Water Pump: Dec 70 build 1971 LT-1.

[John R.]

I'm getting two answers. Which is the correct water pump for a Dec 1970 built LT-1, GM#3782608 or GM#3991399?

[James G.]

Re: Water Pump: Dec 70 build 1971 LT-1.

3991399 is the correct pump
- If not mistaken the 399 pump dates start in I believe August of 1970.

[Mike E.]

Re: Water Pump: Dec 70 build 1971 LT-1.

3991399 is the correct pump
- If not mistaken the 399 pump dates start in I believe August of 1970.

[John R.]

Re: Water Pump: Dec 70 build 1971 LT-1.

Thank you. Was the 608 correct for a Corvette, assume not, in this case it was Manuf Oct 12 1970.

[Joe L.]

Re: Water Pump: Dec 70 build 1971 LT-1.

I'm getting two answers. Which is the correct water pump for a Dec 1970 built LT-1, GM#3782608 or GM#3991399?

John------


That's a good question. I believe that very early 1971 small block could have used the 3782608 casting pump. In that you have a December, 1970 built car and, apparently, have an October, 1970 waterpump, I'm suspicious.

It's easy to tell, though, assuming your car has its original waterpump pulleys. As Mike mentions, the bolt circle for the 608 pump hub is different than that for the 399 (1-3/4" versus 2-1/8"). In addition, the waterpump shaft pilot is different. The 608 has a 5/8" OD pilot whereas the 399 has a 3/4" pilot. So, the 1970 and earlier pulleys are generally different than the 1971+, except for, possibly, early 1971.

The waterpump pulleys used for 1969-1970 HP and SHP as well as some earlier SHP were GM #3848904 (2 groove) and add-on pulley GM #3883235 (1 groove). The 1971+ pulleys were GM #3991423 (2 groove) and GM #3991425 (1 groove). So, assuming that your pulleys are original to the car, which pulley set you have pretty much determines which waterpump your car had originally installed.

[John R.]

Re: Water Pump: Dec 70 build 1971 LT-1.

Thanks - don't know if this also makes a difference, but the block is dated Aug 1970 and the 608 water pump was Oct 1970. As you can tell, this is new to me. Should the water pump date precede the block date or just the build date?

[Joe L.]

Re: Water Pump: Dec 70 build 1971 LT-1.

Thanks - don't know if this also makes a difference, but the block is dated Aug 1970 and the 608 water pump was Oct 1970. As you can tell, this is new to me. Should the water pump date precede the block date or just the build date?

John------


What is the date the engine was ASSEMBLED (from the stamp pad)?

[John R.]

Re: Water Pump: Dec 70 build 1971 LT-1.

That was Dec 1970

[Mike E.]

[Patrick H.]

Re: Water Pump: Dec 70 build 1971 LT-1.

We have documented early (pre-strike, in other words, built before the middle of September) 1971’s with original 399 water pumps. I believe it was a very clear end/beginning of model year change.

I have a 399 with an August 1970 date, so I'd agree with Mike.
In December, it's a 3991399 only.

[Joe L.]

Re: Water Pump: Dec 70 build 1971 LT-1.

I have a 399 with an August 1970 date, so I'd agree with Mike.
In December, it's a 3991399 only.

Patrick------


Just because the 3991399 was being manufactured as early as August, 1970 does not mean that the 3782608 went out-of-production at that time. As a matter of fact, the 3782608 was still being manufactured as late as at least 1972. It also does not necessarily mean that the 3991399 was installed on all 1971 small blocks just because it was available as early as August, 1970. While the 3991399 casting had very limited application (Corvettes and MD/HD trucks), it could well be that its initial application was the MD/HD truck application.

On the other side of the coin, I have long postulated that if the 3782608 waterpump had been installed on any 1971 small blocks, then those engines would have had a different suffix code than otherwise identical engines receiving the 3991399 pump.

[Joe L.]

Re: Water Pump: Dec 70 build 1971 LT-1.

That was Dec 1970

John------

I don't understand how the engine could have an assembly code of December, 1970. The assembly code which is stamped on the engine stamp pad contains no reference as to YEAR, only month and day. So, I have the following questions:

1) What is the complete assembly code stamped on the engine stamp pad (located on the block stamp pad forward of the right side cylinder head)?

2) What is the block casting date located on the block's right side bellhousing flange? It will consist of an alpha character followed by 3 or 4 numerals;

3) What are the part numbers stamped on your waterpump pulleys?

[Keith W.]

Re: Water Pump: Dec 70 build 1971 LT-1.

I have a 399 with an August 1970 date, so I'd agree with Mike.
In December, it's a 3991399 only.

[Patrick H.]

Re: Water Pump: Dec 70 build 1971 LT-1.

Thanks.
I started preparing it early last year for an attempt at a Duntov, and then the pandemic hit.
That plan has been delayed by 1-2 years.