I just tore down the top of the new TT340 after finding that in the first 1000 miles, the rockers were already starting to gall up. I had checked them because I was concerned that the studs may be blocking the oil passage to rocker shafts, as the studs are not relieved like the factory head bolts are.
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When I removed the valve covers, 3 of the rockers on one side had no oil puddled in the rocker, although you could see some oil had made it there, but not much. See the attachment. I took of the rockers and all were scarred up quite a bit, especially for so few miles.
I pulled the heads and did a flow test on the bench to see how much oil could get by the stud compared to the factory bolt. Depending on the location of the stud in the .050 oversize hole, flow could drop to as little as 15% of that of the factory bolt.
I relieved the area of the head bolt hole in the head between the two oil passages by about .060 and retested the flow, which went up to 85% of the flow of the factory bolt. It does appear that the studs can significantly restrict the oil to rockers, but it can be very variable, depending on where in the hole the stud locates, based on tolerances. The rollers that most folks run when they have a studded engine may be less suseptible and help explain why it has not been seen as a big problem by many.
I would recommend that if you are using studs, to relieve the headbolt hole between the passages, as it is easy and good insurance.