Hi.
If changing the original press in 1/2" oil galley plugs over to the screw in type, what actual screw in plugs are used - and who sells them?
Looks like it needs to be at least 9/16" UNF to be able to tap a full height thread?
Thanks in advance for any help.
They are 1/4" pipe plugs, which are threaded with a NPT tap, not a regular bolt tap. You can get them with an allen head at just about any auto supply house that sells standard hardware.
Make sure that the one that lines up with the distributor is tapped and seated far enough in to clear the distributor. You can probably guess how I found that one out!!
-Fred-
Thanks and now understood on NPT threads. Thanks for the tip on the distributor as well.....
However, on the exact sizes, for a 1/4" - 18 NPT thread the major diameter is listed as 0.540" and the minor diameter at 0.442". Now, the holes in the front of the block that take the original 1/2" diameter press in plugs are obviously already 0.50" bores. Therefore the minor diameter of the NPT thread after tapping will be 0.50". That doesn't sound like enough thread.
If you guys say that you simply run a 1/4" - 18 NPT tap into that original 1/2" hole then I'll give it a go, but just wanted to double check.
The pilot hole for most 1/4" NPT taps is 7/16".
Ok, now that makes perfect sense as a 7/16" (0.4375") pilot hole for the 1/4" NPT thread would be just under the minor diameter of 0.442". But that's not possible in the forward oil galley holes as they are already 1/2" bore.
Based on that, a 3/8" NPT screw in plug would be better as it has a minor diameter of 0.578", above the 1/2" bore, so it could be tapped properly to a full thread height.
Maybe the stress on this thread is actually quite low, so over the years if everyone has just been using 1/4" NPT without any problems then it's probably fine to do just that.
I've seen mentioned that some of the Shelby and BOSS blocks had screw in plugs from the start of their life. Out of interest, does anyone know if they were retrofitted afterwards or if extra material was left in the block? You could easily tell if you had one of these blocks by just measuring inside the threaded hole. If it's 0.50" then it will be retrofitted, if it's 0.44" then they must have left extra material in.
Make sure also you get a bottom tap as well as you can start the threads with the standard tap but you'll need to finish with the bottom tap.Also only tap the hole back enough that the plug you use is flush with block as that's all you need for clearance and blow them out good.The top hole of the gallery that the lifter goes in be extra careful as it only takes a few threads and you can ruin that hole.Just a word of caution...Good Luck!
Don’t have an account yet? Register yourself now and be a part of our community!
