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[stinkwheel]

Turns out the slight clattering noise under load on my 612 (full hitchcocks kit, including head and I've got both compression plates fitted) was NOT pinking. It was this:















Inlet pushrod. I'm not sure if it bent then snapped above the adjuster or snapped then bent. In any case, it looks like it was minutes away from total failure. It's shaved a good-ish quantity of alloy off the side of it too. As far as I can see, it's all in the filter (cut it open to check). I checked the rocker blocks where such things seem to accumulate and they are clear of any remnants. Only a very slight metallic haze on the drained oil surface (I've seen worse on a routine service on my VFR).



How did I do this so I can avoid doing it again? The tappets were set with a slight clearance in accordance the fitting instructions, properly checked with a feeler guage. Exhaust one is still bang-on correct.



Overrevving?

[Adrian]

Maybe. Valve springs coil bound? Might be a head-off job to check the inlet valve stem and the guide for signs of partial seizure.



A.

[stinkwheel]

I presume valve seizure would present with scoring/linear marks on the valve stem? I've had a good look at the exposed part of the valve stem with a bright light and magnifying glass and there's no sign of anything untoward. Plenty of oil hanging about up in the top-end too.



I was working on the assumption that the tuned head supplied with the kit ought to have enough clearance for the cams it was suppied with? I checked the piston to valve clearance with blu-tak (ample, didn't even dent it) but I'll admit to not checking the valve guide to collar clearance on the assumption there would be no variation between one bike and the next. It all turned over by hand without any drama.



I supposed the adjuster might have moved, but would it get tighter? I suppose the thread is facing downwards so it could have unscrewed itself? I'd have expected a loss of power if the tappet tightened up though?

[Bullet Whisperer]

Something to do with this, perhaps? ... 'I'll own up to having decided to go for a "Hard" run-in. I warmed it up then put 90 miles of spirited riding on it today, stopping every 30 miles or so for a short cool-down and to see what else has come loose/fallen off. So yeah. It'll do 50mph in first, 70 in second. If you wind on the throttle below 60mph, you have to concentrate to avoid sliding back off the seat. The power deliver puts me more in mind of my old MZ supermoto (XTZ660-based motor) but with all the noise you'd expect from a well tuned brit single.'

[stinkwheel]

Perhaps. I'm still looking for an actual cause though. How would riding it as hard as I intend it to be ridden have caused this? Valve seizure? (as mentioned above). I wasn't aware of any sudden (or gradual) drop in power or misfiring when I was out that I'd associate with a total/partial engine seizure and I made a pont of going out with visor open and no earplugs so I could keep an eye on it.



I did seriously fluff a gear change at full chat at one point and it did over-rev. I wonder if the adjuster could have jumped up onto the top of the follower for a revolution effectively lengthening the rod by 5mm? I can't see any witness marks on the ends of the rod though. It did also have a fairly meaty kick-back at one point when I was trying to start it initially.



Oddly, the fracture above the adjuster opens in the opposite direction to the bend. I'd have expected it to fail on the convex side of the bend. I appreciate things are going very fast and things get pretty malleable at engine speeds but I can't see how the bend caused a fracture in that position (or vice-versa). If anything, the bend in the rod tends to close and compress the fractured part which is presumably why it didn't fail entirely. Unfortunately my good camera got water in it. I've tried to do a better picture. The red line points to the fracture.

[Bullet Whisperer]

I think I would fit a new pushrod and start again, but don't rev it so hard for the next few hundred miles, as moving parts, including rockers and valves will loosen up in this period. Also, the pushrods in a 612 are longer than 350 and 500 ones - which are pretty long themselves - and therefore that bit more vulnerable to bending as a result.

[stinkwheel]

The more I look at this, the more I think it must have jumped off the follower and landed with the edge of the adjuster bang on top of the follower (specifically, the edge nearest the steel rule on the pic above), coilbound the spring, snapped the adjuster and bent the rod similtaneously. It's the only way I can think of to get the bending force and snapping force moving in two different directions.



The bending force due to a coilbound spring will tend to bend the rod away from the rocker. The off-centre force on the edge of the adjuster will tend to load up the stress riser where the adjuster meets the rod opposite where it's hooked up.



I wonder if the shrouded adjuster kit (90235) would be a worthwhile investment?

[stinkwheel]

Infographic. I'm pretty sure this is what happened. Now I can go to sleep!

[Andy C]

Not nice. Odly enough when I bought my 612 back and was going over the engine I found a bent EXHAUST pushrod - nothing like as bad as yours the bend was just perceptible and I ended up replacing it as it proved impossible to straighten. Hope that it does not happen again. Mine has 1 compression late fitted by the way.

[Count Johnny]

I'm had a similar issue (bent exhaust pushrod - twice) with my 612 that was attributable to oil pump failure, causing a momentary seize of the rocker.



We think the oil pump failed, because the oil flow was restricted by non-standard rocker feed pipes, causing back pressure and stripping teeth on the spindle.



We now have standard rocker feed pipes, along with Hitchcock's oil pressure relief valve, and all is fine in the garden.



Not saying that this is your problem. Just putting it out there.