Top end troubles

[simon.holyfield]

I wrote yesterday, in the 'Fitting new valves & guides thread (viewtopic.php?t=11213&start=200) about how I had been cracking on home on the W/NG at a sustained 55-60 when she suddenly cut out with all the symptoms of a blocked jet or failed ignition - no horrible noises other than the drone of an engine slowing and not firing. After finishing the ignition checks I kicked it over to find apparently no compression. I popped the exhaust valve cap off for a look, found nothing and a few minutes later she started OK and carried me home apparently fine.

I thought this might be a stuck valve. She was pretty hot; I didn't ream the new exhaust valve guide 2500 miles ago when I rebuilt the engine and I had replaced the alloy rocker box gaskets with fibre only a couple of days previously, due to horrendous leaks. I understand that settling gaskets could lead to a leaking and overheating exhaust valve.

Today I popped the tappet caps off to check for tight clearances, but the rockers were happily sliding and there was minimal, but some clearance. I'd checked the ignition timing which was set at my normal 5/8" BTDC; the plug is a nice chocolate (50% cocoa solids) colour.

Then I put the camera down the plug hole.

The first picture shows the view directly down the plug hole onto the piston - quite a lot of carbon and it's wet because I had been kicking it over looking at the valve clearances, but you can see a nice shiny polished surface on the bore:

This next one shows the hone marks still visible on the timing side. Folk on the FB group have pointed out that the honing marks are not angled enough:

This is what they looked like fresh from the machine shop:

This view is of the area under the exhaust port. Nice and shiny again.

Here's the money shot. This is the thrust face, under the inlet port - looks like a partial seizure

So, it appears that the top end is coming apart, at least to check the condition of the rings, small end, and to improve the hone. I'll measure the bore and piston diameters to check they are suitable, but the bike has done 2500 miles since rebuild and running at 55 or so is not that unusual - especially if Holyfield Junior is riding it.

All advice gratefully received!

[Simon.Gardiner]

What oil are you using?

SG

[nevhunter]

That looks like a heat seizure where the piston doesn't have enough clearance possibly in the ring land area when it's gotten pretty hot. The type of piston and it's alloy determine the clearance at the various places. It has ovality Taper and relief at the ring land area which should not touch near the rings or it will trap the rings in the grooves. You could also check the head has been lapped correctly to the top of the Cylinder face. This can reduce distortion. BOTH surfaces of the step should be in contact , the inner surface moreso. Nev

[simon.holyfield]

What oil are you using?

SG

Morris SAE 50.

[simon.holyfield]

That looks like a heat seizure where the piston doesn't have enough clearance possibly in the ring land area when it's gotten pretty hot. The type of piston and it's alloy determine the clearance at the various places. It has ovality Taper and relief at the ring land area which should not touch near the rings or it will trap the rings in the grooves. You could also check the head has been lapped correctly to the top of the Cylinder face. This can reduce distortion. BOTH surfaces of the step should be in contact , the inner surface moreso. Nev

I'll have it apart and measure up - it's a NOS piston I got from club spares in a WD box, but maybe the bore was too tight for it.

Seems odd that this has happened now though - it wasn't a particularly warm day, but I guess I don't push the boundaries of its performance very often.

[Mick D]

Hi Simon

Based on your symptoms and photos, I'd be looking for signs of a valve nip up. The signs of the piston nipping up are indicative that things have gotten a little hot.

Regards Mick

[nevhunter]

The top speed of the Bike is only about 72 MPH and less if you have the QUIET cam that was also used in the sidevalve. If the piston was a bit tight It would take a lot of miles to wear much off things. In the OLD days we gave them minimum clearance but the roads weren't as good as they are now and most people only traveled at 30 MPH. Makes a lot of difference to sit on 60. An original WD piston is as good as any unless you want to race it. Nev

[paul.jameson]

When you have the valves out, do a crack test on the stem of the exhaust valve. I had a spell of dropping valves on my 350 after riding at about 60mph for a while. One or two broke at the collets, the last ones at the bottom of the stem. I came to the conclusion it was weak mixture on the slide, changed carbs and had no trouble afterwards. It may be that modern petrol is running hotter, making a slightly weak mixture more critical. For the record, one valve dropped coming back up the M69 in temperatures only marginally above freezing.

[Simon.Gardiner]

OK, I think we can take out oil from the possible causes - straight 50 should be fine.

Higher ethanol fuel runs leaner, so if you're now using E10 that might have contributed to things getting a bit too hot. You'd hope that 2500 miles would be enough running-in though, especially as you've not been just pottering about with it.

I used to blat up and down motorways at 70mph on my old '53 350. Never dropped a valve but I did have a spate of piston seizures to start with - that was eventually fixed by changing from multigrade oil to straight grades....

SG

[simon.holyfield]

Thanks for the excellent replies guys.

So I think this is the job list:

• Strip the top end

• Measure the bore and piston diameters in the land area and upper & lower skirts

• Check and repair any damage to the piston

• Check the rings and their grooves

• Check the small end for damage

• Re-hone the bore

• Inspect the valves and guides

• Crack test the valves

• Grind in the head joint

By the way, I have always used Super E5 fuel in my bikes and incidentally I believe that up here in Norfolk we don't have too much ethanol inflicted on us.