A corner for VBs (and other sidevalves)

[Eero.Korhonen]

December 1994 Pages 16/17 Cheval, available in the Members section of the website

Thanks Pete, my memory seems to work somehow.
Br, Eero

[john.whiting]

A lot of overlap in a SV will likely increase fuel consumption greatly,for no net result.....Incidentally ,warping of the exhaust valve so it doesnt seat properly has been identified as the cause of " SV Fatigue",or a falling off in performance at full power....Sifton cured this effect by including positive valve rotation on the exhaust,and reckoned it was good for 10 mph,and said it was the largest single performance improvement done to a SV......It wasnt his idea ,as White used the rotators from 1940 onwards,and the SV BMWs were designed with tappet and valve axes slightly askew,to cause valve rotation on both valves.

[Mike Nash]

Thank you gentlemen!
To Nev; I see my exhaust valve is shiny and non-magnetic. Stainless steel do you think? (The inlet's magnetic.) Currently my valve clearances are 0.001 inch up on the recommended inlet 0.003 and exhaust 0.006 for settling in purposes. What extra clearance would you recommend for a stainless steel exhaust valve? I'd dearly like a superior material.
To David Anderson; There's no reason an SV cannot use as wild a timing as any in an OHV for there's many OHVs that have parallel valves. But I only aspire to a VH cam which should give me on the inlet 26/77 and on the exhaust 70/33. That should be safe enough!
To John W; on the VB the cam follower is off centre regarding the tappet so the tappets should rotate but I don't expect this rotation to be transfered to the valves. We'll see when its running; I'd dearly like to see a rotator in action.
Regards to all, MikeN.

[nevhunter]

Mike I first found this problem with an M 20 I rebuilt. I stopped at the top of a hill at an intersection and the engine died and when I restarted it had little or no compression. It had run out of tappet clearance. I then removed the cover and took it up a hill with a bit of retard on the spark and open the tappet till it still has clearance when HOT. You have to check it quickly or heat goes out of the valve . I think this is the reason plenty of sidevalves are more sluggish than need be on hills. Nev

[bob.lowery]

I have had a knocking noise in my VB engine which I have finally tracked down to the timing plug protruding slightly into the combustion chamber so the top of the piston was hitting it. This was easily cured, but the plug was only protruding about 10 or 20 thou, which indicates very little clearance between the piston and head. I see from one thread on this subject a minimum recommended clearance of 40 thou (and that's on a tuned engine) so my bike is well below that. I am guessing that the head has been skimmed by a previous owner for some reason - either to correct a warped head or an attempt at tuning? The bike has been troublesome the whole time that I've had it, difficult to start and spits back viciously and I'm now thinking that this could be the cause. Does anyone know what the clearance is on a standard engine? It's a 1952 alloy head bike. I can make up a compression plate to go under the barrel, but what should the thickness be?

[Mike Nash]

Bob,
If you go up this thread to the 17th of September you'll see the measurements of my alloy head VB for the clearance between head and piston. Its much more than yours with the piston being some 50 thou below the cylinder top. But I've no idea why!
MikeN.

[paul.jameson]

There is a lot of variation between different spark plugs as to how far they project into the combustion chamber. There is a theory, probably correct, that the closer you get the plug gap to the centre of the combustion, the more even combustion will be and the better power output. A photo of the plug you were using would help.

[Mike Nash]

VB VALVE LIFT
Thought you might like to see a polar graph of the actual lift as measured on my VB. The picture's a bit cropped; the lefthand line is the inlet and the right the exhaust. (Sorry it's a bit freehand!)

The lift is measured in millimetres peaking at about 8mm on the exhaust and 9mm on the inlet.
Regards from MikeN.

[bob.lowery]

Just to clarify, there is a bolt in the cylinder head above the piston so you can remove it and insert a rod to measure the piston height for ignition timing - the tip of this bolt is what the piston was hitting. As it's a sidevalve the piston can't hit the sparking plug or inlet/exhaust valves.

[nevhunter]

1 MM clearance piston to head face is adequate unless you are doing crazy revs where things stretch. I'd like to see the engine inlet close a bit earlier than above on a mild motor with a long conrod.. Gas velocity in SV ports is not high as they don't SUCK as well due to the lower compression ratio. Nev