Seagull Engine Construction Diary

Making a 10cc twin 4-stroke to the E T Westbury design


As discussed here, I am increasing the tappets from ¼″ diameter to 932″, keeping the original wall thickness. I intend to make them in silver steel, harden them and leave them hard, and to lap them to fit the bushes in the crankcase.

I am not sure which of the supplied materials is intended for the tappets. There are lengths of ½″ and ¼″ mild steel bar in the kit that I am not sure of the purpose of.

2015-04-25 – Lapping a trial tappet

I wanted to knock the bumps off the tappet bushes before fitting tappets, not to polish them, but as a sort of pre-run-in. I turned a piece of mild steel to 0.2813″, a bit too tight for a running fit in the 0.2815″ bushes, and worked it for a few seconds in each hole with some Brasso to give a smooth feel.

I turned a piece of annealed silver steel to 0.02816″ parted it off and faced it to the internal length, swore, decided to carry on with it as a test piece, chamfered, centred, and drilled it 4 mm, and opened it out with a 316″ slot drill to 932″ deep. After hardening, the test tappet is out of round, measuring a maximum of 0.2821″ on the foot, and a minimum of 0.2814″ at the rim.

I had put the piece of copper bar that I use for making small external laps somewhere I couldn't find it. Again. Having tracked it down to where it should have been anyway I made a lap and quickly reduced the test tappet to 0.2811″ diameter, which did not fully remove the machining marks, and which is a bit too loose a fit. I recently obtained a new ceramic oilstone from a market stall. I found it just right for polishing the flat face of the tappet. (4¾ hours)

2015-04-28 – Another trial

Having decided the tappets would be better with a radius at the root of the internal corner, especially if they are going to distort during hardening. I took one of the very first special tools I ever bought, getting on for 50 years ago, a 316″ D-bit, and ground it to cut a flat bottomed hole with a 132″ corner radius.

I faced and turned another length to 0.2821″ diameter, and tried to take another shave off, but the tool was just rubbing. I parted it off and stoned the face flat, though I found it very difficult to hold it so as to stop it digging in and tipping over on the oilstone. I faced the other end, drilled, then opened out with a 3-flute end mill, and finished the hole with the D-bit. I faced it a final length of 0.376″ and chamfered the outer edge. I used a wire basket to hold it for heating and quenching. After hardening the diameter measured from 0.2815″ to 0.2825″. I lapped it to 0.2811″ diameter, without removing the machining marks, and indeed not even touching the surface at one point on the rim. Although it fits smoothly in a tappet bush, there is a little shake when dry, and bigger would be better. (3 hours estimate)

2015-04-29 – And two more

For the next attempt, I used some silver steel without normalising it and tried a different tool as I had been getting a problem with swarf nests round the job. This one had 8° rake rather than 20°, and worked nicely for a while, then picked up. Turmed a piece to 0.282″, which is probably too small. I took a new cobalt hss tool bit (Cleveland MoMax) and ground it to 13° side, and 6° top rake angles, with only a small tip radius. This cut accurately, but produced a rather rough finish. I turned a second blank to 0.2825″, which is about what I am aiming for. Both parts rubbed smooth on the working face, and the other end faced. (1½ hours)

2015-05-19 – Finishing one good one

So, after a pattern making job for another project intervened, we have two tappet blanks, on of which, at 0.2825″ diameter, may be large enough to make a tappet. I clocked this one in the Griptru to 0.0003″ TIR, machined the interior as before, and faced to 0.376″ and chamfered the edge. After hardening and polishing, the diameter varies from 0.2818 at the least, on the rim, to 0.2825 at the most, at the bottom.

I used Loctite Bearing-fit to glue the tappet onto a mandrel of brass rod for lapping. The first lapping to 0.2819″ cleans up all round the foot. It is down to 0.2815″ by the time the top rim is reached in a couple of places, but 90° away the machining marks are hardly touched. At 0.2813″, the lap has taken a little off all over, but some machining marks remain. The rim is down to 0.2811″. At this it fits well in any of the holes. I parted off and drilled out the brass mandrel, picked out the remaining Loctite, and polished the top and stoned the bottom flat again to 0.376″ long. At the fourth attempt, we have a tappet.

Next, I faced, turned and parted off five more blanks. Two of these were too small and the fifth one snagged the chuck jaws on parting-off and disappeared. Unable to find it, I made another, undersize again. So far I have made a start on eleven parts to get four. I don't think I have ever had a job fight back this hard. Finally, I have four blanks, 0.2829″ to 0.2831″ diameter. I rubbed the business ends flat, and found another one bruised in parting-off, so faced and stoned the other end. Finally I faced the tops of all the blanks to about ½ thou over finished length. (6 hours)

2015-05-20 – And the rest

This morning I bored, chamfered, and hardened the four blanks, and polished their working faces. In the afternoon I started lapping. The first started at 0.2830″ at the foot and 0.2820″ to 0.2825″ at the rim. I lapped it to 0.2812/3″. The next one is 0.2828″ to 0.2822″ at the foot and 0.2822″ to 0.2814″ at the rim. I thought it would not turn out well, but although the machining marks are not entirely removed, it is generally pretty good. Two down, and time for a mug of tea. The third one is pretty consistent for roundness, but tapers from 0.283″ to 0.282″ from bottom to top. This one is probably the most completely lapped, but still the turning marks are not entirely removed.

The fourth turned out to be another balls-up, being 0.005″ over size. I am not going to try to lap that off. So, after more than twenty hours work, I have four acceptable tappets, and no spare. With a little oil on them they slide easily in the bushes with neither shake nor rubbing detectable, and they will not drop through under their own weight.

In cleaning the lap, I had to dig out a number of compacted bits of steel forming a partial ring near the centre of the width. (5½ hours)