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OK to Drill and Tap 1/4-20 Hole in Block for Spark Plug Heat Shield?
Hi Guys,
I am in the process of installing spark plug heat shields on my 350 crate motor and noticed that the mounting hole for the heat shield belonging to cylinders #6 & #8 was not drilled or tapped by the factory. The raised portion of the casting is there but no hole or threads. Was this an oversight by GM and is it OK to drill and tap the hole? I attached a photo for illustration but it is not of my engine, it is an engine that has the heat properly installed (photo credit John Z). Any advice or guidance will be appreciated!
Wow - if it were me I either wouldn't do it or go just deep enough for the bolt to 'bite' on a few threads. That bracket isn't really holding much besides a piece of thin metal after all.
I would think that so long as the factory boss is present, it would be OK to drill and tap. Just don't go too deep!
Are the other three bosses drilled and tapped? Should give you a clue. Check the depth if so. I believe that bolt is 1/4 -20 X 7/16 and I would probably reduce that. As others have said don't go too deep and use a depth gauge if you decide to drill.
Thanks Frankie and Swept57 and Randy. Not the ringing endorsement I was hoping for. I figure if the boss is there it “should” be OK to drill but they must have left it off for a reason, however the other three holes were drilled. Maybe it was a shift change?
The other approach I was considering is to fabricate a bracket from the starter brace. I guess also I could just leave the shield in the parts bin and worry about it another time.
Thanks again for your help and advice, I appreciate it!
Gotta admit there are a number of through -drilled holes on Chevy motors that should be installed with thread sealer anyway. I guess the worst outcome is ending up with one more.
I have the same issue with a GM crate motor with the same hole underlined and untapped. I decided to relocate the tab on the shield, because there is the additional 1/4-20 tapped hole adjacent to the untapped one, holding the upper end of the starter brace. I was too much of a coward to drill into the block.
Last edited by 62corvette; 09-27-2015 at 10:05 PM.
Reason: Spelling
If the boss is present then it was meant to be tapped. My only hesitation would be how straight I could drill. You can put a stop on the bit to prevent going to deep. If it were my engine, I'd drill it.
I have the same issue with a GM crate motor with the same hole underlined and untapped. I decided to relocate the tab on the shield, because there is the additional 1/4-20 tapped hole adjacent to the untapped one, holding the upper end of the starter brace. I was too much of a coward to drill into the block.
Were you able to bend the bracket to fit? I was thinking of using the starter brace hole but then I would have to add material to the bracket so that it would reach. This line of thought led me to ponder the drill and tap operation. Ughh! I was read to drill until I saw your message!
I see absolutely no problem drilling that hole to the same depth as the other similar holes.
Exactly, the only thing is to use a bottoming tap rather than a regular tap (bottoming, or plug taps are for blind holes and cut the full threads the whole way to the bottom of the hole).
The position of that hole is directly between 2 cylinders (6 & 8), the metal is at it's thickest point there. 1/4 -20 X 1/2 deep, no problem in that spot. Agree with Mike M, looks like the machine forgot to make that hole, it is present on all other 350's I have worked on in the past.
Were you able to bend the bracket to fit? I was thinking of using the starter brace hole but then I would have to add material to the bracket so that it would reach. This line of thought led me to ponder the drill and tap operation. Ughh! I was read to drill until I saw your message!
I had another old bracket. I cut the tab off that and brazed it where it would make the bracket fit the starter brace hole.
I have the same issue with a GM crate motor with the same hole underlined and untapped. I decided to relocate the tab on the shield, because there is the additional 1/4-20 tapped hole adjacent to the untapped one, holding the upper end of the starter brace. I was too much of a coward to drill into the block.
Mike, that hole for the starter brace bolt takes a 5/16"-18 x 1/2" bolt.
Hi Everyone, thank you for the advice, I was getting ready to drill and tap when I learned that there was not enough room between the engine and the frame rail for my drill to fit unless I purchased a 90 degree drill or jacked up the engine. So in the end, I took the advice from 62corvette and built a bracket. I am happy how it came out and don't think i will have problems with it rattling or coming out of position. I welded it on the backside. Thanks again!
09-27-2015, 01:30 AM
OK to Drill and Tap 1/4-20 Hole in Block for Spark Plug Heat Shield?
