Lister SR2 valve contact with piston crown

[Alex Milton]

I'm stripping down a Lister SR2 and have noticed valve witness marks on the cylinder crowns on both cylinders.  One cylinder is more pronounced than the other.  Is this a known issue and what could be the cause? My initial thoughts are the cylinder heads may have been skimmed too far? I don't know.  One of the piston gudgeon pins also has a crack around it which could be a fatigue crack caused by valve impact? So I'm at a bit of a loss as to what to do - can this be corrected, or is the engine now scrap etc?  Any help greatly appreciated - Alex

[Tony Brooks]

There should be selective shims between the cylinder barrel and head or between the crankcase and cylinder (Too long ago, I can't remember)  You use these to adjust the bump clearance, so this should not happen. We used small lengths of multicore solder on the piston crown to measure the bump clearance.

 

I think that I have seen air cooled Lister piston made with recesses for the valves, but again I don't know which engine. My experience is with the SLs, but I doubt the SRs are any different.

 

Make sure there is no excess play in the big end bearings.

Edited May 22, 2024 by Tony Brooks

[Alex Milton]

Thanks Tony, this is really helpful - I have noticed the big end bearings are worn out, so this isn't helping either and may well be the cause of the issue.  I'll replace these and measure as you suggest.  Many thanks again. Alex

[Steve56]

Are the marks from both valves, or only the exhaust valve. If only from the exhaust then possibly incorrect adjustment of the decompressor. Also has anyone been using the decompressor levers to stop the engine, as this is bad practice and should never be done.

[BEngo]

The other possibility is that it has had an SR2 runaway,  due to a fuel leak into the oil.  This can often cause the con rods to stretch and the pistons to clout the valves.

 

The pistons will probably be OK, but the cracked gudgeon pin is scrap. Check the valves are still straight and the heads square to the stem.  

 

Double check the con rods are straight.

 

Examine the crank carefully, though it will probably be fine.

 

Set the bumping clearance with the shims, some thin multi core solder or plastigage  and a micrometer.  MES (Midlands) may have shims. If not shim stock is available in various sizes from people like MSC Industrial..   Seal them up with Wellseal when finally fitting.

 

N

 

 

 

 

[Tracy D'arth]

I have had this happen in the dim and distant past.

The SR3 engine had run away in a BW work boat. As it was needed pronto I ground shallow recesses in the pistons to restore clearance for the exhaust valves. Why it was only the exhaust rather than both valves I don't know. Ran Ok after.

[BEngo]

19 minutes ago, Tracy D'arth said:

Why it was only the exhaust rather than both valves I don't know.

 It is because of the valve timing and the effect of the exhaust gas.

 

Compression acts against the inertia force of the piston  stretching the con rod,  taking up the big end top clearance and the valves are shut anyhow.

 

On the exhaust stroke the exhaust 0 valve is wide open well past TDC and there is not much resistance from the outgoing gas,   so the inertia load over TDC takes up the big end bottom shell clearance, stretches the con rod and wallops the valve.  The inlet valve is partly open but not as much as the exhaust, so it doesnt get hit until the engine is really over revving.

 

N

[bizzard]

This happened on an external combustion engine in 1938, yes, 4-6-2 Mallard on it's 126 mph record run down the Essendine bank. Running so fast the inside cylinder con rod stretched and bashed out the front cylinder cover, they took the engine off at Peterborough. 🙃

[IanD]

18 minutes ago, bizzard said:

This happened on an external combustion engine in 1938, yes, 4-6-2 Mallard on it's 126 mph record run down the Essendine bank. Running so fast the inside cylinder con rod stretched and bashed out the front cylinder cover, they took the engine off at Peterborough. 🙃

If you want to be accurate, overrun in the Gresley derived valve gear to the inside cylinder (a known design flaw) meant it was overloaded and destroyed the big end bearing... 😉 

Edited May 23, 2024 by IanD

[bizzard]

4 minutes ago, IanD said:

If you want to be accurate, overrun in the Gresley derived valve gear to the inside cylinder (a known design flaw) meant it was overloaded and destroyed the big end bearing... 😉 

Yes. The congugated mechanism for the inside valve gear had too much play in it's bearings. Has been corrected now, more precise with modern machining.

[IanD]

7 minutes ago, bizzard said:

Yes. The congugated mechanism for the inside valve gear had too much play in it's bearings. Has been corrected now, more precise with modern machining.

The problem wasn't primarily too much bearing play, flex in the Gresley valve gear levers that combined the movement of the outer two cylinders to drive the inner one meant that valve travel on the inner cylinder increased with speed as the forces went up, so the inner cylinder developed more than a third of the power -- this showed up on dynamometer tests even on a brand new A4 with no bearing wear. It was a known design problem with the Gresley gear at high speeds, which was one reason his successors switched to three separate sets of valve gear.

 

Better machined modern bearings help, but they don't solve the fundamental problem with the Gresley gear as used in the A4... 😞 

Edited May 23, 2024 by IanD