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10th December 2004


Classic Techniques: Building An Engine

We've said before that restoring really old bikes is more like re-making them. Humbernut proves the point with his painstaking rebuild of a cammy Humber engine...

About three and a half years ago I bought a half-share in a basketcase 1928 Humber 350cc OHC, along with my son-in-law. There were lots of bits missing including the crankcase and lower spiral bevel pinion. One of the few things I have not been able to master in the home workshop is the cutting of spiral bevel gears -- it is a very specialised process and there are few gear cutting companies in the UK who will cut them. Until I found a company that would do some new ones I could not make a start on the engine.

After a lot of searching I found a company called Guest Gears who would cut me a set. In order to cut down on costs they agreed to let me provide the blanks and they would cut the gears. Armed with the knowledge that I could get the spiral bevels cut, I set about making a new crank case, cutting it from the solid. I also made up a new crankshaft from components made by me in the workshop.

If I was making a crankshaft, I'd probably use sticky tape too.

The crankshaft almost ready.

As things were progressing too slowly for my liking I decided (on impulse) at Stafford in 2001 to buy a 1927 Humber sidevalve 350 in running order. The following year I entered this bike in the Banbury Run. It was while on this run that I met up with Phil Heywood and we tagged onto each other on our Humber sidevalve 350s.

Later that year Phil contacted me to say he had also purchased a 1929 Humber 350 OHC from Germany and there were a few bits and pieces missing. Among the pieces missing was the lower spiral bevel pinion. Fortunately I had already ordered three sets of these spiral bevels to be cut from the blanks I supplied. These are expensive gears to have cut, working out over 300. My reason for ordering three sets was so that I would have a few spare sets just in case I made a mess of cutting the splines...

Swarf. What a good word.

Cutting the splines in the spiral bevel gear.

Phil asked me if I could sort his engine out for him and I agreed. Meanwhile, Peter Daniels from Devon also asked if he could have a pair of spiral bevels for his 1930 bike. I was beginning to wish I had ordered five or six sets! It certainly put the pressure on me to get the machining right. However the gods must have seen fit to let me work without problems and all three sets turned out fine in the end.

Peter sent the bottom end of his OHC engine to me and I fitted the new spiral bevel gears. Once I was happy with it I sent it off to Peter who is continuing with the restoration of his Humber with his father.

Both our engine and Phil's were missing pistons. Humber 350 OHC pistons are rare and I know of only one substitute and that is from a Calthorpe -- and this is just as hard to find as the Humber one anyway. As I had already made a new piston for the sidevalve engine I rebuilt and it had proved to be a great success, I decided I would make up three pistons complete with rings, gudgeon pins and wear pads.

Mmm... Shiny. Note the complex shape of the piston crown.

Piston mounted on fixture to enable machining of outside after finishing inside.

Phil had also asked me if I would convert the drive side plain bearing bush to needle roller. This was relatively easy to do and if it proves successful I will probably convert my Humbers to the same arrangement.

When I went to remove the old ball bearing from the camshaft drive side of the crank case I found it was a loose fit in the crank cases. Fortunately there was sufficient meat to bore the housing oversize and fit a steel ring inside it to take the new bearing.

With this done I then set about building up the crankshaft using a homemade crank pin and journals. A new steel bush was fitted to the conrod and a new set of rollers inserted to form the packed bearing assembly. The crankshaft was bolted up and set true on the setting jig, prior to boxing up the bottom end of the motor.

There then seemed to follow a period of inactivity as the summer season of vintaging got under way. However in August I decided I would get it finished as I needed the bench space and it was high time Phil got his engine back.

I started by looking at the cylinder head; there was a lot of work to be done here. First of all I had to make new valve guides and fit them. Next came the problem of what valves to use. A scout about at an autojumble found a few valves that had long stems and slightly larger heads, ones which could be modified to suit. This was done and a quick check revealed that the valve guides and seats were set spot on, so there was no problem grinding in the valves.

Vintage Stuff on eBay.co.uk

I then made new valve spring cups and collets to suit the valves and fitted the valves in the cylinder head. Things were starting to take shape. The remaining items to be made for the valve gear were the wear pads for the rockers and the caps that fit over the ends of the valve stems to prevent them from mushrooming.

A whole new set of engine bolts and screws were next to be made followed by the timing chain case which was cut from solid billet.

From solid. Wow.

Gleaming new timing chain case cut from solid billet and hand finished.

There then followed a whole host of small bits and pieces including the two exhaust clamping nuts that are a prominent feature of this engine.

Can *you* spot the blue gasket goo? (Hint; picture 3)

The finished engine (note the exhaust clamp nuts). I must remember to remove the excess jointing compound next time I pose an engine for a photo! No worries about blocking oil ways with this engine -- tough, there ain't none.

Even though I say it myself, I do think this engine looks great!

1929 Humber 350 OHC Engine Rebuild Work List

Crankcase: Clean both halves and inspect same; machine drive side bearing housing to suit needle roller bearing, manufacture top hat plain bush to suit inboard end of drive side half; found timing side ball bearing to be loose in housing; manufactured steel bush to suit larger bearing and machined crankcase to accept new bush; polished both crank case halves and fitted new bearings.

Crankshaft: Split crank shaft; made new crank pin and securing nuts; had pin hardened and ground to finished size; fitted new drive side main journal; fitted new big end bearing and assembled shaft; made spacer washers for big end; packed bearing assembly.

Cam drive: Manufactured gear blanks for lower spiral bevel gears; machined splines in gear shaft to suit muff coupling; cut keyway in pinnion; had gears hardened; ground square shouldered groove to accept c-clips in place of old style spring retaining ring.

Cylinder: Honed cylinder and repaired damaged fin; cleaned out threads in cylinder head hold down toggle bolts; manufactured piston, rings, gudgeon pin and wear pads; fitted new small end bush to conrod.

Cylinder head: Removed broken grease nipple from exhaust valve guide bore; built up metal by brazing and re-tapped hole; re-manufactured two valves from larger new valves; manufactured upper and lower valve cups and collets; manufactured two new valve guides and fitted same; drilled grease holes in guides once fitted; cleaned and checked cam box; fitted new bearing to cam box; machined old bearing in oil pump to correct length; made new face plate for oil pump to replace badly corroded one; fitted gears to pump and tested same; made new bypass needle valve for oil control to cam box; made new woodruff keys for rocker shafts; fitted rockers; made new pressure washers and bolts for rockers; made new wear pads for valve stems; made new rocker ends; heat treated both wear pads and rocker ends; made two new exhaust clamp nuts to suit thread on cylinder head.

General: Made new cylinder head bolts and cambox securing bolts; made complete set of screws for securing timing chain case and cambox halves; made new set of long bolts and nuts for securing engine in frame; made complete set of new studs nuts and washers for securing crankcase halves; made new set of cylinder hold down studs, nuts and washers including one special for larger thread tapped in crankcase; made new timing chain cover; made new drain plugs for cam drive gear and crankcase; made new oil fittings for oil pump inlet; outlet, overspill from cam box, inlet to front of crankcase and oil breather; made new non-return valve for overspill to crank case from cambox; made new solid copper cylinder head gasket; made spacers for straight cut timing gear on cam side main journal; made valve lifter cable mounting bracket.

And that's it: finished!

Anyone else Building rather than Re-Building?


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