Those heads have been in high demand for quite a few years, due to the popularity of replica trials HT5's being built Being 55 I think your head will have an exhaust stub rather than a push in pipe ?? Later type new cylinder heads are available from Ariel Specialists in the UK
Mine is push-in (it does have “54” stamped in somewhere)... with a cast in iron insert that encompasses both valve seats and the spark plug hole. The valve seats are ground nearly clear through but the head is perfect otherwise. I have a later stub type head but the fins are beat and it’s had some awful weld repair in the past...
I’m slowly working at repairing the later head, but I’m lazy. And busy.
HI Chris, I'm not 100% sure which year had push in or over, The later 57(ish) heads are push in exhaust Mine is like this, The later heads have separate cast in "D" shaped with the flats to the centre, and dovetailed that are cast into the alloy Difficult to renew the seats? But I had new seats fitted into the cast in inserts by a local shop Way back when I first rebuilt this engine I got the seats cut outside the old seats and adapted larger head valves to suit, I got a copy of the pattern makers drawing that showed the shape of the seat inserts After almost 30 years of use the seats were again in poor shape necessitating further work, It has been fine since I had the inserts fitted (Touch Wood!!) Have you enquired about getting inserts fitted into the cast in "skull" I have seen this done on a Norton Head
I haven’t looked in to having inserts put in. I was queasy about having seats pressed in to what would end up being part iron and part alloy after boring for new seats. I suppose it’s worth asking a professional.
I was in the process of goig big on the intake valve and down to 5/16” stems on both before I accepted how bad my seats are, and the project stalled.
I have a couple but one I purchased and extremely happy with was from John Budgen (UK). Very nice bloke to talk and deal with. Easy to find on net. Im down under so probably easier to get direct. They are new and cast in the improved period type you are looking for. Look good enough for the mantle piece I reckon !