Early in 1997, the decision was made to build the special. I had sold the Number 1 to finance it so it was all systems go. In spite of being told not to, and knowing that Honda had abandoned 4 stroke trials bikes back in the eighties, I knew better! I convinced myself that I could build a special to compete with all the latest monoshock 2 strokes, in a shed, with a budget of about £250! At this point you need to realise that I hadn’t actually entered a trial of any sort. I’d only ever played at trials on practice days etc, but I knew it all.....( Who said, He who never makes a mistake, has never done anything...?) I had been persuaded to by a spare frame when building the first TL and I literally stumbled upon an SL125 rolling (sliding?) chassis at a car breakers yard. This has very similar forks and other parts to the TL. So the die was cast...
A friend of mine was building a much more ambitious project off-roader with a 125 Honda based engine bored and stroked to 200cc plus, so some of his ideas were stolen, which also meant that his was the guinea pig. A lot of expensive mistakes could now be avoided!
Bottom end.
I have always been a great fan of the Honda CG125 pushrod engine. For a small unit, it produces torque from tickover speeds, has lots of flywheel, runs forever, and fits straight in the TL frame. It has been said that it was designed for the third world market to only need an oil change once every civil war! It would also allow me to 'stretch' the barrel upwards without worrying about cam chain length, as this can only be adjusted in 2 link steps; pushrods can be made to any length. A complete bike was purchased at a giveaway price and torn to bits. If local rules or a desire for originality dictate, most of the same mods can be done with the OHC engine.
The CG crank flywheels are of a larger diameter than the TL's, as are the CB100 and XL 125, the extra weight being just where it's needed. This however was to cause problems later. It appeared that the crankpin hole could be repositioned by overboring and rebushing 6mm further from the centre without breaking out from the rim. This would give 12mm more stroke. The other camp had experimented with 6.75mm throw increase, but this did break out from the rim and had to be fixed. I decided on the cautious approach. With hindsight, a 185/200 bottom end would have made the job much easier, but one couldn't be found at the time.
The flywheel halves were machined by my father. He hates motorcycle jobs, preferring steam powered engines or timepieces. Each half was set up on a vertical mill and the crankpin hole offset bored on a centre 6mm further from the centre axis to a diameter that just cleaned up the original hole. This enabled a concentric top hat bush to be force fitted and loctited into the new hole to restore the original crankpin dimension, but obviously on a new centre. The tricky bit is to get both halves the same! The crank could now be reassembled and checked for truth. I now had a 61.5mm stroke crank.
At this stage the piston and liner specs had been decided upon, so the crankcase mouth could be bored out to accept the new barrel spigot. This broke into an oilway that feeds the gearbox! Oh dear. After a lot of headscratching and brainstorming sessions, we decided to fill the broken end of the oilway with metal filler and not dwell on this area too much. The longer stroke factory engines i.e. XL185/XR200 have a larger crankcase mouth and the oilway has been repositioned. If you need to use the OHC engine, these are the cases to go for.
At this stage, news was filtering through from the other camp that severe vibration was an issue on these stroked engines. He’d actually got his OHC motor stretched to 230cc, but it tried to shake it’s way out of the frame. When I sat down to think about it, we were moving a large chunk of metal further away from the centre of the crank with no effort made to counterbalance it. Extensive searches ( O.K. 10 minutes on the ‘net) didn’t come up with any simple to use formula for rebalancing. I’d rather not go into the method we used to determine what extra weight would be needed, as someone is bound to shot me down in flames. I would like to know for next time though!
I settled upon the idea of a hollow crankpin to restore matters. Easier said than done. These pins are case hardened and are bloody hard. A tipped tool managed to get through the outer layer of hardening before melting, but unfortunately, because the ‘pin is drilled for oil feed to the bigend, the walls of the drilling are hard as well, and stopped any further progress. However, we had removed enough material to make a difference even after plugging the ends with ali caps to make the ‘pin oiltight again. I hoped that an acceptable balance factor had been restored and kept my fingers crossed.
Top end.
Much thought and research went into selecting the best piston for the job. It seemed that the barrel could be bored beyond the liner to maybe 70mm before interfering with the barrel studs. The CG pushrod engine and the TL and XL 125 barrels appear to use the same liner, but these are cast in situ and can’t be pressed out. A 70mm OD liner would allow perhaps a 67mm bore, giving about 60 thou wall thickness at the spigot. The crankcases will only just bore to this size, breaking into an oil gallery which must be filled as mentioned earlier. ( We found out later that 185/200 barrels are this size!) The only suitable piston in the Honda range of 67mm is from the CB360G5, a rare beast in the UK. This uses a 16mm gudgeon pin as opposed to 15mm in the TL/CG. It is also far too tall and too long. Apart from that, It’s made for the job! After thumbing through parts lists and workshop manuals, an ATC200 piston of 65mm seemed the best compromise. It has a dished crown to keep the compression ratio under control and a short enough skirt to clear the flywheels at BDC. I ordered a + 0.50mm oversize to give a final displacement of 208cc. This thing should fly !! (apart?)
A suitable liner was sourced from a vintage car engine reconditioner, of slightly over 70mm OD. This was given a small step on the outside and steam engines and clocks were cleared from the parental workshop to make way for more bike parts. Various calculations on the back of a cigarette packet suggested that the barrel needed to be 10mm taller than standard. This cannot be accommodated in the traditional manner with a compression plate between the cases and barrel as the valve geometry would be upset. The rockers pivot around a shaft in the barrel. The best solution that I could come up with was to screw a 3/8" ali plate on top of the pot and bore this as one unit with the liner going right to the top. This can just about be seen in the photos. The Honda barrel and plate were bored with a corresponding small step and the liner fitted with about a 4 thou interference and loctite. An OHC barrel could have a compression plate above or below it, or even both. The only bugbear is cam chain length. This can obviously only be extended 2 links at a time, and the 185/200 slipper and tensioner used.
Longer pushrods and cylinder studs completed the job on the CG. There is no suitable larger head gasket for this engine so I didn’t use one! I could always make one if needed. When complete, the CR worked out at about 8:1, this seemed a good enough place to start.