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Model 137 Alpina that had sat for a very long time in the shade, but at the seaside.
David Lahey wrote:Greg Harding wrote:Hi Everyone,Kurt wrote:Other than calculating the change in compression I didn't measure the exact shape of the chamber Greg.
It would be interesting if there would be significant gains to be made by the shape of the squish/combustion chamber on a Trials bike given the low revs at which they operate.
I don't have a split head yet Greg although some might say so.
I am interested as to why you have lobotomised your head though for that silver tray.
There would be a reason why the Two heads are different, my guess is the DT with much higher gearing needed more torque to get off the mark?TY 175s need more bottom end when labouring up hill especially after a really tight turn that limits run up speed. My question in part was: Can the same improvement be made using the original head and that is why I was curious about to volume and shape?
Well I don't have a fancy profile gauge like yours Greg, but I do have something else to add to the story.
Here are photos of the two heads in question. A DT175A (
Measured with my eyeballs, the combustion chamber appears to be the same shape on both and the squish band has the same angle and width on both. Where they differ is in the step up between the squish band and the gasket surface. The TY175 head has a 1.5 mm step and the DT175 head has a 1.2 mm step. So to answer your question, yes you could replicate the compression benefit of the DT175 head by machining the gasket surface on the TY175 head, as many people have done. An even more common trick with the TY175 is to use a thinner-than-standard head gasket or no head gasket to increase the compression ratio.
- Screenshot_20230813-201610_Chrome.jpg (637.37 KiB) Viewed 16592 times
TY 175 head to match the higher compression DT175 head:
.Kurt wrote:It has always struck me as an odd choice to chrome plate the bores of the brake hubs on the :Bultaco:.
Obviously it gives greater wear resistance but at the expense of brake performance and also once you wear through the chrome you then have the brake shoe wearing away sections of the base metal at a much faster rate than the areas with chrome remaining.
did the chrome plated brake drums at a time when manufacturers were going to all sorts of extremes to make their bikes lighter and doing away with the steel or cast iron drum linings made the wheels on Sherpa Ts and Pursangs class-leading in terms of weight. The weight war had been started by 
with their extreme lightweight works MX bikes and everyone else was playing catch-up. 
was pretty serious too and in 1974, the Phantom was the lightest production 250cc MX bike. dabbled with chrome plated aluminium. In 1969, they chrome plated the outer engine casings on Pursang and Matador. It wasn't a raging success and they went back to a polished aluminium finish. Chrome plated the aluminium cases, was it only a decorative feature?Kurt wrote:A wealth of knowledge as always David.
I hadn't considered the overall weight side of things and also the lighter you can make your hub the better your suspension will react to bumps (probably not overly applicable to Trials due to the slow speeds).
Any Idea why :Bultaco: Chrome plated the aluminium cases, was it only a decorative feature?
It would have taken some serious effort to polish up the cases for chroming when the polished aluminium looks as good as it does.
TY rear wheel on the back of my 
KT250 while I was restoring the KT rear wheel and it was quite interesting to feel the difference, and TY wheels are not the lightest twinshock trials wheels. Most noticeable in stream-bed rock riding. The KT250 has the same wheel hubs as the early 1970s KS125 trail bike.
Cota 348 and late model Sherpa Ts have a beautiful ride. Some earlier Spanish trials bikes hubs were not so light. The first Cota 247 had monstrous wheel hubs and the Sherpa Ts from 1969 to about 1974 had "very strong" rear hubs and big diameter rear axles. took the wheel weight reduction theme seriously on the MAR using a 12mm rear axle when it came out in 1972/3.Kurt wrote:One for the Yamaha TY80 Anoraks
What is the function of the Oil scraper No 17 in the parts manual?
It appears to me as if it would redirect oil flung from the clutch gear through a passage to the gear set on the other side of the crankcase but this would be in an oil bath anyway.
engine outer casings. shiny plated the outer engine casings of some models. I have no idea why, but I do know that the plating eventually starts to flake off and I haven't found a way to make them look as pretty as polished aluminium looks. primary drive casings were made close in time. The plain aluminium casing I'm thinking was made in 1967 or 68 and the chrome plated casing in 1969. I don't think either of them has ever been refurbished so it is a pretty good comparison of how well they have fared over the years. logo.
