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Sidevalve Seizure Sorted

Humbernut had just taken his newly-restored 1927 350 sidevalve for a spin, when it seized! Here's what happened once he got its top off again...

I promised at the end of my article on the restoration of a 1927 350cc Humber sidevalve engine that I would let you know how things went with the inquest. You might remember that the engine seized after doing about a mile. After the article appeared in This Great Ezine I received a lot of good feedback through the Message Board about it. I also was given the names of a couple of websites which detailed how much ring gap there should be for a range of different size bores. First of all I must say thanks to everybody for the feedback and advice - RealClassic assistance!

Strip Down

I had suspected the ring gap to be the most likely cause of the seizing but I couldn't rule out the piston having insufficient clearance. The only real way to find out was to strip down the top end of the engine.

Now this is where us vintage boys have the upper edge on you modern classic riders, even more so if the vintage engine is a sidevalve. These old things were designed to be stripped down at the roadside. It's simply a matter of removing the spark plug, taking off the exhaust pipe, taking the carburettor off, and then loosen the four nuts which hold the cylinder to the crank case.

Sound effect of sink plunger being pulled away from blocked sink

The whole cylinder and cylinder head assembly are lifted clear of the studs and, with the crank at bottom dead centre, the cylinder is lifted to reveal the gudgeon pin in the piston. Push out the pin and push the piston back up the bore a little - this then leaves sufficient clearance for you to be able to remove the whole assembly from the frame.

The whole thing takes less than 30 minutes.

What Did I Find?

Surprisingly little, to be honest. There was some sign of wear on the piston but it was evident that there was sufficient clearance, as this wear did not extend round the piston for 360-degrees. I am sure if the piston clearance had been the problem there would have been scoring through 360-degrees.

The rings were bright on their outer surface and showed some wear but no scoring.

They were removed from the piston and the ring gap checked. This was approximately .005"-.009" which, from the information given by Nigel Barraclough and also corroborated by a commercial piston manufacturer's site, was obviously insufficient.

While I had the rings off the piston, I compressed one to close the gap to see how well the opposite ends seated together. As I compressed it, it snapped. I think that the ring must have been partially broken due to the stresses it had been subjected to when the ring expanded in the bore and became tight.

Just to round things off I dropped the piston after I had put the rings back on it, and I managed to break another ring!

'It's a good job they are easy to make...'

It's a good job they are easy to make, so much so that it pays to make two or three sets of rings. (Vintage bikes with total loss lube systems don't have oil control rings). I had enough spare parts so all that had to be done was to gap the rings at 0.016" [RealMart: The original said .0016" which would be a Very Small Gap Indeed, so I've guessed it should have been 16 thou - let me know if I'm wrong].

Rebuild & Results

Rebuilding is carried out in reverse order to the strip-down. However it is a pain in the neck to fit the piston to the conrod. You have to hold the weight of the cylinder assembly with one hand, while you try to locate the conrod in line with the gudgeon pin hole, and push the gudgeon pin home in the piston. It's one of those jobs which I find goes well if a little profanity is used as you guide the pin home. I mustn't have used enough the first time, because I managed to push the pin through the fresh air either side of the conrod. I didn't find out until I kicked the bike over and wondered what the strange clanking was, coming from the crankcase. The little end of the conrod was flailing about, connected to nothing...

An extra dose of profanity worked wonders and the second time around I managed to get the gudgeon pin through both piston and little end. The four nuts were tightened and the valve clearances checked. The exhaust, carburettor and spark plug were fitted and the bike was ready to try again.

I wasn't going to leave anything to chance so I started the bike and let it run for one minute. Next time I started it up I let it run for two minutes. I let the engine cool between each run, until I had the engine running for six minutes - by which time it was getting very hot and not showing signs of seizing.

Convinced I could at least get round the block without trouble, I set off and went to get some petrol at my local filling station which is only about 400 meters away. No problems.

I let the bike cool down again, then set off on one of my favourite shorter routes of about 10 miles. I stopped in a car park after three minutes just to feel whether the engine was hot. It felt not too bad so I set off again without switching off the engine. I completed the run without any problems and the engine was still fairly cool when I got back.

The bike is much more responsive now the correct profile piston has been fitted. I am sure that, once I have run it in for a few hundred miles, it will have a better all-round performance than it did before the rebuild. That blissful feeling that disappeared so quickly when the engine seized was slow to return, but with each mile covered on the first test run a fraction more returned until, at the end of the run, it was fully restored.


Making your own engine parts is worth the effort and it is very satisfying knowing that you aren't so dependant on the autojumble for those hard-to-get bits. I am as pleased as punch with the bike's improved performance so far. Dare I say it... but... maybe the extremely rapid 50mph barrier can be broken when it is run in. I will have to wait to see if my crankshaft components can stand the test of time, but I see no reason why they shouldn't.

That's about it for the time being but watch out for future 'How to make articles' which I am now preparing for RealClassic!

Probably not listed in the M&P Catalogue.



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