why the large tappet clearances?

[snail]

Could someone please explain the theory behind the large tappet clearances on my 12/2? The book says .017 and .032 Which is the same as the original Lister.If I set them both to .005 cold they open up to .013 hot.This implies that they could be set to zero cold as per some other motors (But I'm not game to try it! ) Is there any advantage  to the bigger numbers or did Mr Lister just like the sound?

brian

[hotater]

snail --

Under prolonged running the clearances change depending on what parts heat up and expand.  My engine runs next to a curtain to the outside.  In windy weather the pushrods cool off enough that I can hear the difference in clearances.  In the summer time I had one go hot with it pre-set cold to .013.

I was experimenting with a new infrared remote thermometer and changing all the clearances and specs to see where the problems might crop up under long hours and days of no supervision.
 Put a FLAG on valve train components!!
  Valve train clearances are VERY important.  Too tight means too hot too fast to catch, sometimes. (90 deg. to 410 in less than thirty seconds set cold at .005)

Consider the clicking noise as a good indication of proper lubrication clearances on the lash caps and pushrods.   

[snail]

Hotater,

I bow to your superior knowledge! I'll bear your experience in mind. Funny, isn't it, with all that iron around, I'd have thought that temperature transients would have been the least of our problems!


Cheers

Brian

[Sc0]

Does your Lister have slightly warped/bent pushrods?  That might be another reason for the generous clearances.

[SHIPCHIEF]

I'd be willing to bet BruceM's carbon fiber pushrods have more thermal stability?
Bruce, maybe they are worth the effort, just in a different way than you initially thought.
Scott E

[hotater]

ScO---

Pushrods should be plumb straight.  I refinished the ends on the lathe and they both ran true.

It would be fairly easy to bend the intake, I think, if the head was too low on the piston and the adjuster screwed down.  Otherwise what would fail?  Rocker arm break?  nah... rocker shaft bend?  maybe....it would make smoke first, I think.

[BruceM]

Scott,  yes, it's true the carbon fiber is very stable dimensionally.  I thought that might be a detriment, since they wouldn't grow along with the cylinder/valve stems, but once again a Hotater report shines new lght on things (pushrod temp affected by outside temp/wind). When I'm in operation on my off grid property, I'll do as Hotater did and check valve lash under a variety of conditions, then pick a safe value for cold set lash.

[Halfnuts]

For what it's worth, I just calculated the expansion of a steel pushrod to be 0.27 mm (0.007") going from 0 C (32 F) to 45C (113 F).  That clearance still seems excessive. 

[snail]

I suppose the reason my clearances appear to be even worse than most is that "cold " is around 30 deg C here!( I mostly use my 12/2 for aircon so I tend to do the routine maintenance on warmish days. )
Halfnuts:Thanks for the 7 thou figure( Great handle BTW!). Could I trouble you for the figure for the amount of cast iron between the cam and rockers?(sorry' my computer's at work so I cant get the distance immediately) Presumably this is greater than 7 thou (between 20 and 100C) so the clearance increases with heat.Don't panic,Hotater I'm not into burnt valves, this is purely academic!
I've just found the discussion of the carbon fibre pushrods (logically ) on a thread about air filters. You guys sure love to play with your motors.I was thinking of going the opposite way, pushing a sleeve of soft silicone rubber over the pushrods( I have some soft 40 duro stuff available) to stop them "ringing"after impact. This works on a small boring tool in a lathe (stops chatter) .Strikes me that it might be quicker than making up the carbon fibre variety.I know it will add weight but with the cam only doing 290 rpm (on my motor)I'm not sure that this will be a problem.If it is , the tube could probably be removed in under a minute.

Cheers

Brian

[Halfnuts]

Hi Snail, 

The pushrods on my 6/1 are 15" long, and with the lifters, it's about 19" or 0.5 m, which is the figure I used.  Now, certainly that 4" of lifter is going to be lots warmer than the 15" of pushrod, but the cumulative difference isn't much.  Hence 0.27mm or 0.007" is pretty close.  Now, of course as the engine warms up the cylinder and head are going to expand more than the cooler pushrods, so I expect the clearance would actually loosen up rather than tighten as the engine is run as you surmise. 

Some sort of semi-resilient pad (or a progressive valve spring) might decouple the mechanical noise.  I think the carbon fiber pushrods were initially the brainchild of someone trying to reduce reciprocting mass which isn't really a problem at such a slow speed.  The byproduct that works here would be a quieter engine.  I think I'd just swap out the pushrods for a couple of hardwood dowels to see if it makes any difference before going to the trouble to mess with carbon fiber.

Halfnuts