-SG- I have known Juris Ramba in Latvia for about thirty years and I would say we are good friends. Over the years he has provided plenty of useful information as well as lending me his Model 25 to participate in the Latvian rally a few years back. Here is a recent message and photos from him which we feel are worth posting.
"In the beginning of September my two sons and I went to the Polish fiva world motorcycle rally. Robert took our 1926 Model 25 and Reinis was on our 1930 Model 18. The Model 25 engine had been fully rebuilt with a newly cast cylinder which I bought in Britain and we machined it at our workshop.
The piston had been cast in Slovakia, the crankpin was bought NOS from a standard Norton and oil ways were erosion cut in it at a local factory. As the old crankshaft had been extensively hammered and out of alignment by 0.30 mm, we decided to make a new crankshaft from scratch. The webs were machined from heat treated steel billets fully adhering to the original measurements, even down to balance drillings. After assembly and testing the run-out which was 0.02 mm the balance factor came out to 67% which is quite good. The engine does not vibrate almost.
The shafts were made from case hardened and tempered 3 % nickel chromium steel, these were ground to 0.04 mm oversize. I bought a liquid nitrogen container from China, filled it up and used to shrink the shafts in cryogenically. To avoid any possibility for the shafts to turn in the webs, holes were erosion cut partly in the shafts and partly in the webs and pins were pressed in to safely secure these. The original drive side shaft had turned in the web in the past, so we chose to be safe rather than sorry.
One new intake valve was cut and machined from a Chevrolet valve which is nickel steel, with chromium plated stem. That was bought from the US. Then we had a look at the plunger oil pump. Its body was hard chrome plated and ground on the od. The inner plunger was made new, tempered and ground to size in our circular grinding machine to obtain minimum clearance in the bore. The old plunger had worn and had a running clearance of 0.04 mm which would not work with hot oil. We got the running clearance down to 0.005 mm or so. The pump now works as it should and I am happy we paid attention to it, otherwise all the efforts and new parts would be in vain if the pump did not deliver enough oil to the engine components…
Finally upon assembly i found that the roller bearing central sleeve was too tight in the bearing. So i put it in the lathe and ground down 0.02 mm with fine abrasive paper. Thus i avoided seized bearing…
I measured the crank end play with a piece of plasticine and a shim.
Robert had done 300 kilometres of running in before the rally, going out with the M25 on early mornings before the summer heat came in. One day the inlet valve spring on model 25 broke. We did not want to take off the cylinder head to replace it, so the old spring was cut with an angle grinder in pieces and removed and another old spring was fitted. I immediately ordered a batch of new springs from Alberta springs in the UK.
The distance at the rally was 420 kilometres. All 3 machines finished the whole distance."