I am curious about how you are getting 35mm valves for the M42. Cause I remember reading from another thread that M62 valves are 35mm but shorter in length, thus it won't fit.
Yes M62 valves are few mm's shorter than M42 valves...
There are few main concerns and possible solutions:
1. Valve springs. They need suitable installed height to have right amount of seat pressure. Also, coil bind must be avoided (situation where spring is compressed so much its coils touch eachother and spring becomes solid). When valve is shorter, spring plate at the end of valve is closer to cylinder head causing spring to be shorter than normal, causing higher seat pressure (well, harder to have leaks which is good but also higher stresses on valve) and smaller lift before coil bind occurs.
- Aftermarket springs with different lenght etc could be fitted
- Cylinder head could be machined so that springs will be located same amount deeper in cylinder head than valves are shorter
- Different spring plates
2. Lifter / camshaft clearance. When valves are say 2 mm shorter lifters will be 2 mm deeper in the cylinder head. Hydraulic lifter *might* be able to adjust that away but many agressive camshafts also have smaller base profile so the difference might be 5 mm and that will not be adjusted automatically away. With solid lifters you need to adjust the difference away by using shims.
- Really only one solution here... Solid lifters with sufficient shim adjustment range.
