Stuart Sirius from a machined kit

[smfr]

This will be a short build log, but I thought it might be interesting to document the assembly of a Stuart Machined kit from c. 1978. I bought this on eBay as a casting kit, but on opening the package it turned out to be machined, and was almost certainly purchased as a machined kit; the parts were still wrapped in brown paper sealed with paper tape, and the box was full of vintage shredded office documents.

The obligatory box photo:



And the kit unpacked:



Initial reactions: the crankcase and cylinder casting was really nice, and well machined. The aluminum base not so much, with one of the foot holes visible off-center in the lug, and some minor casting issues. Oddly one of the threaded holes in the crankcase was filled with some kind of hard glue, which I had to dig out with a pick and then chase the threads.

The piston valve shipped inside the valve chest, and was a snug sliding fit. None of the edges had been eased. The value chest was pretty flat on the mating surface, other than a minor ding which filed out.

The flywheel was nicely finished.

The connecting rods were only machined on the working surfaces so looked a little rough, but were a good fit on the crank. The caps had oil grooves cut into them.

The bearing caps were also only machined where necessary. The coupling side had a crosscut which raised a significant bur which had to be filed away to get it to fit, and the drilled holes also had raised edges that needed to be filed down. The bores seemed nicely perpendicular to the mating faces

The valve eccentric shaft was hardened and ground, and a very close fit to the reamed hole in the crank case. The gear was held on with a tapered pin, which had to be removed for assembly.

The banjo is a stamped metal part, and the holes on each end needed to be reamed to size; some straightening was also required. The eccentric end has a phosphor bronze bushing, and the other end a brass bushing.

The crankshaft was again only machined on the bearing surfaces, and the gear seems to be permanently attached (shrink fit?). The crankshaft only has a few thou of runout.

The pistons were well finished, but one needed a few tenths taken off with emery paper to fit in the cylinder bore. The gudgeon pins have copper ends to avoid scuffing the cylinder walls, which is nice.

There’s an extra gear, and an extra taper pin; I’m not sure what they are for. The dipstick (not shown) is a rather poor fit in the hole in the crankcase. There are inlet and exhaust fittings (inlet is 1/4-32). There are two blanking plugs for the drain cock holes.

The bit and bobs:


All fasteners are slot-headed screws.

So that's the kit! In later posts I'll talk about putting it together, and the fit and finish required.

[Roger B]

That looks to be an interesting little project 

How many 'unique features' can you find in a machined kit 

[Jasonb]

As all the parts would have been machined in batches to speed production I doubt anything will be unique, if there were an error it would likely be in a whole batch.

Following along as you don't see many write-ups of the machined kits going together. So interesting to see what "Fitting" is needed as there are bound to be a few tweaks needed as the machined to size parts are made to fit together.

[Charles Lamont]

I had one as a kid. I manged to crack a piston with ham-fisted insertion of the gudgeon pin. They were tight. A replacement from S-T was no problem.

[Chipswitheverything]

I can't remember from your other note on this engine whether it came with drawings and instructions. A builder working from castings might be assumed to know about tolerances and fitting, and interpreting the machine drawings for that matter.  ( Not that it always worked out quite that way, as some of the featured restorations of wonky efforts made by others in the past demonstrates ...! )  But I wonder if Stuart Turner provided any helpful notes to guide customers assembling the machined kits? Dave

[smfr]

Hi Dave. The kit came with exactly the same plans and parts list a a casting kit would, with no other instructions that would help someone assemble the machined kit! I feel like you'd still have to be a pretty experienced machinist to get a good result at the end.

[smfr]

OK, let’s start the assembly!

I already mentioned that one of the bearing plates needed a large burr removed to fit the casting, but with it cleaned up, they were a good fit, and seemed well aligned. The bores were a very tight fit on the crankshaft, so I made up an internal hone accurate to the slightly odd 11/32” size, and used some Timesaver to open up the bores a smidge.

I then turned this hone into an expanding mandrel by drilling down the center, threading, making a countersink about half way down, and turning down a flat-head bolt to fit into the recess, then slitting. The very slight expansion from tightening the bold was enough to hold the bearing plates for finishing:



I machined the external surfaces and outer edge which had been left rough, and tidied up the inner boss a bit. Later I’d use the mandrel to hold the bearing plates for length adjustment.

The valve eccentric shaft came fully machined, hardened and ground. Oddly they hadn’t machined the outer rim of the eccentric disk, so I tidied that up with a carbide cutter (it was hard!). It has a timing mark ground into it, and the plans show how to line that up when mating with the crankshaft gears. I knocked out the tapered pin to release the gear, and could then insert it into the main casting. The hole was reamed, but needed a little work with some Timesaver before the shaft span freely. I also had to countersink the top to allow for the radius on the shaft.

One inserted and the gear re-attached on the inside, there was about 20thou of vertical play in this shaft:



I wasn’t sure if this shaft was meant to float a little to let the gears mesh, but made up a bronze washer to take out the play. Initially I placed that on the inside, figuring that the banjo arrangement was aligned with the shaft riding on the top of the casting.



Then I re-attached the gear with the tapered pin, which is quite accessible:



However, on reassembly (once I had the bearing plates fitted, which came a bit later), I found that with the gear and shaft pushed down, the gear meshing pushed the crankshaft too far to one side. Crankshaft placement was better with the gear up against the inside top of the casting, so I moved the bronze washer to the outside under the head of the valve eccentric shaft. With that arrangement, the connecting rods were properly centered in the pistons (there was a bit of play there). The bearing caps also needed just a little adjustment to fit the crankshaft without binding, and no end-to-end play, measured like this:



The mandrel came in handy to take about 13thou off the seating surface of the plate on the opposite end from the flywheel. I also needed to clean up some dings off the inner end of the flywheel-side bearing, and taper it to avoid interference with the gear on the crankshaft.

Let me know if you think the valve eccentric shaft should just float.

Simon