I am just doing some research of my own on the cosworth cylinder heads.
recently its been brought to my attention that there are 4 differnet types of 2wd cylinder head.

I was wondering out of the guys on the forum with a 2wd head and three bolt thermo housing what is the YN0*** part number on the front of the head. and do you have coscast between the cam gear?

This is something Im interested in myself my head is supposedly from a saphire its got the 3 bolt housing but no coscast and the part number is yn0 450. The 4 you know of are they
RS5000
2 bolt thermo
3 bolt thermo
3 bolt thermo with coscast

im talking about 4 differnet 2wd heads that have the 3 thermo bolts! (i forgot to mention that in the first post)

im still working out whether your part number is an early sapph head or a later? i know that the yn0925 yno935 is a later head.

i was told there are slight differences between certain heads.
its nothing to worry about. its just interesting as to why the changes where made and when etc... im not sure if it effects power output???

i wasn't aware of the yno450!!! interesting though. i will try and find out more.

Far out be interesting to see how many there are

ok some more information

the rs500 (have checked this out on my car also) uses a YNO627 3 bolt thermo and no coscast.

apparently there were a few crate engines (spare rs500 engines made for the race cars or just spare engines) they have the part no. YNo627 with coscast

then you have the early sapph head (this could be the head you have santa???) still got to work this one out. YN0450

and the later heads YNO925 YNO935... I THINK??

also some heads have the coscast some dont? not sure why????

Santa does your head have 4 bolt holes at the back of the head or only 2???

Its a bit hard to see in the car but I can only see 2 bolts up top and this would mean what?

Will check the two I have got and mates two original rs500 ones.

Check the profile of the water ways on the exhaust side you have. Are they holes or slits?

Found this quote from the rsm site talking about 2wd cyl heads.

"To my knowledge everything from the V89HF casting on has the closed in exhaust side water jackets. So Sapphier on I think. This version on also has the more hemispherical combustion chamber with slightly smaller CC volume. Only the V86HF part number casting got the open water jackets."

Mine is V86HF with holes. So that means its a early head correct?