Crankcase pressure

[robin rose]

I am developing a lot of pressure from my 39 VH crankcase with a strong pulse from the breather and also via
the joint on the mag to dyno cover. The engine is recently rebuilt and is otherwise running well with no problems apart from blowing oil from the head bolts, perhaps influenced by the pressure in the engine. I am running a JP piston and am aware of various horror stories but no trouble at all with siezures etc, bored it to .004" as per data sheet.
The bike has done 700 miles, just wondering if it could be the piston blowing bye or a non return valve issue.
Any ideas? Rob

[brenton.roy]

Hi Rob,
If you have it bored at 4 thou and it hasn't nipped up along the way (a miracle), then I would be looking elsewhere.
I am running double that clearance (same mileage), with no problems - so I doubt it's blow by.
Were the rings 120 degrees apart at assembly?
Does the motor burn oil?

I'm a bit intrigued by oil coming from head bolts.. Both or just one?
The first place I'd look would be for good valve seating - riding tappets, bent valve, bad seats, worn guides.
The second would be around the top end for cracks - and then maybe broken rings.
regards, Brenton

[nevhunter]

When you rest your foot on the kickstarter pedal does it hold compression well.? A good 500 ohv is pretty impossible to move over compression without using the valve decompressor or bouncing it off compression to use the flywheel momentum. Your head should be lapped onto the cylinder which has two stepped surfaces. Lap both and then wipe and lightly oil the inner one and continue to lap the outer one. This won't affect the amount of blow by. I would suspect that the rings have not seated or are distorted/broken. If the bore is true and honed nicely the rings should bed in quite quickly. You can distort them fitting them to the piston if you are not carefull, or they may not be good quality. I don't think .004" is enough clearance on a JP. It is possible if the top (ring land area) of the piston is a bit tight to move a bit of aluminium and trap the ring in the groove. This is a partial seizure, and will cause compression loss. Nev

[david.anderson]

Robin
I have to agree with Nev in that if you have suffered a minor seizure some of the aluminium may have dragged into the ring groove and locked the rings to prevent them from sealing properly resulting in excess blow by. The quoted piston clearance for a VH iron cylinder is 5 to 7 thou. I am running 4 thou clearance on a JP with an alloy cylinder.
I guess the real question is what was your crankcase pressure like initially.

You should also realise that every time the piston comes down 500cc of air must be expelled from the crankcase, through the timing chest and out a tiny breather. Every time the piston goes up the same breather tries to suck 500cc of air back in. That is a lot of air to move at 6000rpm. That is 6000 inhales and 6000 exhales per minute of 500cc volume through a ¼” breather pipe. The end result is that pressure builds up in the crankcase and tries to escape wherever it can, generally around the magneto seal, but also including around pushrod tunnels and rocker boxes. You can help this by fitting a reed type pcv valve to the breather pipe. Make sure you take out the ball valve in the elbow to help air flow. The pcv valve lets the breather expel air but stops it being sucked back in, there by greatly reducing crankcase pressure. My Ariels always left an oily mist around the magneto due to excess crankcase pressure until I fitted a PCV valve. Try one of these http://www.mikesxs.net/products-40.html#products The reed valve on this web page works wonders.
David

[Eero.Korhonen]

Hi,
Do you mean this one:
http://www.mikesxs.net/product/15-0677.html
How do you install this to Ariel, inlet goes to timing case, right?
Br, Eero

[john.nash]

Were the rings 120 degrees apart at assembly?

I am always mystified by this.
I can understand that gaps lining up may lead to blow-by.... (or blow through) ..
However, every time I take a piston off the rings have moved round (unless they are pegged 2-stroke type ones ) so I stopped lining up the gaps.

cheers

[brenton.roy]

Hi John,
I don't know if we do this because that's the way we were taught or whether there is some science to it?
I've never checked to see whether they've moved..

[david.anderson]

Eero
Yes that is the valve. As the inlet to the pcv valve is large I remove the breather pipe from the back of the timing chest and use a short length of pvc hose to connect between the elbow and pcv valve. Inlet side of pcv goes to timing case.
David

[nevhunter]

Unpegged rings rotate during use. Staggering the compression rings at assembly should give a bit more but it may be a bit academic. When an engine has all the grooves to one side ( thrust or the other but not front or back) it will give better ccompression retention in one direction than the other. This is due to the piston moving and reducing the effect of the gap(s). This is observed when checking the compression on aircraft engines by pulling them through by hand. You can't turn a motorcycle engine in both directions normally unless the primary drive case is off.
The breather adequacy on the singles is lacking. If you were racing, you would have to supplement it A one way valve probably helps but it wouldn't want to increase the restriction. A quick test of whether there is not enough gas escaping is to run with the valve cap(s) off for a short time or turn one of them into a breather. Post 54 ( or therabouts), there was an extra one placed in the timing side crankcase, but due to it's positioning, I would think some oil gets out too. Nev

[Simon.Gardiner]

There's always been a touch of the 'black arts' about engine breathing, I seem to remember something about people putting big breathers all over Vincent V-twins and massively increasing oil consumption. I've also always been curious as to why the original Ariel timing-chest breather with a nice valve so it only blew and never sucked (so in theory I guess creating a depression under the piston on the upstroke that reduced the chance of oil getting past the rings and into the combustion chamber) was added to with a plain pipe to the open air. A change in fashion or a reflection of better piston ring design? But why leave the original valved breather in place at the same time?

On piston rings, I've moved to the dark side....received wisdom is gaps 120 degrees apart but without a gap directly lined up with the 'thrust' face (the back for Ariels). I've taken to ignoring 120 and using more like 170, so basically all gaps to the side of the piston, staggered side-to-side and in front of/behind the gudgeon pin so nothing is in line. Seems to be fine so far (150k miles on an air-cooled BM with only the normal BM piston 'features').

(Apologies to all for using the 'B' word )