differences between 69 and 70 3927186 heads?

[Richard R.]

I need to replace a head on my 69 L46 (350/350). Story as to why at another time. Anyhow I have a fellow who has a 3927186 head cast in Jan 1970. Is there any mechanical or cosmetic differences between my April 69 head and this one? I know the cast dates are not judged for heads but want to make sure this would be a reasonable swap.

Thanks,

Rich

[Duke W.]

Re: differences between 69 and 70 3927186 heads?

The ...186 heads were used on both '69 base 300 HP and L-46 and 302 Z-28 engines, and 70 base, L-46 and LT-1. The base engine version were processed for the 1.94/1.50" valve size and the L-46 302 DZ and LT-1heads were processed for the 2.02/1.60" valves and included a machined inlet valve relieve with either a 2.40 or 2.34" diameter cutter.

If this '70 head is off an LT-1 I believe it should include have pushrod guideplates, which would mean the pushrod tunnels are opened up. It would be okay to use this head on a L-46 even though I don't think the original L-46 heads have pushrods guideplate/opened up pushrod tunnels.

I'm not aware of any discernible differences in the raw castings. The only differences are how the heads were processed for the final applications and those differences are not visible with the valve covers installed.

Assuming you can't visually inspect this head just have the seller send photos of the combustion chamber and rocker box, and post them here if you can't make a positive ID.

Duke

[Richard R.]

Re: differences between 69 and 70 3927186 heads?

Thanks Duke. I am attaching pics from seller. He says it has been mag and pressure tested with 40#. probably take me a couple of replies

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[Richard R.]

Re: differences between 69 and 70 3927186 heads?

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[Richard R.]

Re: differences between 69 and 70 3927186 heads?

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[Dave P.]

Re: differences between 69 and 70 3927186 heads?

EDIT/ADD II: That head has had 'hardened?' exhaust seats installed. I am usually leery of added valve seats because they can cause problems, like getting loose, breaking up, and destroying the piston and head. Hardened seats are not necessary in an application like a Corvette where the engine is not run at high output for extended periods of time. Like a G30 Motorhome Chassis, or a C30 pickup with a service bed that weighs 10-12k pounds.

[Joe L.]

Re: differences between 69 and 70 3927186 heads?

What about the spark plug holes? 69 was last year for gasketed spark plugs. 1970 was the first year for tapered seat spark plugs. I do not know how 'complete' the change-over was between all engines and the two model years.

The OP mentions "judging". The two spark plug types have different dimension wrench hex. This probably isn't observable because the plugs are concealed by the ignition shielding?

If the new head has tapered seat plugs, you have to purchase two different plug types for the engine. So something to check.

EDIT/ADD: That head appears to be "angle plug". I don't remember that the 350/350 had angle plug heads. I don't think it did. Angle plug heads were tapered seat. I also don't recall that the angle plug head was used on a production engine. Only available over the counter.

EDIT/ADD II: That head has had 'hardened?' exhaust seats installed. I am usually leery of added valve seats because they can cause problems, like getting loose, breaking up, and destroying the piston and head. Hardened seats are not necessary in an application like a Corvette where the engine is not run at high output for extended periods of time. Like a G30 Motorhome Chassis, or a C30 pickup with a service bed that weighs 10-12k pounds.

Dave------

From the photos I cannot tell what sort of spark plugs these heads are machined for. However, the first year for tapered seat spark plugs for small blocks was 1971. Big blocks did use tapered seat plugs for 1970, though.

I agree that no Corvette small block utilized angled plug heads, at least through 1972, if not throughout C3. However, I do not think that any 3927186 heads were machined for angled plugs, PRODUCTION or SERVICE.

The 1969-70 L-46 cylinder heads utilized press-in rocker studs, which these heads do have. My preference, though, is to convert these to screw-in studs, with or without guide plates.

I agree that valve seat inserts are undesirable and almost always unnecessary. However, I once unwisely went this route and, so far, have escaped the possible negative consequences.

[Duke W.]

Re: differences between 69 and 70 3927186 heads?

Those heads appear to have been processed for the standard 1.94/1.5" valve set. I don't see the unshrouding cut for the inlet valve. The ...186 heads for the L-46 were processed for the 2.02/1.6" valve set that included the inlet valve unshrouding cut that would be obvious.

So these heads were originally installed on a 350/300 installed in a Corvette or any other Chevrolet model that was available with the 350/300 engine.

I don't like the exhaust valve seat inserts either, and my recommendation is to keep looking.

Duke

[Joe L.]

Re: differences between 69 and 70 3927186 heads?

Those heads appear to have been processed for the standard 1.94/1.5" valve set. I don't see the unshrouding cut for the inlet valve. The ...186 heads for the L-46 were processed for the 2.02/1.6" valve set that included the inlet valve unshrouding cut that would be obvious.

So these heads were originally installed on a 350/300 installed in a Corvette or any other Chevrolet model that was available with the 350/300 engine.

I don't like the exhaust valve seat inserts either, and my recommendation is to keep looking.

Duke

Duke----


Yes, I missed that. With 2.02/1.60 heads the valve seat machining for the 2 valves slightly overlaps These don't show that so they are 1.94/1.50 heads. As a matter of fact, I don't think that hardened exhaust valve seats can be installed on 2.02/1.60 heads. At least, a lot of machinists won't attempt it (unless one "holds them harmless" if a head is ruined and one agrees they'll pay regardless).

[David M.]

Re: differences between 69 and 70 3927186 heads?

My 2 cents...now is the time to upgrade. Dont cheap here. Why do one side with the other not fresh.

Heads are one of the single best improvement areas you can do with near zero outward appearance modifications.
Aluminum even batter. 2.02s with 1.65 or 1.7 roller tip rockers even better! Small inexpensive upgrades with a bang.

Back to the iron heads. Have a matched set prepped. Mag fluxed, PSI tested, with mild porting and blending and a proper 3 angel valve grind. Do the screw in studs. Guide plates are advised for moderate to higher RPM engines.

You will likely be into a cam lifter timing set. You are in there just do it.
Head work with the right cam profile and cam advance (or not) timing etc. The car will pick up solid seat of the pants performance gain.

Even with the single head replacement be cognoscente of deck height, intake port matching final CR ratio and chamber CCs. Measure the head gasket thickness and piston TDC etc. If anything in the equation is off... it will be... well off.

The whole package needs to be assembled with matched parts.

How many miles on the bottom end? You could be in the MAW mode where the head leads to a general refresh of the whole engine.
When will you have or want the chance to do it again. Then its done and reliable. IMO I would never replace just one head unless it was a farm truck.

[Richard R.]

Re: differences between 69 and 70 3927186 heads?

Thanks for all of your advice. I'm still looking for a head. Thought I had one locally but the shop said it's surface was too pitted to use. This engine has only 40 miles on it since it was totally rebuilt. So we will replace only what is needed to repair the damage and to make sure all components are matched, balanced properly.

OK. I will finish the rest of the story.

This past May, I was preparing for the NE regional for a PV. I had checked and double checked all the functions, correcting any problems along the way. It was perfect, sort of. The car just wasn't running right. After a 10 mile enjoyable drive, including running the engine up to 90% of redline, it would stall just as I was pulling into the driveway. Then it wouldn't restart. Drifted into the garage. Engine seemed to be running hot, but the gauge didn't indicate a problem. Next morning, the day before I was going to leave for the regional, I started it up and was trying to figure out what wasn't right. I thought I might have found the problem. The heat riser valve wasn't opening and in fact was closed with the engine fully warmed up. I was holding the valve open while fiddling with the throttle when the engine let go. antifreeze coming out the exhaust, heart falling out on the floor. I was just crushed.

Next day, instead of leaving for Boston, I pulled the right head off the engine and discovered that #6 cylinder (which is right above the heat riser) dropped a valve head into the cylinder. destroyed the head, piston and ruptured the cylinder in 3 places. After waiting in line a few months to get the engine looked at by my trusted machine shop, it was determined the block can be saved (original block) which is the great news. But the head is a goner. That's where I'm at now. So anyone out there with a spare 3927186 head laying around, drop me a line!

The question that hasn't been answered though is what caused this. I believe the engine had been rebuilt properly so the only thing I can think of was that stinking heat riser valve (an original) was pushing hot exhaust at that spot in the head and the temperature sending unit wasn't working properly by not indicating an overheating problem.

The therapy sessions help but it's still painful to talk about it........

[Duke W.]

Re: differences between 69 and 70 3927186 heads?

Did the valve head break off the stem? Retainer fail? Pictures of the parts would help. I'm surprised you were able to drive even ten miles with the heat riser valve stuck closed. We really don't need that thing if we only drive the car in mild to warm weather.

That's why I recommend wiring them open, especially if the car is not being judged. This is a good reminder to all. If the valve is not wired open, next time you go to start the engine or out to the garage to do some work, check that it works.

With the engine cold you should be able to push on the counterweight and it should freely open with little effort. Long periods of sitting can cause it to stick and invariably that means it will stick closed.

Duke

[Bill B.]

Re: differences between 69 and 70 3927186 heads?

A couple years ago, when I still had my heat riser installed (I removed since), I purchased AC Delco Rust Penetrant and Inhibitor from my Chevy parts counter, P/N 10-4020, GM 88862628. I had heard good things about for heat riser valves years ago, although like anything else, the orig formula probably changed to me it less toxic, although my can still has a long list of chemical names on the back in red print that I can't pronounce!

[Joe L.]

Re: differences between 69 and 70 3927186 heads?

A couple years ago, when I still had my heat riser installed (I removed since), I purchased AC Delco Rust Penetrant and Inhibitor from my Chevy parts counter, P/N 10-4020, GM 88862628. I had heard good things about for heat riser valves years ago, although like anything else, the orig formula probably changed to me it less toxic, although my can still has a long list of chemical names on the back in red print that I can't pronounce!

Bill------

This is the original product, GM #1052627, the predecessor of the 88862628:

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[Richard R.]

Re: differences between 69 and 70 3927186 heads?

here's a couple of pictures of my broken heart......

The valve head broke off from the stem. Spring, retainers etc were still attached, but the head of the valve went for a wild ride. Interesting in that I put a torch on the heat riser valve spring and it opened immediately. But it definitely wasn't opening on a hot enging. It will be replaced and wired open for non judging. I can't think of another reason this would happen and it seems too much of a coincidence that it would be the cylinder right above the heat riser.

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