Big Bore TL125

Following are two articles that anyone planning to do a bigbore TL125 should read. Mark's article shows how an ultimate big bore TL125 can be done. And Al's article shows that even with these small engines common sense is still a valuable asset, there is definitely a right way and a wrong way to do a big bore engine.

Mark Worsfold has made the following article on BIG bore TL125:

'Big' TL Engines

In the quest for more 'grunt' from a TL motor, the phrase 'you can't beat cubes' keeps coming to mind. This led me to build my biggest engine so far, of 223cc, which has now done several events, including a 100 mile 'enduro/trial'. I'm currently building a second version for my Sammy Miller framed bike to compete in the "Manx Classic 2 day trial" in September.

In the UK, the only classic classes are for Pre '65 machines or Twinshocks, so the TL has to compete with quite modern machinery. This does mean that I'm free to use any parts that I fancy. In the US, I believe that the standard crankcases must be retained, so a bit more work is required to achieve the same result (ed.note: under AHRMA rules modifications are limited to what was typical for the period. I have not heard of anyone using XL185 cases, doesn't mean it wasn't done. If you know of major mods like this done to TL125 in the '70s please ).

The project started with a chance remark by a guy running an XL185 in a Long Distance road trial. He mentioned that a 67mm piston from a Honda stationary engine, usually used to power generators and ditchpumps, fitted the bike engine. I duly ordered one and the project started. This piston has the same gudgeon pin diameter as a TL and the deck height is 1.5mm lower. It's also half the price! This started me thinking about a long stroke crank to go with it. Loads of calculations on the back of an envelope led me to the dimensions I finally used as follows.

I have used the stronger XL185 crankcases and crank (ed note: TL125 crank won't drop into the XL185/200 cases, the main bearing is of smaller diameter). These fit a TL frame with a small mod to it. The later cases have a bulge at the lower rear engine hanger bolt for the auto decompressor mechanism. The frame was ground away to allow clearance and the rear mountings drilled out to 10mm to match the new cases. The 185 5 speed gearbox can be used, but the UK spec. XL125S 6 speed cluster is better. If you are constrained to use the original cases, the mouth needs boring to 70mm by 32mm deep to accept the new cylinder barrel. This will break through into the gearbox oilway, which will need welding to seal the end up again. The TL gear cluster will not fit without modification to top gear, either remove it or use the CG125/CB100N cluster with 2nd gear from the TL fitted. You must use the clutch cover from a TL if using the 185 cases as the later cover fouls the rear brake. A small amount of weld is needed to fit this. The clutch lifter plate must match the gear cluster used.

The 185/200 57.8mm stroke crank has to be pressed apart and the crankpin holes redrilled on a new centre 2.75mm further out than standard. These are then rebushed to the stock size with a top hat shaped bush. I can supply dimensions if required. The crank is then rebuilt. This sounds easy, but a lot of work is required to do all of this! This gives a crank of 63.3mm stroke. The side of the bigend eye may need to be 'cleaned' on the outside with a grinder to clear the gearbox pinions on trial assembly.

The best cylinder to use is the 185/200, as this will bore to 67mm with no other mods. A spacer plate need to be made from 1.5mm (.060") ali plate to go under this with a stock gasket either side. This has the effect of raising the barrel by .081" to help accommodate the extra stroke. I also use a head gasket made from the same material. I have aimed for a CR of 8.2:1, the same as stock TLR200.

The ditchpump piston has had 1mm (.040") removed from the crown. Bear in mind that the crown markings will be lost in this operation, but as I don't know which way the arrow pointed in it's native application, this didn't matter to much. The gudgeon pin is offset, so make sure that this is fitted the same way round as the stock piston. The skirt will also have to be shortened with a flycutter set to the radius of the crank.

This just about wraps it up for the 'heavy' mods. I use a TL125 head with small valves. My first bike used an XL125S carb, as this was the only one I had, but the second version with be fed with a TL carb. The stock generator fits, although I will weight to rotor on the next one for more plonk.

I hope this makes everything clear. I would welcome feedback and comments. It may be a good idea for anyone thinking of embarking on a project similar to this to mail me, in case there are any details that I have forgotten!

Parts as follows. (Note. These are UK part nos. They should be valid world-wide, but....)

crank1.jpg 34kB

Al Johnston sent me the following cautionary tale:

'Too Big' TL Engine

There has been comment recently on oversize TL engines. I do not wish to unnecessarily prolonging the debate, but I would like to illustrate a point I made on the Message Board:

When overboring with the intention of using a CB750K2 piston the liner MUST be replaced with one of a larger outside diameter. I know that not everybody agrees with this, the implication being that the old liner will suffice. This is untrue, it will fail. Sometimes it is claimed that a 122cc barrel wont do but a 124cc barrel will. Again, this is untrue; both liners are of the same outside diameter and when bored to accept a 750 piston the wall thickness is left at 0.018" (less than 0.5mm). Enclosed is a picture of a standard barrel bored to take a 750 piston. As I bought the bike in this condition I cannot say how many miles or hours the engine ran for before it failed. However, it does not appear to have run for long as the piston is relatively unscuffed.

What is not clear from the picture is that the crack runs a quarter of the way around the liner at the line where it emerges from the alloy barrel, then migrates to the bottom edge. Even where the liner isnt cracked it can be squeezed oval between finger and thumb. In addition, because it is not possible to put a 'lead in' at the bottom of the liner there is a high risk of breaking the piston rings on assembly. So don't ruin a new overbored engine by not replacing the liner.

barrel.jpg 23kB

Back to TL125 homepage

Last modification February 16, 2003
kailasmall.gif 2kB

Big Bore TL125