drill, tap hole size for Yamato 302 cooling?

[halestorm]

While I have the power head off my y302 tower, I thought I would add a hole in the back of the tower I've seen people use to facilitate flushing and cooling with a small electric bilge pump. This is where I'd put that hole, right? (see attachments) And what size? It seems someone once told me to use 1/4 NPT but that requires a 7/16 drill -- that's a pretty big hole! The hole I'm meeting up with is about 5/16 in diameter.

[dwhitford]

Why not use 1/8" NPT? Correct tap drill is a Q from an alphabetic index, 0.332" diameter.

[halestorm]

Well that's kind of what I was wondering. 1/8 NPT seems a better diameter but that's why I was asking, to see what other people have used and if it really mattered. So you've done 1/8 NPT? I suppose the only possible down side is that the thread pitch is so fine. 1/8 is 27 tpi while 1/4 NPT is 18, so they might be a little more durable.

[dwhitford]

Put in the brass or steel 1/8" NPT fitting into the aluminum ONCE, and with some good sealant, but select a fitting into which you can insert a plug -- perhaps 1/4" NPT -- that you remove and reinstall repeatedly for flushing. Doing the installation this way will protect the aluminum threads in the driveshaft housing.

"Put in the brass or steel 1/8" NPT fitting into the aluminum ONCE"...... right! but I would suggest using anti-seize instead of sealant so if you have to take the fitting out, the threads doesn't gall into the aluminum. The tapper thread of the fitting does the sealing.

[halestorm]

Where you say "fitting" do you mean bushing? I.E. insert a 1/4 NPT bushing into the tower and then insert a plug into the bushing when racing and a hose barb adapter (into the bushing) for flushing. Still, a 7/16 hole is pretty big and seems like it will just barely fit onto that flat pad if centered on the 5/16 channel in the tower. Would be good to see a photo of what you've done. Thanks.

[Steve77t]

Just put in a standard 8 mm x 125 thread and use a Konig water tube that is about 1/4 id, or use the 10 mm one that has about 5/16 or so size

[dwhitford]

Here are photos about how I routed water from my "hatchet" Yamato gearcase to my Quincy alky powerhead, a similar application to your problem. Turns out, I used plumbers' thin teflon tape while screwing in the initial fitting into the aluminum. So the advice you got about not using sealant is correct. My main point is to protect the integrity of the initially-tapped (potentially weak) threads into the aluminum. You can add numerous options for fittings after the initial installation into the aluminum.

[jswain3]

Hey Halestorm, get ahold of TJ concerning this matter. He's much more familiar with the 302 and common procedures.

[DeanFHobart]

I would second that.