Piston Contacting Exhaust Valve

[blaat!]

Hi All,
I completely rebuilt my Mach 1 engine and started it up recently. It was a little hard starting, but ran reasonably well on the handful of times I started it in the driveway. Still working out some other bugs so wasn't able to take it for a spin just yet.

On the last time I ran it (to show it to a friend) it started immediately but was bogging and had no pep. After a few minutes of running it just quit rapidly - like something internal got jammed, or it backfired and quit. Immediately I tried kicking it over, and it did so nothing was locked up, however I couldn't feel the stiff compression at TDC like before. I did a compression test which got no reading at all. I could however still feel the puff of air from the piston out the tail pipe when kicking it over.

I put it in top gear and rotated the tire with the spark plug out to see if everything seemed good. On the top of the exhaust stroke I thought I could feel the piston hitting something, but I could still rotate through. At TDC I could easily spin both valve shims so I knew the rocker wasn't pressing on either valve. I left the engine at TDC and pulled the head, and I could see a piston mark on the valve where it was making contact and removed the carbon:

valve.jpg

The measured gap on the exhaust valve shim was 0.35mm so that was in spec and not causing the interference.

At this point I'm thinking the problem is a bent valve leaking out the exhaust, but reading zero on the compression test seems like too much to leak out from a bent valve, no? I released the springs/catch/collets, and tried to slide the valve through the guide. The valve would move smoothly at first, but got stuck about halfway out. I had to twist and pull to get it out and the barbed catch at the top of the valve stem scraped the inner diameter of the guide taking bronze chips with it. I think this confirms the bent valve theory?

When I reassembled the engine during rebuild everything went exactly as the manual said, and it ran well the couple times before (5-10 minutes each time) so I don't think the bevel gears are off by a tooth or anything like that.

I don't mind replacing the valve and guide, but I don't want this to happen again.

Anyone have any ideas how this might have happened?

[Bevel bob]

Non orriginal piston?, Too little clearance in the valve guide ?

[Eldert]

0.35 mm exhaust clearance is way to much . i use 0.20 for racing . std camshaft should be less .

the top of your exhaust valve is mushroomd i think .

Eldert

[Duccout]

I think that Nigel Lacey recommends always checking with plasticine the valve to piston clearance, and the squish clearance. Possibly these days with spares coming from various sources, clearances are always an issue and have to be checked. I think that you will need a new guide and valve, and gently open up the valve pocket in the piston and check with plasticine that the clearance is correct. Hopefully Nigel or Eldert will chime in with more expert advice.

[blaat!]

Non orriginal piston?, Too little clearance in the valve guide ?

It is a non original piston, but it came from Lacey (along with the valve and guide) so should be pretty safe. What do you mean by too little clearance in the valve guide? Like it got hung and didn't return and was hit by the piston? Not a bad theory but everything was new, fit together smoothly when assembled, and was from a reputable source Still a possibility I guess.

0.35 mm exhaust clearance is way to much . i use 0.20 for racing . std camshaft should be less .

I was going by the manual when I set it up which says 0.30mm for the exhaust shim:

clearance.JPG

Cam is original to the engine. Plus a smaller clearance would bring the valve closer to the piston making the condition worse, right?

the top of your exhaust valve is mushroomd i think .

Good eye and thought Eldert. I was thinking that is how it came and should be. Maybe Nigel could comment? That would elongate the valve and cause this issue, but if it mushroomed the clearance would likely have gone away or at least gotten smaller which isn't the case. Plus I only ran it a couple times for 5-10 minutes each and wasn't revving it too high. The return springs are the only force that could cause mushrooming which seems unlikely. The only other possibility is too much heat (incorrect timing?), but I didn't run it that long and didn't let it get too hot.

The only other thing I can think of is that I got the cylinder used and maybe someone shaved the height to increase the compression? I could possibly add another base gasket or two to add clearance?

[Jordan]

> I could possibly add another base gasket or two to add clearance?

Yes, some SCR models had aluminium plates (plus two gaskets) at cylinder base to reduce compression.
You probably wouldn't need to have the plate, just gasket(s).
An advantage of vertical cam drive shafts is that it is quick and easy to do this, nothing else needs doing.

Providing more clearance by machining the piston's valve pocket has less effect on compression ratio, but is a bit of a job.

[Geordie]

"I completely rebuilt my Mach 1 engine and started it up recently...a few minutes of running it just quit rapidly - like something internal got jammed"

If (please excuse my assumption here) you replaced the valve guides, their bore reduces because of the interference fit in the head. After I replaced mine, a local machine shop honed/ reamed them to a correct clearance for the valve stems as advised by Nigel Lacey. quote "The factory clearance is in the manual but generally speaking between 1 to 2 thou for a road bike will be fine."

As it ran ok initially, it may well be a temperature cause.

[Duccout]

Good point Geordie, if the exhaust guide did not have enough clearance then possibly the valve partially seized. I remember talking to Vic Camp years ago about replacing guides and he warned me against the clearance being too tight as they may seize.

[Geordie]

Yes Colin and Bevel Bob also mentions the possibility in his post above. If this is the cause then the guides may be correctable in-situ (with the exhaust guide replaced if necessary). Even if the valve is still true, replacing it would be prudent.

[themoudie]

MMMmmm!

During my machinations with the '74 450 MKIII, when changing the cam from the 173A to a 'White' cam, I noted that there was some scuffing on the exhaust valve and that there was also a small area of blueing on part of the exhaust valve seat. I had built the engine with the 'Viton' valve oil seals, the original valve guides had been lined with tempered phosphor bronze K-liners and the clearance to the valve stem was 0.0015". The valve was a new steel valve, but not a stainless steel valve.

So, before rebuilding the head and after consulting Nigel Lacey and the engineer to whom I took the head, I agreed to have the clearance between the valve stem and valveguide reamed to 0.002" and that I would run the engine without the valve seals in place. This with a new stainless steel exhaust valve. There would be some oil burning, as oil would be drawn down the valve guides on the overun. However, this would also ensure that the bronze guides/liners would have lubrication that the seals prevent. Bronze valve guides (N.B. not all bronze are the same, there are numerous 'recipes' and you need to use the correct one fit for purpose!), unlike cast iron valve guides that contain graphite and hence are self-lubricating, in fact lubrication can cause cast iron to wear quickly, whilst phosphor bronze requires lubrication, some types more than others.

The material favoured for valve guides, especially exhaust valve guides, appears to be COLISBRO bronze, but I believe that its machining is not straight forward. Link to COLISBRO info: COLISBRO-trademark-alloy

Also, when drifting the new valve guide into place, it is possible to drive it into position "off true", with the valve seat. This needs to be checked and rectified, before re-cutting or dressing the valve seat with the appropriate cutters.

If the replacement valve guide was driven off centre and the valve seat not re-cut or dressed, the clearance between the valve head and piston crown not checked, after a short running time, thermal expansion could cause the valve head to start hitting the piston crown. This would appear from "blatt!'s" paragraph to be the case.

At this point I'm thinking the problem is a bent valve leaking out the exhaust, but reading zero on the compression test seems like too much to leak out from a bent valve, no? I released the springs/catch/collets, and tried to slide the valve through the guide. The valve would move smoothly at first, but got stuck about halfway out. I had to twist and pull to get it out and the barbed catch at the top of the valve stem scraped the inner diameter of the guide taking bronze chips with it.

The mushrooming of the valve stem tappet end would indicate some sustained and forceful blows to the valve head from the piston. The fact that the

"....barbed catch at the top of the valve stem scraped the inner diameter of the guide taking bronze chips with it.

In my mind this means that the head needs to be stripped back again and measured up, before purchasing new guide(s), valve(s), maybe even rocker pin(s) if the present ones are not concentric and then ensuring that they are fitted and finished acurately. With the sort of damage found in the head, I would be wary of the damage that might have been caused to the piston, connecting rod, little and big ends and would check this before a dry run re-assembly of the engine to measure the valve to piston clearance.

Sorry to be a Jonah! But, Ducati singles engines are not the same as Japanese engines for manufacturing tolerances and not as tolerant as some British engines for operating, with engine component inaccuracies!

I hope you feel able to keep going with the wee beastie and can then enjoy running it on the road.

Good health, Bill