Bolt size on thermostat housing?

1963 Series IIA with 2.25 petrol. I removed the three bolts holding the thermostat housing on and broke one. I looked at Rovers North to buy new ones and they are 1/4". My machinist friend looked at the old bolts and thought they looked like 6mm. Does that make sense? Are the three bolts that go through the housing into the head 1/4" or are they metric? Further, the RNC bolts are 2.5" long, but the old bolts I removed are 3" long and someone put several washers on them to shorten a bit. Thanks in advance.......:confused:

I would think the 1/4" x 2 1/2" would be correct, unless someone re-tapped the threads to metric. Take your bolt to a hardware store or home center, they have displays with nuts and bolts affixed in both Metric and SAE (inch), you just screw in your bolt until you find the size it fits. I think the length issue you solved by saying that extra washers were used. You could always bottom a wire or similar in the bolt hole and use your thumb to mark the depth and then measure what you have.

'99 Disco II
'76 Series III Hybrid 109
'95 R.R.C. Lwb (Gone...)
'70 Rover 3500S (Gone...)

I just did the same repair. My 1960 series 2 was a 1/4-28 X 2 1/2. I had the later model thermostat housing but it should have the same length. Just be careful with the torque. I think it is 10-15 ft lbs. I couldn't find the spec int eh books, but that is about correct for the bolt size.

You posted the year of your truck but not the year of your engine. The original bolt in the original engine would be 1/4unf but a very late engine may have a metric block and use metric fasteners.