Rocker Tunnel Clearance

Martyn Goodwin

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Am in the process of fitting a Mk2 Cam and have come across an interesting issue. I have already fitted valve stem seals so there is no part of the lower guide projecting into the rocker tunnel; all that's there is the top of the guide seal ET 122/s in the exh. and ET 122 s in the inlet.

I have measured the available valve travel, with the valves in place, by measuring the distance between the underside of ET34 on each valve. On my Exh it is 0.410" and the inlet is 0.365". I need a minimum of 0.400" in both.

So I am wondering what it really should be. I am yet to strip town the valves to measure the height of the shoulder (on the valve stem) that ET35 sits on.

So ..I am wondering:
1. What should be the height of the valve stem shoulder for ET35 above the base of the valve and
2. What should be the clearance between the top of the guide seal carriers and the underside of ET35?

M
 

litnman

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Sounds like you need to sink the valves .035 deeper in the head or make a special valve with the shoulder farther away from the valve face.
 

davidd

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Martyn,

I measured a few used valve shoulders recently and I found no two that were the same. I would presume that new valves are consistent, but I do not have any from the Club.

I have the exact opposite where my intake has more range than the exhaust on the most recent head I was testing for fitting beehive valve springs. The valves from the racer have the shoulders positioned higher. As a note, the big port heads were designed for the MK2 and the clearances in the head are greater. The guide has no lock ring either.

From the top of the valve stem to the top of the shoulder:

2.475"
2.450"
2.512"

I put the valves on a plate and used a height gauge to measure. I measured the three valves from the top because the margin (the cylindrical part under the seat bevel) varies quite a bit from valve to valve. Of course, the top of the stem may vary also, but there is little reason to grind it down compared to the seats.


This may or may not be helpful, but the drawing specs would be what you need. Like Greg, I have to make new valves, so I was just gathering as much data as I could.

David
 

Martyn Goodwin

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Hi David,

Here is a bit more of the story - but up front let me say problem has been solved by retaining the Mk1 camshaft!

On a recent run the piston in my Comet “nipped up”; not a total seizure but on later examination the second compression ring had become jammed in its groove and there were significant witness marks on the sides of the piston and the surface of the barrel liner (in line with the head bolts). Subsequently I found evidence of poor contact between the liner and muff.

Immediate fix was a new liner and piston (as the original set was already 0.025” oversize) and on a whim I have had the crown of the new piston ceramic coated and its sides Teflon coated (www.hpcoatings.com.au TBC1 and DF1/DF3). Based on favourable reports from some folks I also went down the path of attempting to install a Mk2 Cam that I had obtained.

Installing the Cam was a succession of frustrations: The first thing I found is that the cam would not rotate in the case without hitting the underside of the pivot portion of the valve lifter – some work on the valve lifter with my grinder and subsequent polishing soon fixed that issue. Next up I found that as the inlet valve approached maximum lift that the top of the inlet rocker arm made contact with the underside valve rocker bearing – more grinder and polish work on the upper surface of the rocker (being careful NOT to remove all traces of the web) plus some judicious filing of the opening in the valve rocker bearing fixed that. But now, having given more room for the rocker to move I found that the Valve stem collar on the inlet was hitting the top of the valve guide seat (the screw in type with a valve seal that I obtained years back from the Kemps). There was plenty of “free” room in and around the exhaust valve.

If I was to continue I need to find some extra space for the inlet valve to move. There are a number of options, the most likely being replacement of the lower valve guides BUT….

At this point I called on some help from one of my trusted advisors, an qualified automotive engineer, who has had over 20 years’ experience rebuilding all sorts of exotica including Vincents, Exotic European car and bike motors and even F1 race engines. He took one look at the attached diagram of the two Cams – Mk1 and Mk2 and advised in the strongest terms against the use of the Mk2 that I had obtained. He reckoned that the bike would be a bugger to start, would idle like crap and then only over 1,300 rpm and would not give any additional usable power till the motor was doing more than 3,500 to 4,000 rpm. He also expressed concern that at high rpm that the valve may actually hit each other if there was the slightest trace of valve bounce.

I am now putting the motor back together with the original Mk1 cam in place. At least I know that there is plenty of “freedom” inside the head and timing chest.

It would be really interesting to see the graphs of other Vincent cams such as megacycle, Gary Robinson’s or even the VOC’s Mk2’s if such graphs were available.

Oh - I am responsible for the production of the attached diagrams including the unintended 'shudder' I introduced in the Mk2 exhaust profile.

M
 

Attachments

  • Cam Comparison.pdf
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greg brillus

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Martyn, If your Mk 2 cam is one of Terry's, I had two sets of them installed in 2 Shadows I had carried out complete restorations on, and found the engines were a complete pain to start and could not get either to idle successfully. This due to the inlet valve closing with the piston half way up the stroke...........Fine in a race engine, but a pig in a road bike. I replaced both sets with factory spec Mk 2's and it completely transformed both engines. At any rate you will need a good 0.400 travel of your valves as the original spec Mk 2's have about 0.350 at the lobe, so you will need to do whatever is required to the lower guides to give you this figure, if not the new cam and followers will be destroyed very quickly. The original Vincent cylinder head design does not lend itself to easy mods when it comes to extra valve travel, not unlike you found reworking parts of the timing chest for the same reason, but it pays to check everything at least twice to be sure less carnage is the end result. Keep on asking and we will help where we can, cheers...............Greg.
 

nkt267

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I have Gary Robinsons MK2 cams fitted and have done since i built the Comet..Once warm she ticks over beautifully,nice and slow, pick up is good too.
I run an Armours stainless exhaust which is not toooooo noisy:oops:, straight through but with reasonable absorption material. It is a LOT quieter than the Toga 'silencer' I had before.
The Comet will pull well in all gears and I can pull away in 2nd if I have too, even 2 up..John
 

timetraveller

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MartinG, I strongly suspect that whatever Mk2s you were using they were not factory spec ones. None of the problems you mentioned occur with original spec ones,and I have worked on lots of bikes with Mk2. Top of the valve guide problems yes, piston to valve head clearance yes.
 

Robert Watson

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We tried some Teflon coated pistons in a few bikes around Vancouver perhaps 15 to 20 years ago, and unless they have got better at it I would stay clear. It's a bugger when the Teflon peels of and jams in the rings!
 

davidd

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Martyn,

I agree with your adviser, although I don't see the consequences as lurid as he does. I wrote an Egli Section review recently about some of the oddities of the MK2 compared to the MK1 in terms of compression ratio and starting ability. I have noticed that starting is more difficult with the MK2, but I could usually get it done in a single kick. Coburn Benson ran the Flash to an 8300 RPM red line with the MK2 and he would occasionally hear valves floating when the rider was not watching the tach. I have run to 8500 on my cam with no float, which tells me that the valve train does quite well at avoiding float, but I have always used good springs.

Here is a graph of the Megacycle MK1:

MK1 Graph.JPG

I have the ex lump first and the inlet second.

Here is a shot of the big port valve guide:

004.JPG


I would think that the coating technology is pretty good now as there is much more emphasis on preparation. Of course, the stock cylinder has never done exceptionally well at dispersing heat, but it almost always does well enough, particularly on the twins. Singles are a tough case as they have more trouble with highly localized heating. The coating may help, but it may be the liner constantly changing shape that is causing the issue.

David
 

roy the mechanic

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Martyn, are you sure that your cam diagram is "round the right way"? In over 50 years I have never seen a set-up with more lift on the exhaust. I bought a pair of N O S mk2s for my latest, took one look at them and threw them on the back of the bench(the word that was said was anglo saxon. Went for T P V mk 4 was expecting starting issues. On rollers it starts before I have fully let go of the clutch. It takes 6 paces to bump it. It has loads of torque an idles like an old stocker. So i guess its all in the set-up.
 
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