TO35 manifold

Have an early 1955 TO35 I am reassembling after major teardown. Have replaced the exhaust manifold with new one from this site. Down flow exhaust pipe connection had adapter I removed to accommodate new exhaust pipe. 3 bolts holding on adapter were counter sink heads. New exhaust does have that feature, so am trying to use 7/16 NC thread to install new exhaust pipe. This is correct bolt and thread according to parts manual. Bolt will start about 2 turns and then resists further turning. Have tried finger inserting a 7/16 tap. It also stalls. Should I force the tap? Got me puzzled.
 
I believe the originals were fine thread, but you have an aftermarket so who knows? Could conceivably be metric. I wouldn't tap new threads until I checked the alternatives.
 
I've lost count, of how many times the MALE threaded bore has METRIC threads...

Same said for bolts, and Cap Screws...

Not all metric fasteners have markings on the fastener to identify as a metric thread.
cvphoto137638.jpg
 
Thanks evryone. Took original bolt to my son. (works at fire truck manufacturer) He's going to check it tomorrow for pitch. He agrees its probably metric.
 
Had this happen many times with aftermarket parts witch are made anyware from Bulgaria to China as they can only work to metric sizes. MJ.
 

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