I don't think any of the caps would have a thread size the same as the plug but the adaptors are all standard easily available and not expensive......look at any website that sells the caps and they list details of sizes of caps and adaptors.
The bath plug outlet I used had just the right size flange to drill the fixing holes and fit over a standard size Nippy rad cowl hole. At the back where the flange protruded slightly over where the bonnet fits the flange was sawn back till flush with edge of cowl step and clear of rubber surround. It was the smallest size cap available and does look in proportion and as if original fit.
In case you are wondering why it was needed, I am using an Austin A35 (or was it A30) radiator core fixed to a flat steel frame which fixes to the bodywork ahead of cow-horns and also supplies stronger brackets to the cowl where the wing/headlight brackets bolt onto the cowl, with a flat strip also running from side to side so giving full support to the wings and no strain on the cowl....no more cowl splitting in future. The radiator pressure cap sits close under the cowl opening and to add water I had to open bonnet and use watering can with a spout bent down on the end that just fitted under cowl. Now just open bonnet, remove pressure cap and open Monza cap to fill from any container to hand. That radiator has a metal plate surround that bends up nicely to fit to the mounting frame at the bottom and the wing stay bracket at the top....easy cheaper core renewal if required.
bottom bracket
top wing stay bracket
Rad mounted in shell
mounted on car with flat bar between wing stay brackets
The bath plug outlet I used had just the right size flange to drill the fixing holes and fit over a standard size Nippy rad cowl hole. At the back where the flange protruded slightly over where the bonnet fits the flange was sawn back till flush with edge of cowl step and clear of rubber surround. It was the smallest size cap available and does look in proportion and as if original fit.
In case you are wondering why it was needed, I am using an Austin A35 (or was it A30) radiator core fixed to a flat steel frame which fixes to the bodywork ahead of cow-horns and also supplies stronger brackets to the cowl where the wing/headlight brackets bolt onto the cowl, with a flat strip also running from side to side so giving full support to the wings and no strain on the cowl....no more cowl splitting in future. The radiator pressure cap sits close under the cowl opening and to add water I had to open bonnet and use watering can with a spout bent down on the end that just fitted under cowl. Now just open bonnet, remove pressure cap and open Monza cap to fill from any container to hand. That radiator has a metal plate surround that bends up nicely to fit to the mounting frame at the bottom and the wing stay bracket at the top....easy cheaper core renewal if required.
bottom bracket
top wing stay bracket
Rad mounted in shell
mounted on car with flat bar between wing stay brackets