I have a 261 head bolt question. Evidently some 261s have a head bolt that is drilled for oil passage as I have read on other posts.I want to be sure what I think I have figured out.Someone who knows please confirm or correct me.According to the old Chevrolet parts book this special bolt is larger in diameter than the other head bolts in the same engine.1/2-13 compared to 3/8-13.Im figuring it will be obvious where the special bolt goes by one hole in the block being bigger.Right?Also if using a 235 head on a 261 will one hole in the head need drilling oversize or are the holes in the head big enough as is?Thanks.
Drilled 235-261 Head bolt
From 1938-62, only two head bolt part numbers are listed for 216, 235 and 261. Long and short except for a special drilled bolt, used when a 59-62 block is used with earlier top end* to reduce the oil flow to the rockers.
Note: Use with drilled bolt
Plug # 37 64 579
Install in block oil port near right center head bolt.
(This plug blocks the original oil path and sends it through the drilled head bolt)
* The cylinder head did not change but the rockers did.
What Dave's saying is that Chevy never used this bolt on any original engines. If the engine you have hasn't been molested, then It doesn't have, and will not need this special bolt.
If it has one of these bolts, it would be the center one on the passengers side of the engine, right next to the oil tube that goes to the rockers. I think they were marked with an "O" on top.
To be sure of the situation, check the year of your engine block. if it is a 59-62, then the rocker arms should have an oil groove that is offset to one side. If the groove is dead center, it has the older rockers, and would need this bolt and plug, OR preferably get the right rockers. The cylinder head itself has nothing to do with this. If it is a 54-58 block, then the bolt isn't needed and the rocker type doesn't matter as the flow is controlled farther upstream in the system.
If you have a 59-62, then take a look at a shop manual. It has some good info and pictures in it concerning how the oiling system works.