Simon,
The 12558162 is a 496 casting number for sure. In your two original pictures above the number on the head in the right hand picture is hidden by a rocker arm. It almost looks like it ends in a different number ? That is what I was wondering.

https://cimg4.ibsrv.net/gimg/www.off...7ece067995.jpg

Same number just followed with a CA. I wonder what the CA denotes?

Looks all good anyhow, so I’m in progress rebuilding.

Thanks for your interest and help.

Simon

Simon,
Interesting. Kind of strange the the same casting number heads would use different head bolts. I will see if I can find the GM head bolt part numbers for the early engines compared to the last ones and see if they are the same. Looks like the old one is 2002 and the new one 2007.

Going back there was some head changes on these because of the different apps the 8.1 - 496 had. ***** 1 you had marine - 2 you had industrial - 3 you had truck and the Avalance - 4 you had California emission for auto and marine that had to be met. The California auto side took place in 2001 for tough emissions and the California marine side took place late 2007 for model year 2008 for the tough emission standards. I am talking about pollution control catalyst engines here for both sides. Many forget the 496 marine was catalyized in late 2007 for CA and the rest of the USA got in late 2009 model year 2010. I sat in on the emission control marine classes in 2007. SO imo you got the CA heads of the emission eras.

I have worked on all there was for the marine side from every where including CA. even the catalyst 496 inwhich is not even the same as any other 496. Just saying. Seen and worked on all from every marine manu as well - Indmar - GM - Volvo - Merc and so on.

So last weekend I received the new bolts and re fitted the heads. BTW BUP that was interesting info you posted re the different heads.
I found that the new bolts worked to take a toque and felt ok going up, but it became noticeable that the 2 that was in the position that used to be shorter are on these heads actually requiring slightly longer than the other 16 long bolts as there was less amount of bolt exposed than the other 16 when dropped in the hole before screwing up. I would say maybe 3/8” of thread wound in.
Hopefully this will be ok but it would be great to know the part no for the correct bolt sets if anybody can find out.

Thanks again