Topic: Valve to piston clearance with webcam 41a and big bore ??

[Hummel]

Hello

I just assemble my cb750 engine with

836cc kit ( cheap from eBay, looks like original piston)

Beehive valve springs

Bigger intake valves

Webcam 41a

Adjustable cam sprocket

Would this normally works together without cutting in the pistons ? I have tried to measure valve to piston clearance by setting the valve lash to minus 0,50" and minus 0,80" like webcam recommend but the engine will not turn around like this.

If I set the valves to  about the half it work fine, so do you know how important the clearance is ?? I really would hate to tear down the engine again for cutting the pistons


Another question is how to set the valve timing with the adjustable sprocket ? Is there a good and easy way ??

Many thanks

Tommy Hummel

[MRieck]

Those pistons will not work with OS intakes. That is what I found.

[Hummel]

Nothing Else to do than tear down engine again ?

[lucky]

Hello

I just assemble my cb750 engine with

836cc kit ( cheap from eBay, looks like original piston)

Beehive valve springs

Bigger intake valves

Webcam 41a

Adjustable cam sprocket

Would this normally works together without cutting in the pistons ? I have tried to measure valve to piston clearance by setting the valve lash to minus 0,50" and minus 0,80" like webcam recommend but the engine will not turn around like this.

If I set the valves to  about the half it work fine, so do you know how important the clearance is ?? I really would hate to tear down the engine again for cutting the pistons


Another question is how to set the valve timing with the adjustable sprocket ? Is there a good and easy way ??

Many thanks

Tommy Hummel

This does not make sense.
Why do you thing changing the valve lash would make the piston clear the valve?
BTW...it is .05mm=.0019 thousandths, not quite .002 thousandths. Not 0,50mm.?


You need to forget about valve lash and just focus on piston to valve clearance.
Don't forget to add .010 thousandths for expansion of the metals.

Put clay on the top of the pistons, install the cylinder head  and turn 
the engine over by hand and check you piston to valve clearance.

Don't forget that webcam needs .003 and .003(thousandths)  for the valve lash.

[PeWe]

You need to take it apart anyway to check clearances, double check everything. It's very easy to make a mistake despite correct parts.
You can test just for sure before giving those pistons up. The worst case means that you must use 10.5-1 CR pistons with pockets for valves ....more fun when driving :-)
Remove head, clay the pistons, assemble, double check cam timing again while you turn around the engine. I used 0 valve  lash on cyl 1,4 (checked IN, EX timings again) 2,3 had the lash they will get (tight 0,15mm).

[Hummel]

How much clearance between valve and piston is good enough. ??

It's hard to explain how i measure it, but by turning The adjust screw Down on The valve 0,5mm and then turn The engine around, if it can turn there is 0,5mm clearance.

Make sense ?

[Hummel]

Can i reuse gaskets when they have been torgue down ?

[MRieck]

I had to open up and add depth to the intake valve relief. The only way I could accurately index the valve was to use a spare head with no valves in it. Bring the pistons up to TDC and run a punch through the guide to mark the piston crown. I use an old valve with the head cut off which is sharpened to a point

[Hummel]

Now I open the engine again, laid I layer of clay (modelling dough) on the pistons, assemble the engine again at TDC, and then pressed the valves into the clay with the adjustment screw. Take the engine apart and marked up the pistons where the clay was formed like the valve.

Now my plan is to get my machinist to grind 2 new pocket in the valves, 2mm on the exhaust and 1,3 at the intake, and 1mm larger diameter than the valves is.

Hopefully this will make enough clearance for the valves, and not take to much of the CR.

Would you guys reuse a bottom gasket ??

Thanks for the help


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