Difference in 823 v/s 821 castings on L92 heads?

[DavesG8GT]

Anyone know the difference? So far the only differences I have read about are just the fact that one is cast in Mexico while the other is cast in Michigan. My '09 G8 GT has 821 casting heads which everyone is saying are LS3 heads but with the solid stem intake and exhaust valves. My question is, if I end up having to break down and get a new head for my car last minute tomorrow, will an 823 be identical to my 821 on the other cylinder bank? Has anyone ever researched this?

Head shop "should" be able to fix my head but if not I might have to get one ASAP. All I am finding right now are 823 castings.

[Crazy Paul]

Search on ls1tech.com for one of the many threads about L92 head casting numbers. Search "821", "823", and "5364".
In particular read the threads from head porters and blokes who see a lot of cylinder heads up close.

[Mier]

Speaking of heads and the associated valves. I have a link to an LS engine primer which essentially gives details on all the LS family engines. Listed under L76 car it says we have in particular high-flow heads with large bore intake and exhaust ports but the valves are listed as hollow stainless intake valves and sodium filled exhaust valves. Is that true or false?

Here's the link (http://www.pirate4x4.com/tech/billavista/LSPrimer/Part1/)

[-Ray-]

Speaking of heads and the associated valves. I have a link to an LS engine primer which essentially gives details on all the LS family engines. Listed under L76 car it says we have in particular high-flow heads with large bore intake and exhaust ports but the valves are listed as hollow stainless intake valves and sodium filled exhaust valves. Is that true or false?

Here's the link (http://www.pirate4x4.com/tech/billavista/LSPrimer/Part1/)

It's false. Corvette's have the hollow stem intake valves.

[johnbell2]

False on sodium-filled exhaust valves from the factory, also.

[GRRRR8]

5364 for some reason is the favorite for most serious builds for some reason.

[DavesG8GT]

I've read a good number of threads so far, problem is I'm not finding any 821's and I don't know if it's ok to mix lol. Hopefully this won't be an issue and the machine shop will tell me tomorrow that they fixed my head successfully. But if not, would it adversely affect my performance if I for instance used an 823 passenger side and my factory 821 driver's side? This question was asked in other threads too but nobody ever answered or said anything about it.

If I can't find an answer if it's ok to mix w/o adversely affecting performance, and IF the machine shop can't fix my head, I might have to break down and buy a similar pair if I can't find another 821. Hmmmmmmmmmmmm............

[G8-4-Speed]

I've read a good number of threads so far, problem is I'm not finding any 821's and I don't know if it's ok to mix lol. Hopefully this won't be an issue and the machine shop will tell me tomorrow that they fixed my head successfully. But if not, would it adversely affect my performance if I for instance used an 823 passenger side and my factory 821 driver's side? This question was asked in other threads too but nobody ever answered or said anything about it.

If I can't find an answer if it's ok to mix w/o adversely affecting performance, and IF the machine shop can't fix my head, I might have to break down and buy a similar pair if I can't find another 821. Hmmmmmmmmmmmm............

821's and 5364's are identical as a bare head. You can swap them without a difference. The only difference is the 821's can be a LS3 head and have the lighter intake valves and yellow valve springs but still them same head and flow.

The only time you are going to see a performance difference is when there is a casting shift that affects the head flow. You could have a bad head and get something better or just the opposite.

[johnbell2]

Should be no magic between 5364 and 821. Get some 5364 castings, put in some GM hollow stem intake valves / dual springs / trunnion upgrade, done.

If it were me, I'd go ahead and have 'em worked over. At least a valve job.

[DavesG8GT]

Should be no magic between 5364 and 821. Get some 5364 castings, put in some GM hollow stem intake valves / dual springs / trunnion upgrade, done.

If it were me, I'd go ahead and have 'em worked over. At least a valve job.

I would if I had a little extra time and money. I just need to get the car back on the road since this is my DD and I only have a car borrowed through the end of this week.