Valve Tappet adjustment

SPRIDDLER · Post by **SPRIDDLER** » Wed Sep 07, 2011 12:40 pm

Been musing on this too and although I'm only a Batchelor of Spanner Rash here are my lay thoughts:
The coefficient of expansion of Aluminium alloy (23) is twice that of iron (11), forged or cast. Say the pushrod (which is only in contact with hot bits at its ends and is also cooled by oil draining from the rocker box) only gets half as hot as the iron bits it'll only expand about the same as the hotter barrel....and contract similarly when it cools - assuming that the 'length' of the iron and the ali bits are roughly the same.

Likewise an alloy piston in an iron/steel barrel will generate more piston slap when cold than when hot because the piston expands more than the bore. If they both expanded the same amount we'd never get seizures . Simples! (Or naive, of course).
It works in practice, so why does it have to work in theory? Edited by - SPRIDDLER on 07 Sep 2011 2:20:22 PM

GOLDSTAR · Post by **GOLDSTAR** » Wed Sep 07, 2011 4:42 pm

By the lord 'arry, just wish to be a simple batchelor, do'nt want nuffink after it, anyways oop I just set them so they rotate freely? engine cold, the old thing rattles a bit when its hot but it is an all all alloy motor and ' They all do that sir' kind regards

Wavy · Post by **Wavy** » Wed Sep 07, 2011 9:52 pm

Set them as mentioned, then ride the bike, take of cover, burn fingers, then check that the pushrods spin with no shake. make sure the plug cap is removed as when it fires with yours fingers in there it can be scary. turn the engine over with kickstart, to compression and sometimes over checking that the rods spin if not reset!

tr7dan · Post by **tr7dan** » Wed Sep 07, 2011 10:52 pm

quote:
Dan - some thoughts....

I see your point about what happens to nil clearances when the engine cools, but this probably thinking about it too much! Stick with whatever method is known to work, and just accept that it works because everyone previously found that it works. And as a Doctor of Engineering, that's a difficult thing for me to say!

James

...... ah, so you haven't the foggiest either, James!!

Jus' kidding.... I'm aware of the way metals will expand when heated and the complex geometry of the components may indeed affect the way the clearances vary with heating and cooling. Perhaps, no, in fact probably, the rate of expansion may change exponentially (or may even be variable), rather than linear but, if I've got to think about it so hard, then it's making my brain hurt - even though I've got an 'O' level in woodwork !!

Whatever the reason I'm sure that the designers will have decided, through calculation and testing, that a warm engine is the optimum for tappet adjustment so, I'm jus' gonna do what everyone else does.... set 'em with the injun cold !

Dan

itma · Post by **itma** » Wed Sep 07, 2011 11:08 pm

The main thing is that they spin freely, rather than just turn it with your fingers in the hope that you will achieve some kind of fine tuning, flick it with your finger so it spins.
Its served me well for 40 years.
Better a little too loose than a bit too tight.

en7jos · Post by **en7jos** » Thu Sep 08, 2011 3:10 am

Reminds me of a story one of my lecturers told me years back:

A man works all his life in a factory, keeping all the machines running, doing maintenance, making adjustments and that sort of thing. But one day, the powers that be make him redundant.

Not so long later, one of the main machines in the factory goes wrong and no-one can get it going again. The whole factory grinds to a halt. They get in various specialists, consultants, etc but no-one can fix it.

Reluctantly, they eventually decide to ask the man they made redundant to come in and see if he can fix it. He agrees, comes in to the factory, has a quick walk round the machine, pulls out a chalk and marks a small X on one of the components. "Here's your problem - that bit needs replacing".

A few days later the man's invoice arrives at the factory for £1000. "A thousand pounds!" they say "just for a few minutes looking a putting one small X on the machine". "How on earth can you justify £1000 for drawing a chalk cross? We'll need a breakdown of your invoice please".

Sure enough, a few days later the man's itemised invoice arrives at the factory with only two items listed:
- Small chalk cross on machine - £1
- Knowing where to put it - £999

Kind of sums up old motorbike maintenance really, doesn't it? Theory NILL, practical experience ONE.

MatthewC · Post by **MatthewC** » Thu Sep 08, 2011 9:30 pm

Ken de-Groome told me that he sets the clearances with a cold engine - if it's good enough for him it's good enough for me!