How to identify an R block

I just happen to have an R block sitting on the other side of my desk. This is an older block and has been around for a while. First picture shows an R inside the front timing cover area, Second picture shows the casting number, notice the "A4" part of the casting number, Third picture shows the standard style freeze plugs and 3 billet center 4 bolt main caps. Hope this helps..

90lxcoupe said:

R-351 blocks did not have screw in freeze plugs. The new boss 302 and 351 blocks do, the picture you posted is a Boss 351. The R-351 blocks also have billet 4 bolt mains on all 5 main caps. The blocks are not 9.5'' deck height like a standard 351w, they are 9.2" deck height and have dry sump oiling.

There is a little bit of info on it here.
http://www.yellowbullet.com/forum/showthread.php?p=8333564

Click to expand...

didnt know all that...

we run a Z351 RDI block which is a 9.5 deck

guess it doesn't have 4 bolt mains on all 5. mine didn't either. and I guess the one I had had the freeze plugs machined cause they were screw in and I also had the R cast in the front of the block as posted above. I was just trying to post visual cues from what I remembered.