Mindsigh

Postie bike engine strip-down

Posted by jerry on February 25th, 2007 — Posted in Journal, Motorcycling

Well, after about six years faithful service, my daughter’s postie bike (Honda CT110) began getting hard to start, and when it did, it started blowing smoke. I checked the compression and realised that the engine was down to about one-third the compression it should have. The dial read about 32 psi, when it should have been over 100. With a couple of weeks to go before my daughter was due to head overseas we just poured in some treacle-like ‘smoke free’ which brought the compression up to about 42 psi – still very low.

With the bike in storage I decided it was time to find out what the problem really was. My suspicion was a broken oil ring on the piston. I spent one evening last week removing the footpegs, bash-plate sub-frame and the exhaust pipe.

And today I set to work in earnest. It took less time than I expected. I began by removing the carby (two 10mm bolts) then followed the Clymer manual beginning with ensuring the piston was at top dead centre on the compression stroke.

I had also previously made a ‘piston holding fixture’ which you can buy as a special tool or make one from wood. The instructions were in the manual – for info you need a piece of wood 1/2inch x 1-1/4inch x 4 inches. and you just drill a 1/2inch hole in the centre and then cut away from one end until you have a fork – like this

And it worked perfectly.

But I digress… After removing the camshaft and with care to ensure that the cam chain did not slide down into the crankcase the head was removed. I used wire threaded through the cam chain to ensure it remained above the cylinder.

There was some carbon on the piston crown, and some deposits on the valves. The piston holding tool worked very well, but I needed to explore further. On removing the cylinder I found that the piston rings were quite worn, and that the oil ring was in three parts – clearly that is where the problem lies.

So it was time to remove the piston. This is secured to the connecting rod by two spring clips – not easy to see, but observing a small cutaway on the side of the piston, I noted a hint of a clip. With a pair of needle-nosed pliers it was a brief job to remove the clip from each side. I used a short piece of 1/2inch dowel to push out the piston pin and the piston was free.

After cleaning the worst of the carbon with a wire brush and fine emory paper I saw another issue – the top compression ring had at some point worn so thin that it had gouged the groove in which it sat, allowing for movement.

So it is likely we will need a new piston. But the bore seems unscratched and with luck I will be able to find a new piston to match. And that is a story for later in the week.

Meanwhile, there is a rather sad looking Honda CT110, with rags to prevent the entry of dust, sitting in my garage awaiting some new parts.

And the rest awaits reassembly on a nearby table

In the meantime, you can read more about this wonderful bike People actually race these things, and every year there is a Postie Bike Challenge in Australia where people ride these things from Brisbane to Adelaide (2005), or Ayre’s Rock (Ulluru)(2006). And this year 16-27 October the Postie Bike Challenge is from Brisbane to Cairns (over 3000kms) on some of the roughest and most beautiful country in Australia

And here is an online owner’s manual 🙂

Cheers
Jerry

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