Mk3 R4 Head

The RH4 heads were probably cracked, from new, from the factory.

I found this around 2000, on several heads with incredibly low mileage and had never been disassembled from new.

Without proof of course, I believe the big 3/4" guide AND the 32mm ports and a ham fisted factory assembly guy busted the very thin port wall. about 50% of the heads I've inspected have one or two intakes cracked. I've never found a RH10 cracked!

Upon valve removal, clean the intake ports real good, then 3 part die penatrant test them before going any farther. The crack will go from the guide toward the intake port (not the valve seat).

good luck

http://atlanticgreen.com/nhth.htm

Previously David Comeau wrote:

The RH4 heads were probably cracked, from new, from the factory.

I found this around 2000, on several heads with incredibly low mileage and had never been disassembled from new.

Without proof of course, I believe the big 3/4" guide AND the 32mm ports and a ham fisted factory assembly guy busted the very thin port wall. about 50% of the heads I've inspected have one or two intakes cracked. I've never found a RH10 cracked!

Upon valve removal, clean the intake ports real good, then 3 part die penatrant test them before going any farther. The crack will go from the guide toward the intake port (not the valve seat).

good luck

http://atlanticgreen.com/nhth.htm

Bye the bye ,leaky RH4 heads, I"ve got a" 74 850 Mk2a , and ,yes there is oil leaking mysteriously from the Head area." Yes Sir they all do that" but I was interested enough to read the Atlantic Green link. This seemed to say that RH10 heads were superior in this respect. It also went on to say that the RH10 head had 30mm rather than the usual 32 mm ports. As it turns out, I have an RH10 head , after years of not bothering to look. Can someone explain why please.

Because MK2a's came with RH10 heads as does mine. RH4 was on 73 and 75's, RH10 on 74. Dave Comeau's site says that.

My RH4 head has had guides two or three times over the last thirty-five years and the head is as dry as a bone. Perhaps I've just been lucky ?

I've seen a lot of people troubled by third fin up oil stains who haven't sealed the inlet rocker cover properly though.

Also have RH4 heads on both my '73 Mk 1A and '76 Mk 3 without sign of leakage or cracksfor their entire lives.