Seagull Engine Construction Diary

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

Inlet Manifold

Surprisingly, this fiddly little casting looks as though it is pretty accurate. The passage will want a bit of careful treatment, as it is knobbly and has quite a bit of sand still adhering to it. This wants doing before machining to ensure the delicate sealing face does not get scratched. There is very little metal at the bottom edge of the passage, meaning the casting wants to be set as high as possible. But here is also very little metal to clean up the carburettor mounting flange, meaning the casting wants to be set as low as possible. Machining the face first, and then the flange to a just sufficient area will determine how this works out. The positions of the cast-in bosses on the face will also have a bearing on the matter. To get the flange an equal thickness all round, it will want supporting on narrow parallels. It may be necessary to make a cradle to hold it. The top edge will probably need machining to match the exhaust manifold. The top row of bolt holes will be taken from the exhaust manifold, spot-faced from the front afterwards.


2010-07-17 - Making a cradle jig

I decided a fixture was the way to get an even thickness. After cutting out a piece from stock yesterday, I milled the cradle all over today. The support legs are a bit on the thin side so I used a ball-nose mill to avoid a sharp section change at the root. (3¼ hours)

2010-07-18 - Facing the bolting face

Used the fixture to machine the face to finished thickness. As expected, the cast shape of the port cavity has resulted in a sealing face width that is a shade under 332″ in one place. (1 hour)

2013-02-02 - Investigating the Carburettor flange

While doing preliminary work on the Carburettor casting, and while I had the 4-jaw chuck on the lathe, I decided to investigate the manifold to see if there was any scope for increasing the bolt hole centres on the carburettor flange. Machining this close if not to finished dimension was just sufficient to clean up a face long enough for the original flange, and still a bit short on the width. Exactly how best to get the finished part out of the casting is still in question, and the next feature to contribute to the decision is the boss at each end for the outer-most bolts. These will probably give a bit of vertical latitude by cleaning up larger than necessary. If so they can be subsequently cosmetically dressed to be concentric with the bolt holes.

I have decided to mill the inlet tract, to get the full flow area, a smooth surface, and as nearly equal distribution as possible. I am thinking of using some JB-Weld to fill the deficient parts of the cast passage. (¼ hour)

2013-02-04 - Spot-facing the end bosses

To allow this casting to be further assessed, I machined the spot faces on the outer bolting bosses. As it turns out, they will allow very little scope for adjusting the casting position. The tops of the two inlet tract bulges look as though they will provide a datum for getting the casting level.

2014-09-29 - Milling the top edge

Still exploring this casting, I milled 20 thou off the top edge to get a datum, then after further careful measurement, I took another 0.0075″ to final size. (1¼ hour)

2014-10-01 - Drilling

Firstly, I made a pair of filing buttons for the outer holes. Using a collet, the 732″ silver steel ran out by only half a thou so they just needed facing, drilling and parting off.

Next I set up in the mill to do the mounting holes. Being a bit suspicious of the milling machine y-axis dial readings, I set up a DTI to check it. It is about 0.010″ out on the small distance from the top edge to the hole centre line. There seems to be something wrong, but I will deal with it when the mill eventually gets a proper strip down. I drilled the six holes in the top row, spot-facing the middle four, and then went back over the spot faces, taking another 0.005″. After drilling the bottom row, cleaning up and deburring, I tried the manifold over the studs on the partially assembled engine. The holes are drilled 2.8 mm so they have very little clearance on the studs, but, with a bit of wiggling, it fits over all eight. I am happy with that. Also the carb. mounting flange is accurately centred.

Comparing standard 6 BA nuts with ones having 7 BA hexagons, I conclude the manifold will look better with the small ones. (3 hours)

2014-10-04 - Filling

Looking at the joint face, the cored inlet tract comes close to one of the holes in the top row, with only about 0.020″ between. Taken with the shortage of metal above the carb. flange, this confirmed my decision to use some epoxy to make up the missing metal. I gave the manifold an ultrasonic clean and filled with JB-Weld. (¾ hour)

2014-10-07 - Tooling

I made some spacers to support the manifold on a jig for milling out the inlet tract. (1 hour)

2014-10-08 - Milling the inlet tract

Continuing to tool up for milling the inlet tracts in the inlet manifold, I made a 316″ diameter centre locating pin for the rotary table, and drilled and tapped the jig plate. The tapped holes are for mounting the manifold on the plate, and there are two 316″ holes to fit over the locating pin, positioned at the centres of the inlet tract arcs.

The milling went well, with the jig justifying itself in making virtually fool-proof an otherwise extremely difficult set-up. The external shape of the casting meant that the cavity needed to be 1132″ deep in the middle to fully uncover the inlet hole from the carb, but no more than 516″ at the ends to allow a safe thickness. I used a ¼″ ball-nose slot drill, and cut the left hand side first, using increments of 0.005″ in depth for every 15° of rotation of the rotary table. I found this looked too coarse, so I filled in the intermediate 7½°, 2½ thou, steps. The right hand side I cut with the smaller steps from the start, and it looks a bit neater. Even with the jig, this all still needed concentration.

The last job was to rub the face flatish (final lap later) on some 1200 grip paper to remove the small amount of epoxy projecting from the face. (5¼ hours)


2014-10-09 - Finishing

I worked on smoothing out the machining bumps in the inlet passage. The best tool I found was abrasive paper wrapped round a match stick. This will need more work to produce a nicely polished surface.

I hardened the 732″ filing buttons, finished a special bolt to fit tightly in the 2.8 mm bolt holes, and made a start on fettling the round the end lugs, finishing to the final overall length. (2¼ hours)


2014-10-18 - Finishing

I did a little more filing round the right hand end lug, and started on filing buttons to make filing templates for the carburettor flange and exhaust manifold end flanges. More of this work at the Exhaust Manifold. (2¼ hours)

2014-10-18 - Drilling the carburettor flange

After more work on the flange templates, I drilled the port and mounting holes in the carburettor flange, and milled a finishing cut of 0.003″ across the flange. Using a916″ end mill at 800 rpm, the finish could have been better.

2014-11-11 - Fettling

After finishing shaping the exhaust manifold earlier today, I had a little time to get back to the inlet manifold. (¾ hour)

2014-11-15 - More fettling

Today I was mostly working on concave shapes round bolt hole bosses, and on refining the shape of the inlet tract. (1¼ hour)

2014-11-21 - More fettling

I continued working round the right hand side, and in the concavity at the top centre. The right hand side is now almost done. (4½ hours)

2014-11-29 - More fettling

This morning I finished work on the inlet manifold. I think it looks a lot better than it did, and is ready to be bead blasted. (2 hours)


2015-01-24 - Bead blasting

Both manifolds will be blasted together, so I have dealt with this at Exhaust Manifold.