427/390 Pushrod Diameter

[Owen L.]

Am in the process of ordering parts for rebuilding my '67 L-36 and have a puzzle to figure out. My old pushrods and guide plates are 5/16" diameter. All are the style with no discernible tip, except for one which has ball-tips. The heads are original, dated April '67. Were 5/16" pushrods standard on lower HP 427s?

I ordered new 5/16" intake and exhaust pushrods and they measure ~0.050" shorter than the old pushrods. The only way to get the same length is to go with 3/8" pushrods (and $140 worth of 3/8" guide plates!).

My goal is an engine restoration and would like to use whatever was factory at the time. What should be correct to my engine?

Thanks all!

[Joe L.]

Re: 427/390 Pushrod Diameter

Am in the process of ordering parts for rebuilding my '67 L-36 and have a puzzle to figure out. My old pushrods and guide plates are 5/16" diameter. All are the style with no discernible tip, except for one which has ball-tips. The heads are original, dated April '67. Were 5/16" pushrods standard on lower HP 427s?

I ordered new 5/16" intake and exhaust pushrods and they measure ~0.050" shorter than the old pushrods. The only way to get the same length is to go with 3/8" pushrods (and $140 worth of 3/8" guide plates!).

My goal is an engine restoration and would like to use whatever was factory at the time. What should be correct to my engine?

Thanks all!

Owen------


1967 Corvettes with L-36/L-68 engines manufactured before about 9/1/66 were originally equipped with 5/16" pushrods. These pushrods were so-called "one piece" and were of GM #3904376, intake and 3904377, exhaust. These were replaced for SERVICE by, respectively, GM #346280 and GM #346282 in April and June, 1975. The latter pushrods were also used in PRODUCTION for 1973-74 LS-5 and were of the "welded ball" type. So, one of your pushrods has been replaced with the later style pushrod, GM-sourced or otherwise. The latter pushrods are GM-discontinued.

By the way, regardless of when your engine was manufactured, I am pretty confident that it was originally equipped with the 5/16" pushrods. But, I would be curious to know the actual assembly date (determined from the stamp pad date code and cylinder head casting date).

You can, of course, convert to 3/8" pushrods by purchasing an engine set and, as you mention, a set of 3/8" guideplates. However, I think that new 5/16" pushrods will function just fine even if 0.050" shorter.

No 3/8" pushrods for your application are currently available from GM but are available in the aftermarket. 3/8" guideplates are still available from GM under GM #3860038 for a current list price of $13.08 each.

[Owen L.]

Re: 427/390 Pushrod Diameter

Thank you, Joe.

I'm a bit confused on two of your statements:
1967 Corvettes with L-36/L-68 engines manufactured before about 9/1/66 were originally equipped with 5/16" pushrods.
...regardless of when your engine was manufactured, I am pretty confident that it was originally equipped with the 5/16" pushrods.

I've also got something else with the heads that appears out of the ordinary from what I read in the GM '67 specs AMA doc: my closed chamber heads should have the small valves (2.06"/1.71") but they actually measure 2.19"/1.88". On CF, one of the replies was, "No oval ports ever came with 2.19/1.88's from the factory."

I've owned it since '79 and am nearly certain the engine has not had any major work: still has standard bores and GM steel shim head gaskets.

Here are the dates of the heads and block:
IMG_1208.jpg
IMG_1207.jpg
IMG_1211.jpeg
IMG_1209.jpeg

Block cast and assembled on the same date
IMG_1214.jpeg
IMG_1213.jpeg

[Mark E.]

Re: 427/390 Pushrod Diameter

I ordered new 5/16" intake and exhaust pushrods and they measure ~0.050" shorter than the old pushrods. The only way to get the same length is to go with 3/8" pushrods (and $140 worth of 3/8" guide plates!).

The hydraulic lifters can handle 50 mil difference.

[Joe L.]

Re: 427/390 Pushrod Diameter

Thank you, Joe.

I'm a bit confused on two of your statements:
1967 Corvettes with L-36/L-68 engines manufactured before about 9/1/66 were originally equipped with 5/16" pushrods.
...regardless of when your engine was manufactured, I am pretty confident that it was originally equipped with the 5/16" pushrods.

I've also got something else with the heads that appears out of the ordinary from what I read in the GM '67 specs AMA doc: my closed chamber heads should have the small valves (2.06"/1.71") but they actually measure 2.19"/1.88". On CF, one of the replies was, "No oval ports ever came with 2.19/1.88's from the factory."

I've owned it since '79 and am nearly certain the engine has not had any major work: still has standard bores and GM steel shim head gaskets.

Here are the dates of the heads and block:
[ATTACH=CONFIG]123689[/ATTACH]
[ATTACH=CONFIG]123690[/ATTACH]
[ATTACH=CONFIG]123691[/ATTACH]
[ATTACH=CONFIG]123692[/ATTACH]

Block cast and assembled on the same date
[ATTACH=CONFIG]123693[/ATTACH]
[ATTACH=CONFIG]123694[/ATTACH]

Owen------


First of all, the seeming contradiction in my statements is not what it seems. Note that I emphasized the term "ABOUT 9/1/66". That date comes from some GM information that I have been long suspicious of. Your engine would indicate that the 5/16" pushrods were used for a LOT LONGER than that date. As I said, I am confident that your engine was originally equipped with 5/16" pushrods. Why? Well, because I HIGHLY doubt that anyone would have converted it from 3/8" to 5/16" pushrods especially using original style "1 piece" pushrods. Now that I know the other history of the car, including your ownership tenure, I think it's even less likely that such a conversion would ever have occurred. So, if it wasn't converted, then your 5/16" pushrods must be original.

As far as the valve size goes, what makes you think that these are 2.19/1.88" valves? The 3909802 head was manufactured with 2.06/1.72" valves and that's exactly what yours appear to be. Once again, the chances that the heads were converted to the larger valve size is virtually nil, if it's even possible, at all. The CF posters were correct; no closed chamber, oval port heads were ever manufactured with 2.19/1.88 valve size.

[Owen L.]

Re: 427/390 Pushrod Diameter

Owen------


First of all, the seeming contradiction in my statements is not what it seems. Note that I emphasized the term "ABOUT 9/1/66". That date comes from some GM information that I have been long suspicious of. Your engine would indicate that the 5/16" pushrods were used for a LOT LONGER than that date. As I said, I am confident that your engine was originally equipped with 5/16" pushrods. Why? Well, because I HIGHLY doubt that anyone would have converted it from 3/8" to 5/16" pushrods especially using original style "1 piece" pushrods. Now that I know the other history of the car, including your ownership tenure, I think it's even less likely that such a conversion would ever have occurred. So, if it wasn't converted, then your 5/16" pushrods must be original.

As far as the valve size goes, what makes you think that these are 2.19/1.88" valves? The 3909802 head was manufactured with 2.06/1.72" valves and that's exactly what yours appear to be. Once again, the chances that the heads were converted to the larger valve size is virtually nil, if it's even possible, at all. The CF posters were correct; no closed chamber, oval port heads were ever manufactured with 2.19/1.88 valve size.

Thank you Joe. You are correct - I somehow mismeasured the first time, they are indeed 2" & 1.7" valves.