an "over the counter" head from GM in straight or angle plug configurations.

1969-1970 1.94 intake/1.60 exhaust with camel humps, 350, 64cc chambers, straight plugs or angle plugs

The part number is 3973370 and they ARE "fuelies", however, there's one "catch". They only came with the 1.940" x 1.500" valve option. They do carry the "camel-hump" markings at each end and have 64cc chambers.

These were "over-the-counter" pieces for use by G.M. when a replacement was need for the original 186's with the small (1.940") valves. After G.M. ran the supply of the 186's out they used either these 370's if they needed the 1.940' valve or they used the #492's (straight-plugs) if they needed the 2.020" option. All these over-the-counter 492's had studs/plates also.

To make it easier, the 186's had either valve option, the 1.940's AND the 2.020's. As I said, after these were "exhausted", G.M. used the 370's for the 1.940 deal and the 492's for the 2.020 one's. They couldn't put the 370's, with the small valves, on anything that had the 2.020's originally!

None of the 370's had the original screw-in-stud/guide plate option from G.M. either!
 
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Gary S

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Apr 14, 1999
24,419
Bismarck, North Dakota
I have not found any documentation that shows that the 3991492 head was ever used on any factory production engine. Everything I have shows that this head was an over the counter part only. The casting is basically the same as the 3927186 and the 3947041 heads that were used on the high performance engines in 1970, so it was the replacement head of choice for those engines when repair was needed.
 

70lt1z28

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Oct 3, 1999
4,726
Beavercreek, Ohio, USA
Can you flip the W243 head over and send a photo of the combustion chamber?

'492's were cast and machined at Tonawanda and Flint. However, I am pretty sure that Flint was the only plant that had the tooling to unshroud the combustion chamber for the larger 2.02 intake valves. That is one reason why only LT-1's had the V prefix in the engine code. V=Flint, T=Tonawanda. The W head may or may not have the unshrouded intake but it may tell us who made it. If it is shrouded, than definitely it wasn't an original LT-1/Z28 head.
This also follows the Corvette LT-1's. I don't think anybody has found a Vette LT-1 with anything but '186 heads.

What would be strange is having 2 '492 heads with such differing date codes. Your block date is right in the middle of 70 Z28 production so an anomaly like you are describing is very, very rare. Maybe if it was one of the last ones built and they had run out of '186 heads (since the '71's used a completely different head) they may have scrambled to find a suitable head and special shipped '492's in.

Looking at the Mortec page, I'm not so sure if it doesn't list heads by "application" compatibility rather than actual OEM fitments.
 
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70lt1z28

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Oct 3, 1999
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Beavercreek, Ohio, USA
One other thing you MAY see (if the combustion chamber is clean enough) look around the exhaust seat. The '492's I have seen have flame-hardened exhaust valve seats and you can see a blue ring around the exhaust seat. They did not start doing this to heads until sometime in 1971 model year and did it for unleaded gas. 1970 heads did not have this.

If the engine has been run awhile you may not be able to see this ring.
 

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
Can you flip the W243 head over and send a photo of the combustion chamber?

'492's were cast and machined at Tonawanda and Flint. However, I am pretty sure that Flint was the only plant that had the tooling to unshroud the combustion chamber for the larger 2.02 intake valves. That is one reason why only LT-1's had the V prefix in the engine code. V=Flint, T=Tonawanda. The W head may or may not have the unshrouded intake but it may tell us who made it. If it is shrouded, than definitely it wasn't an original LT-1/Z28 head.
This also follows the Corvette LT-1's. I don't think anybody has found a Vette LT-1 with anything but '186 heads.

What would be strange is having 2 '492 heads with such differing date codes. Your block date is right in the middle of 70 Z28 production so an anomaly like you are describing is very, very rare. Maybe if it was one of the last ones built and they had run out of '186 heads (since the '71's used a completely different head) they may have scrambled to find a suitable head and special shipped '492's in.

Looking at the Mortec page, I'm not so sure if it doesn't list heads by "application" compatibility rather than actual OEM fitments.
 

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l16pilot

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Dec 13, 2004
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Hurlock, MD, USA
Here is a couple more unexplained 186 with M104
and 492 with Y183. I read that when they made a additional run of heads they used the date area to mark it.
(I wonder ? )

From the CRG on "M" coded blocks:

"M"-Coded Blocks
There is one exception to the normal block casting date convention that has been observed: some blocks have been seen with "M" cast codes. Research by the Saginaw Metal Casting Operations of GM Powertrain Division has found that these "M" codes are not date codes, but actually a foundry code used to indentify parts that were made to evaluate a casting process change. After evaluating the parts, the blocks would either be destroyed or released into production. There is no way to determine the date an "M"-coded block was poured.

Gotta wonder if "Y" and "W" were possibly similar foundry codes for evaluation purposes?
 

70lt1z28

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Oct 3, 1999
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Beavercreek, Ohio, USA
Sounds reasonable. Best explanation so far. Based on the research I have done, it looks like the W243 head went through the Flint machining operation to unshroud the intake valves. Still not definitive that they are original heads to an LT-1, but we may never be able to tell with certainty. The only other way I can think of to tell is to see if the flame-hardening of the exhaust seats is present-absent. Even that may not be definitive as it could the an "experiment" to do a few sets before making it mandatory in 1971.
 




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