A corner for VBs (and other sidevalves)

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bob.lowery
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Re: A corner for VBs (and other sidevalves)

Postby bob.lowery » Tue Nov 10, 2020 1:08 pm

Thanks to all for the fast responses. I will take the cylinder head off again today and take some accurate measurements. I have limited experience with sidevalve engines, I never knew that the clearance between piston and head was so tight! You live and learn.....

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James Hickman
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Re: A corner for VBs (and other sidevalves)

Postby James Hickman » Tue Nov 10, 2020 3:31 pm

nevhunter wrote:You are not likely to ever get a SV to a very high compression. ..Nev


Can anyone put an approximate figure to what compression we should be able to achieve, kicking over a Black Ariel Sidevalve?
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Re: A corner for VBs (and other sidevalves)

Postby nevhunter » Wed Nov 11, 2020 8:34 am

You get to a point where raising the compression impede's gas flow. Long stroke to bore ratio helps. Under 6 to one probably.. They are inefficient because of a poor combustion chamber shape where it has too much surface area for the Volume of it and the flame travel is bad.. Nev

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Re: A corner for VBs (and other sidevalves)

Postby bob.lowery » Wed Nov 11, 2020 6:30 pm

I checked the clearances in my engine yesterday. The cylinder head above the piston is flat, the piston is 18 thou below the top of the barrel at tdc. The copper head gasket is 34 thou thick, giving a total clearance of 52 thou. This is above the 1mm min clearance recommended by Nev, so I guess it's ok, although it still seems very tight to me! I have some 18swg alloy sheet and I am still considering making up a plate to go below the barrel. This would put the clearance up to about 100 thou 2.5 mm. Am I being over cautious?

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Re: A corner for VBs (and other sidevalves)

Postby JohnnyBeckett » Wed Nov 11, 2020 7:05 pm

hi are you measuring a new head gasket or a old one because a new gasket will compress wen its fitted and it will be a bit thinner :?:

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Re: A corner for VBs (and other sidevalves)

Postby bob.lowery » Wed Nov 11, 2020 9:52 pm

That's a valid point, a copper gasket will compress a little but I think it's going to be a relatively small amount. The compressive load from tightening the head down is spread over quite a large area.

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Re: A corner for VBs (and other sidevalves)

Postby nevhunter » Thu Nov 12, 2020 12:02 am

1 MM is enough. If the big end goes in a big way, it may touch but nothing else is likely to cause an issue. It's not like Valves bouncing and touching in an OHV engine. If your engine carbons up it's easy to decoke. Don't have the gasket expending over the piston if you can avoid it, Nev

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Re: A corner for VBs (and other sidevalves)

Postby Mike Nash » Sun Nov 22, 2020 5:36 pm

CLEARANCES IN A VB CAM CHAMBER

Dear friends, I’d like your advice on a clearance problem in the cam chamber of my VB.

If I firmly secure the 5 set screws of the cover that goes over the cam and its followers something jams and holds the tappets up and the valves off their seats when I rotate the engine. However, if I back the set screws off by a quarter turn the jam is released allowing the tappets to drop and the valves to snap back on their seats. As the screws are quarter inch Whitworth and so 20tpi, a quarter turn must back the screws off by 12.5 thou. I assume that somehow the cam followers are being trapped on the cam follower pivot pin (part number 1475-29. Note I’d had 19.5 thou end float on the cam itself which I reduced with a 10 thou shim behind it.)

I noticed that the "gearcase to crankcase gasket" (part number 2030-55) that I’d fitted was 8 thou thick and that new ones from Draganfly are some 7 thou thicker at 15 thou so I fitted one – but the same thing happened with the tappets and valves hanging up. The next step is to fit two gaskets (with a thicker shim on the camshaft) and see what happens; do you think I’m on the right track? Has anyone else had this problem and if so what was the solution?
Thanks in advance, MikeN.

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Re: A corner for VBs (and other sidevalves)

Postby dave.owen » Sun Nov 22, 2020 5:51 pm

Have you got end float on the cam.
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Re: A corner for VBs (and other sidevalves)

Postby Mike Nash » Sun Nov 22, 2020 5:57 pm

On the camshaft, yes Dave, some 5-10 thou. I fear there's none on the cam follower pivot pin - but no idea why! If I'm right how could that happen?
MikeN.


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