GM aluminum 842 heads open chamber?

[BLACKLS5]

Has anyone seen a set of 842 big block aluminum heads that were open chamber design? I have to believe these have been modified? Alan Colvin's book says the 1968 and early 69's has a screw in plug on both ends and late '69 had a plug on only one end. These don't have a screw in plug on either end. The date looks like 7 15 69.

[SuperNovaSS]

They all came closed chamber from GM. I have had many sets that we opened up to the open chamber design.
Those also had the extra(ZL1) head bolt boss added to the underside. The later service heads had no plugs on either end.


Jason

[BLACKLS5]

Thanks Jason. That was what I assumed but wasn't 100% sure. Are the stories true about the seats coming loose on some of these early factory aluminum heads. I've had a few sets pass through my hands but didn't have the nuts to roll the dice and use a set for fear of ruining an engine.

[SuperNovaSS]

I have had many sets that have been grenaded due to loosing a seat. Most of those were pushed to their performance limits so it’s hard to say the cause of the seat loss. I have also had many sets with no damage. When playing with the most high performance parts available at the time, it is expected to see some modifications and breakage.

Jason

[olredalert]

----I was looking for a new 69 435 at a Columbus dealer when the sales guy said that they had a black convert L89 new, but in the service department. I asked why and he said they had one head off. When pressed he took me to the service head who told me it had dropped a seat. I decided to pass on L89's at that moment!....Bill S

[BLACKLS5]

Lol... Good story Bill.

[Zman1969]

Originally Posted by SuperNovaSS

I have had many sets that have been grenaded due to loosing a seat. Most of those were pushed to their performance limits so it’s hard to say the cause of the seat loss. I have also had many sets with no damage. When playing with the most high performance parts available at the time, it is expected to see some modifications and breakage.

Jason

My Machinist buddy made it sound like was every day occurrence on these heads and warned me not to get it hot

[big gear head]

I hope the problem was fixed with the 077 heads. I take mine to 7200-7300 pretty regular, with plans to go 7500 next year.

[markjohnson]

The problem is that the expansion rate of aluminum is greater than the expansion rate of the steel valve seat insert. The aluminum head expands and the valve seat just falls right out if the machining tolerance isn’t dead on. The high end solution is copper beryllium valve seats that actually expand more than the aluminum so they really lock themselves into place.

[Igosplut]

Originally Posted by markjohnson

The problem is that the expansion rate of aluminum is greater than the expansion rate of the steel valve seat insert. The aluminum head expands and the valve seat just falls right out if the machining tolerance isn’t dead on. The high end solution is copper beryllium valve seats that actually expand more than the aluminum so they really lock themselves into place.

Funny you say this, as I had a valve seat fall out of an older Harley a while back,. and the machine shop used a copper/style seat to replace it, stating the very same reason.