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Vintage Engine Restoration

What do you do if you can't buy those vital spare parts which your old bike needs? Humbernut has the answer, and it involves a milling machine, an oxyacetylene torch and boiling up the occasional piston...

Do most classic motorcyclists out there realise how lucky they are to be able to buy spares for their mounts? If only they realised! Owning a couple of really old Humbers has made me jealous of the classic riders who are able to buy more or less anything they want, to enable them to keep their bikes on the road.

Probably not listed in the M&P Catalogue.

As you may be aware from my article about my 1927 350cc sidevalve Humber, I cover a fair amount of miles on it. It has been totally reliable for me, and always gets me home after a run. I want it to stay that way so I decided to build up a spare engine so that I could retire the original and re-condition it as a spare.

I had a spare pair of crankcase halves, and a badly battered cylinder / cylinder head without valves. It had many broken fins as well as the exhaust stub missing. The cylinder was internally in good, relatively unworn condition, and only required a clean up with a cylinder hone. The valve seats also appeared to be in very good condition with no signs of pitting.

For many vintage machines, you can't just obtain these sort of things mail-order from your average classic spares supplier. The parts hardly ever turn up in a condition where they can be fitted to the bike without having to do any work on them. The only way to get my spare engine up and running was to make or repair the required components.

Cylinder Restoration

The cylinder had been bought for a fiver at an autojumble and was given to me by my son-in-law to see if it was fit for restoration. As already mentioned the bore was in good condition but a rebuild depended upon me being able to repair the broken exhaust stub. I made up a new stub from a lump of cast iron, and cut away the remains of the original one from the barrel in the milling machine. I used some bronze brazing rods which I bought from Halfords at an expensive 9, and brazed the new stub into position using oxyacetylene brazing equipment.

Heavy Metal
View showing repaired/replaced exhaust stub

The results were encouraging, and after blending in and painting you wouldn't know it had been repaired. I then cut some discs of cast iron and filed them to shape to replace the various broken fins.

The results were really good and by now the cylinder was looking like new so, heartened, I proceeded to think about the valve gear.

Valve Gear

Anyone remember the mystery object bit of 'Ask the Family'?
View showing modified oversize valves

I scoured the autojumbles for valves but couldn't find any that were suitable so I decided that maybe I could re-cut some larger valves to suit on my recently acquired tool and cutter grinder. With the larger valves purchased for 2 each I re-ground the heads to the correct diameter and profile, and ground the shoulder in the stems for the valve collets.

I knocked out a couple of valve guides from some scrap cast iron pieces and fitted these to the barrel, then ground in the valves to produce a nice thin line on the valve faces which showed them to be seating properly. New collets were made from mild steel along with valve spring cups and retainers. And that concluded work on the cylinder!

Did he just say that the inlet pipe was one he prepared earlier?
View of completed cylinder with valve gear fitted and new inlet pipe which had been made earlier to this restoration as a spare

Piston & Rings + Gudgeon Pin & Wear Pads

There are reasonable supplies of pistons available for most British bikes of the Sixties, but try going back to the late Twenties -- things become even more scarce! Despite all efforts it was not possible to find a piston to fit the cylinder.

I have made 'model' engines up to 100cc capacity per cylinder and have made the pistons for these with success. So I thought I would have a go at making one to suit the Humber cylinder, as I had an oversized one that I could use as a pattern to copy.

The problem of expansion and how much clearance is required is difficult when it comes to pistons made from normal alloys of aluminium. I estimated a couple of thousandths of an inch clearance should be OK with the piston at working temperature. But what is 'working temperature'? At a guess I decided that it would be about 100-degrees Celsius. I machined the piston close to finished size and then placed it in boiling water to heat it up, then measured the expansion. I heated the cylinder up to a similar temperature and measured the expansion of that. From these two figures I was able to decide how much material needed to be removed from the piston to give the required clearance at working temperature.

There are bound to be purists out there who will say this is not the right way to do this and will go into great mathematical contortions to prove that I did it wrong! All I say is... how many pistons have they made recently?! I'd also remind them that we are talking steam age machinery and practices, not up-to-the-minute oval machining of pistons with relieved skirts, etc.

I have made piston rings with success before, so I made a set to suit the piston and bore. The one thing I am still not sure about is how to estimate the required gap when placed in the cylinder bore, so once again a guess at about 5-thou was made. Maybe someone can tell me if there's a better way?

A new gudgeon pin was made and case-hardened, then trued up with an external hone. The bronze wear pads were a straightforward job. With these parts made, my attention was drawn to the bottom end of the engine and what to do there.

Hands up everyone else who has made piston rings before...
View showing old piston used as a pattern with new piston, rings, gudgeon pin & wear pads on left

Crankshaft & Con Rod

I had obtained a pair of flywheels with the crankcase which were in reasonable condition but the main journals were badly pitted. A few months prior to going ahead with the restoration of the engine, I decided that I would make up a spare set of journals to suit the sidevalve, along with a new crankpin, while I was making a set of journals for the OHC Humber which I am restoring with my son-in-law. The idea was to keep some as spares, and also to get a batch heat treated and ground to size professionally. This was done and so they were readily available to enable me to make up a new crankshaft.

The only thing I didn't have was a conrod, so... you've guessed it! I set to and made that as well Machined from a piece of 2" diameter EN9 bar, it was a relatively straightforward process (if a little wasteful on the material side). I am sure that the rod will prove strong enough in service as EN9 is a fairly tough bit of steel. Again, the purists may attack by saying that it should be forged from steel but we are dealing with a low output engine, here not a 10,000rpm rice-burner!

The big end bearing on this engine is a packed bearing assembly, which uses two rows of rollers separated by three hardened and ground spacer washers, one in the middle and the other two on the outside against the flywheel. These were easily made on the lathe, and were heat treated and finished off using a flat surface and varying grades of wet or dry paper to get a good finish on them.

Mmmm.... Shiny.
Home made crank pin and con rod

All that remained was to make a little end bush and fit it to the conrod, and manufacture two new main bearing bushes and fit them to the crankcase [All? RealMart]. With this done I then built up the crankshaft. After much profanity and a lot of tweaking, I finally got the crankshaft assembly fully bolted up and running as true as I could get it.

With it installed in the crankcase, I was able to turn the engine over -- only just -- by hand. The whole assembly was run in the milling machine at 200rpm, with the spindle set horizontal and the conrod supported using a rubber bungee, until it was all running reasonably freely.

How much time went into the parts in this photo?
View of piston/con rod assembly and spacer washers with rollers in foreground

The First Trial

I had all of the rest of the bits and pieces required laying around as spares (like cams, timing gears and timing covers), so it was time to put the engine together. I decided I would get the various parts I'd made for the outside of the engine nickel plated to make the engine look pretty, so I sent them off to be plated.

I got them back a few weeks ago so I was finally able to assemble this engine and swap it over with the original engine.

I finally got the engine swapped out last weekend after a lot of messing round getting the valve timing right. (The timing marks on the cam gears were 180-degrees out...).

At last, I was ready to try out the new engine on the Monday.

I started up the bike first kick. The difference the correct profile piston has made was evident immediately in the exhaust note, which was much more crisp than before. I set off into the unknown, wondering how things would go. Again it was apparent that there was an improvement in performance with much better acceleration -- and again, that nice crisp exhaust note. It was also evident that the advance/retard on the magneto was having much more influence on how the engine runs than it did previously.

Great ! I was happy.

About a mile from home my state of bliss was spoiled when I felt the engine tighten up and seize. I got the clutch lever in and the engine died immediately. I let the by now very hot engine cool down and gave the kick start a prod. She burst into life and I set off for home, only to have the engine seize after a few hundred yards. I pushed the bike home (the first time it has let me down) and locked it away in the garage where she will have to wait for my return from offshore to investigate the cause of the seizure.

Am I disappointed, angry or downhearted? No. I am really chuffed because I know it will not take much to sort out the problem -- it will be a simple matter of either increasing the ring gap or taking a couple of thou off the piston, and I will have the bike running again. At least I know the conrod is strong enough to withstand a seize up without snapping, and the big end bearing is in good shape.

I will keep you posted on the investigation as to the cause of the seizure and how things progress with the engine once I get it running again. In the meanwhile, the next time you pick up the phone to order that hard-to-get part, think about the many people out there who have to make it themselves!

Homemade Happiness?


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