Ariel Owners Motorcycle Club

The valve guide shrouds in the head I have been rebuilding are in a poor way. My intention was to renew them, but Draganfly's website says that these are 'low stock - may need to be backordered.'

I don't want to hold-up getting this engine back together if I can avoid it, so I thought I would ask what other members think about these shrouds. Are they essential? Do they really prevent oil going down the valve guides? Has anyone deleted them in the course of a rebuild and, if so, what was the result?
I sort of understand the logic of these things, but I don't understand why the exhaust valves need them. In other applications, valve guide seals are often fitted just to the inlet valves to prevent oil being sucked down them on the intake stroke. On the basis that there is unlikely to be such negative pressure above the exhaust valves when open, no seals are fitted there.

I would add that the engine (a Mk2) will have a Morgo pump installed, so there is a good chance of there being more oil around under the rocker covers.

Thanks in anticipation for the enlightenment I am hoping for.

The short answer is that you would be unlucky to need them if you have replaced your valves and guides. In the main, I have got away without them, but I did have a longstanding problem with exhaust guide no2 on my first Healey. They did help with that problem, to some extent.

Thanks, Paul.
I have fitted new valves and guides, so clearances should be ok. I think that I'll place an order with Draganfly and see what happens. If they only have four in stock, I'll fit them to the inlet valves and get on with the rebuild.
(Not to ignore your experience with an exhaust valve guide causing a problem; that does sound unusual. If I go ahead with fitting the shrouds on the inlets and have a problem when I get the engine running, I've only myself to blame!)

It occurs to me that it may be an idea to look at oil drainage from the head, given that the Morgo will be sending more oil up there than the standard pump. I wouldn't want to restrict the oil supply to the rockers because the more that drains down to the camshaft, the better, but if there's a way to speed-up oil drainage, it could be helpful.

What do you think?

The amount of oil running down from the rockers is purely a function of the amount of wear on the rockers and their shafts. Surplus oil from the Morgo pump simply flows out of the pressure relief valve on the pump into the timing case.

That's a good point!
My rocker/spindle clearances seem to be fine so at least that shouldn't cause a problem.

My Mk2 experience is that the rockers are over oiled, even more so when a morgo is fitted. There is an oil pressure increase when a Morgo is fitted, so there is actually more oil being pumped around the engine including to the rockers. I believe one of the problems is that the top of the valve spring retainer is a dish which collects the oil, and the oil drains (and is sucked when the inlet valve is open) through the gaps between the split retainers and into the guide. My 4 was a complete rebuild involving the replacement of virtually every moving part including valves and guides. It is fitted with original umbrella shrouds to all valves and yet while there is no visible blue smoke from the exhaust, anyone riding behind does smell burnt oil. If I was to build the 4 again I would fit positive valve guide seals to all valve guides. High temp viton seals are probably the go. This would involve machining the guide to accommodate the seal. There was a thread on this forum some time ago where someone did just that. Then again I do not smell the burnt oil when riding the 4, and I mostly ride my singles.
David.

More food for thought....thanks David.

My initial thoughts were that the Morgo would send more oil to the top end, but Paul's comment about the OPRV releasing excess pressure into the timing case sounds entirely reasonable.
If the OPRV on the Morgo is set to 'blow-off' at the same pressure as the Ariel oil pump, then there can't be an increase in pressure at the rockers, can there? I've read that oil pumps do not generate pressure, they generate flow. Pressure arises when something downstream of the pump outlet, such as a narrow oilway feeding a small clearance between moving parts, puts a restriction on that flow.

I assume that the Morgo pump provides a much higher flow at all rpm than the standard Ariel pumps. Even with the OPRV operating in a similar fashion as the Ariel one, the Morgo would be able to maintain pressure as the oil gets hot and thins.

Whatever the science may be, my 'gut' inclines towards better pump means more oil, but I could well be wrong (again!)

One thing does seem clear to me, though, is that if Ariel saw fit to introduce the shrouds, there must have been a problem with too much oil finding its way between the valves and guides. The 'oil drainage paths' ground into the rockers is another attempt at solving it. All this was done before Morgo pumps existed, too.

My Mk1/2- engined bike (Mk2 top end on MK1 bottom end, with Morgo pump) produces no visible smoke, but does smell smokey on occasion. This sounds like the description of your bike. I'm not intending to pull the head off to try and solve this 'problem,' but I am rebuilding the head on another bike (a Mk2) and hence my original question about the shrouds.

Thanks for reminding me about Mike Loehr's post (in Ariel talk) on valve guide seals. I had forgotten about that. I've replied to his post and asked for details about how he machined them in-situ and what brand/grade of oil seals he used. (I only did metalwork for two years at Grammar school, so my skillset is lacking. I do still have a smattering of Latin, though!) It may be that a friend of mine could do the necessary machining.

Watch this space....

Allan.

Alan
Actually the morgo pressure relief valve is set at about 52psi whereas the original Ariel pressure relief valve is set at 40psi. At least those are the readings on my oil pressure gauge. But the really noticeable difference is at idle with a hot engine where the original pump only records about 5psi but the morgo is pumping at 50psi.
Also the later shrouds are fitted to a different valve spring and a different top spring retainer than the earlier mk2 models used. So if your head is not fitted with the shrouds it would be as well to check that you have the correct springs and top spring retainer.
David

Good to know the Ariel factory pressure figure, thanks David.
I have a pressure gauge on my bike, but the bike came to me with a Morgo pump fitted, so I could not make the comparison.
I believe that the engine I'm working on is a later variant (valve adjusters over pushrods) and it had the shrouds fitted when I stripped it down. The trouble is that four of them are split and worn at the bottom where they cover the valve guide.
The valve spring top retainers look like those I've seen in photographs and I assume that they are the later ones.

I have ordered some new shrouds from Draganfly anyway. No news so far. (It's a pity that they don't update their website to show when orders have been posted.)

Allan.

Something odd there Allan. I always get an email from Drags telling me when they have posted things to me.