Adapter Rear Cylinder Heater Hose Help Needed

[S11A]

Ahh, thanks that clarifies my confusion.

[o2batsea]

I can't figure out what the reason for saving the old valve is. There's like ten gazillion other possible heater control valves out there that are easily adapted to your system. You'd just get a metric fitting that has a hose barb on it, then plumb the HCV in between the head and the heater core.

[Pop Alexandra]

This is very typical on older Series IIa engines — heater ports often end up a mix of BSP, imperial, and later metric repairs.

What you’re seeing (a slightly smaller, finer thread than RNC663) strongly suggests your head is not using the same standard as the replacement part, likely BSP or M16x1 rather than the coarser imperial fitting.

I would avoid re-tapping the cylinder head unless you have no other option — it’s easy to damage the casting and lock yourself into that modification.

A better approach is to identify the exact thread and then use an adapter or custom fitting. Suppliers like Aerocom often handle these odd metric/BSP/imperial conversions and can save you from modifying the head itself.