Tap and die approach for Vintage OMC

[chri0891]

As I move through the carb>powerhead>magneto>lower unit on my two vintage (late 50s) OMCs I've been finding several bolts/threads in need of TLC: gunked up, rough, and now a lower drain plug looks cross-threaded...

On the 'cheap' - considering this and an old Lawnboy are the extent of my wrenching and the primary need here is to just clean up threads, not cut new - a couple options appear to be:
- ~30pc SAE tap & die set, though it probably doesn't have all I need, and other sizes I don't
- Buy individual tap/die as needed as bolts are pulled and sized, but I don't know I have the patience/time to go this route
- Try the youtube method of buying a new bolt to fit the various thread, grind two slots in its threaded end, and use it to ream out the hole. Seems to good to be true...

It'd be of great help if the Johnson service manual had a quick reference to bolt sizes, but alas I see none.

What are other folks doing?

-Brent

 

[F_R]

The vast majority of the fasteners on motors of that sort are 10-24 1/4-20, and 516-18. Those three taps will take care of over 90% of the needs you need. Wanna know something? In all my life, I've never owned a complete tap and die set. Sure down through the years, I've run across something else and had to add to my collection That 3/8" for your gearcase plug might be an example. As for dies, I still have only a few. Clean up your bolt threads on a wire wheel on a bench grinder that you put the tap and die set savings towards. Be sure to use eye protection with that wire wheel.

 

[jakedaawg]

What you really want are thread chasers, not taps. Some slight differences. Taps are for cutting new threads, chasers are for repairing old threads. Many old timers will argue and say they've never used a chaser, my response is that they used to bleed people with leaches...

 

[Vic.S]

Thread chasers are the proper tools for cleaning up existing threads. With a tap or a die there is always a risk , especially with a taper tap, that you will start to cut a new thread. That is far less likely to happen with a thread chaser.

If using a tap to clean up threaded holes use a plug tap because that is not likely to start to cut a new thread.

 

[jonesg]

I've made a chaser from a new bolt, grind a taper and cut some vertical slots with a dremel cutoff rotary blade.
The taper allows you to reach inner threads first, bypassing the bent thread at the start of the hole.
It worked good.

 

[chri0891]

Great advice - thanks all! I'll start with those few chasers, and as F_R suggested look for the wheel and extra pair of safety glasses.

 

[oldboat1]

Depends on what's messed up and where. You might find a stainless bolt, and try that as a chaser. Some marine grease, and line up/square up the bolt very carefully.