What size are the exhaust manifold bolts? I know they are 12point..thanks bunches

 

Early DI's were 6 point 14mm. The 12 point on your '97 should be 12mm.

 

but the threads are NOT metric......3/8-16 ( old Buick heads)

 

If the threads are 3/8-16, then the head will be 9/16. You won't find a metric head with UNC threads. It's not that hard to change the tool that taps the threads on the Buick casting to whatever Rover wants so it's quite conceivable that they are metric. If the heads are stamped 8.8, 10.9 or 12.9 then they are metric.

If the heads are 9/16 or 1/2 then they'd probably be 3/8-16 or 5/16 respectively but they could have an odd head to shank combination to provide clearance.

 

3/8x16 UNC if you mean the ones that fasten the manifold to the head.

 

Thanks Tom