Marine Compound Engine

February 2010.

John R. Bentley 2010.

Some Excitement
While Tapping the LP Bottom Cover
-  for the Stuart Compound Launch Engine  -

Yessir ! - that's a LynxTM 333 high speed dental handpiece digging out a broken 7BA tap.

I don't often break a tap.  This one was used for the sixty-odd holes in the engine's cylinder block, not to mention many others previously.  I can't blame anybody else - there was no one but me with a hand on the tap at the time.  I am a bit miffed that the plans call for simple mounting holes to be made to a depth within microscopic distance of the stuffing box cavity, when it is not necessary.  The drawings say: "Tap 7BA x 3/16" deep".  They also indicate that the face for the crosshead slider should be milled to a 3/8" distance from the center of the piston rod hole.  The rest is up to the shape of the casting.  I would strongly advise anybody faced with this task to simply shorten the four screws, drill the holes exactly 3/16" (0.1875) and tap down as far as is practical.

It was my own responsibility to ascertain that the suggested task was reasonable before embarking.   I didn't do that.  Instead I tried to tap a 3/16" blind hole exactly 3/16" deep. ...I guess "blind" is the key word - I blindly followed an impractical instruction.  When a child, I once took a friend's bad advice and used it as my excuse for a error of some consequence.  Under the interrogation by my dad which followed he curtly asked, "if someone told you to jump in the lake, would you do it?"  He said no more then, and neither will I now.

These British taps are expensive to acquire in this country and I regret losing this one.  I am travelling to Britain in a couple of months and you can bet that my first shopping stop is not going to be the postcard kiosk (unless they sell 7BA taps).

The Repair:

The LynxTM 333 dental handpiece operates at 350,000 RPM using about 30 psi of air. To power it, a compressor of 4 CFM or more is advisable and a decent size reservoir tank would be an asset.  My little compressor has only a 1.5 gallon tank and hence the unit starts quite frequently - the noise can be startling and this type of work is as much guided by the sound of the diamond burr cutting inside the hole as it is by the visual scene.  The handpiece sends a nice jet of air to the tool tip for cooling and to keep the work free of debris.  It was a very useful instrument to have on hand in this case.  It took two or three hours of drilling to totally reduce the remnants of the tap to dust.

After grinding away the threads in the hole, I decided to give it
some regularity by passing a 1/8" straight reamer down into the hole

I found a small triangular scrap of cast iron to make a thread insert

Turning a length to size (about 0.126" DIA.)

Providing a pilot hole

Drilling prior to tapping

Threading the hole 7BA

Checking the screw fit - just to be sure

Ready to press in the insert

I used the drill press vice to press the insert into position

I covered it with threadlocker before pressing

There! - "good as a better one!"

NOW more of that foolishness!
- I'm milling the screws to a practical length

I drilled and tapped a piece of 1/8" sheet and milled the screw ends
- just proud of the sheet

All done.

The crosshead slider plate is held nicely in position
by the shortened stainless flathead screws

I should note here that this is only one very minor point in an excellent set of castings and drawings containing hundreds of dimensions.

The plans that I have are not actually wrong in giving this instruction, but do unnecessarily require a level of precision for a simple fastening which is unusual in the course of making the engine. One expects this sort of thing around the valves and various other critical areas of the engine. I feel it would have been better all round if they had either supplied shorter screws - or simply indicated that the builder should shorten them. There are ten threads on the screws, reducing that to six or so should still provide full strength from a 7BA screw. Checking how few threads are engaged in the cylinder block for the cylinder cover studs would appear to confirm this statement.

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(c) John R. Bentley 2010.