Trying to get a better understanding of what might happen to all these critical engine parts as they heat up and get to operating temp. i.e. how much do they expand with heat?
Experiment: I put the cylinder and piston in my oven and checked sizes. Unfortunately oven dial numbers worn out so temp is a big fat guess. Thinking maybe about 150 C.
Keep in mind standard piston/bore clearance spec 0.055mm (Talking KTM EXC200 2 stroke)
Piston skirt - measured at bottom, below you normally measure for size: Skirt expanded more and quicker due to much thinner material, and grew by over 0.1mm (If cylinder was to stay at ambient temp you'd get a big seize up as piston at this temp larger than bore by 0.05mm)
Skirt measured across the wrist pin: grew the least amount - 0.04mm
Top of piston: grew by 0.08mm but this measurement not related to real world where top of piston much hotter in engine
Cylinder ID grew by approx 0.06mm
Nothing very accurate or scientific with oven baking but looks to me like piston/bore clearances on hot engine will be very very small.
Engine rebuilder just told me some very expensive temp sensors that transmit live data have been fitted to engine parts before. He reckoned cylinder and bottom of piston 100 C, top of piston 350 C.
Upshot of this is I'm starting to think the main problem with excess wear on cold start-up might not really be due to what I thought it was - ie lack of lubrication on the bore at moment of startup, but rather a rapidly expanding piston against a more slowly expanding bore. Seems to me the old air-cooled cylinders had a possible advantage as they'd heat up a lot faster than a water cooled one. I have a feeling the major wear might be somewhere around a minute or three after starting, so low revs/low load til after this time?
Good question as im in the process of fitting a big bore kit to my 92 blade.I got the kit several years ago in england, but there was no specs on piston bore clearance.The liners i have fitted are composite material which need to be honed with a diamond hone.The pistons have a coating on the top only.Im thinking on .002" but all i can do is guess really.
If the factory have the clearances too tight from new then going with a forged piston in a standard bore might help, I can't believe they would screw that up though. - forged piston will expand less when heated than a cast (if they not forged from factory that is) Whats the piston to bore clearance (with a feeler guage) if you get the barrel up to 50 degrees or so and drop the hot piston without rings into it? You could do this at a few different temps and see how tight the clearance gets?
I recall reading an article ages ago by Kevin Cameron (who knows what he is writing about). The article was about lubrication and he wrote that a piston will be very close to a press fit in a cylinder at operating temp, and it's only the lubrication film that prevents everything seizing solid.
Also pistons (good ones anyway) are shaped to allow for expansion differential,eg they might be smaller across the pin bosses than front to rear. They might also be bigger at the bottom than the top
Last edited by pete376403; 6th February 2009 at 19:14.
Reason: additions
it's not a bad thing till you throw a KLR into the mix.
those cheap ass bitches can do anything with ductape.
(PostalDave on ADVrider)
can help u there but not until Monday as I am racing this weekend , I have some stuff that will answer your question as i do this stuff every day ,,hey its my day job!!!! ..Ive attached a photo of a 2 stoke piston ( used the wrong mesh so , its a bit short in the heat flow ,,,I cant find the final copy off the top of my head ,, but this will give you a good Idea...
I was modelling heat flow on a piston After detonation had compressed the boundary layer ,,,,I am not sure if this picture is before or after compression ( after I think )
anyway the formula is something like Dela L = (alpha ) +(1+original length ) where alpha is the co efficient of expansion ......( sorry pissed ,,cant remember , when sober remind me )
yes you are correct about the lubricant , though some parts rely on boundary lube ,,,ie down to the chemicals in the oil ,,,
I run my Enfield Race bike . alloy barrel , piston , high comp at 4 thou clearance , ,,, I say this because Enfields are prone to seizure when you up the thermal loading ....
one season under me belt no prob,,,but I reduced the thermal loading ,,a lot ...
Stephen
"Look, Madame, where we live, look how we live ... look at the life we have...The Republic has forgotten us."
forged piston will expand less when heated than a cast
Not true, they actually expand MORE, look at your cold clearances, much more for a forged jobby, that's why they rattle lots more when cold.
But it comes back to the fact that warmup is still the most critical time for an engine, especially older one's. The newer one's with the plated bores are a little better, you get more uniform expansion....
Not true, they actually expand MORE, look at your cold clearances, much more for a forged jobby, that's why they rattle lots more when cold.
But it comes back to the fact that warmup is still the most critical time for an engine, especially older one's. The newer one's with the plated bores are a little better, you get more uniform expansion....
I agree.
Now. So long as you're up, you might as well pop over and chuck that gearbox back in
Not true, they actually expand MORE, look at your cold clearances, much more for a forged jobby, that's why they rattle lots more when cold.
But it comes back to the fact that warmup is still the most critical time for an engine, especially older one's. The newer one's with the plated bores are a little better, you get more uniform expansion....
Ah ok had that arse about face... backyard mechanic bullshit
I was talking to Steven Briggs today about this and he told me they'd had sensors in exhausts headers before - 800 degrees! Hot enough to vapourize an egg let alone fry one. He also pointed out heat is a different deal with a stroppy two stroke as they're cooking up a storm with firing just a bit more frequently than easy going 4t. Two stroke cylinders are also a problem with the transfer port holes. Mine had obviously gotten a bit hot by prev owner. Piston had pressure marks from port bridges. The bridges being quite thin they expand more.
Hey bsasuper, be real careful with that cylinder. What is your coating type? My Nikasil is apparently only about .15mm thick. I just had it honed. Now regretting this as I lost about .02mm - doesn't sound like much but wear factory tolerance is only .025
Human hair is about .05mm so wear tolerance literally half a hair.
Jonbuoy, I put the piston in the bore with everything cooling down still pretty hot. Piston was quite snug in bore but still free enough to drop through. I'm told you have to be more careful about heating engine up first with forged pistons, denser material expands differently.
Pete, hmm I find that degree of tightness pretty easy to believe. Scary stuff. Seems like way too much friction working so hard against the film of oil. My piston is about .1 wider at the skirt than the top. I don't doubt pistons eventually expand to be quite even but feeling the thin skirt might swell up quicker. Maybe a couple of pinholes for extra lube during warm up might help longevity, cant see it causing any harm? There must be a theoretical perfection of clearances that will just be impossible to achieve - too many vairables. Dirt bikes have pretty crude cooling systems, they only got around to fitting a thermostat on my model on 2004. I'm considering other options for cooling improvements, eg overflow bottle, and a fan I can switch on in the slow and gnarly stuff.
Brain d'marge. I like the image and now wondering how a piston with heat sink fins cast under the crown would go. looking forward to this data. Go easy on the formulas and the turps though. Too much science will cause seizure if my brain expands too rapidly.
You're right about the relative crudity of bike cooling systems. My KLR has a thermostat and waterpump, but the operation of the 'stat means the temperature bounces between hot and cool as the water flow starts and stops.
This guy http://members.cox.net/watt-man/TB%20Testing1.htm has developed a more sophisticated system and has the numbers to compare against the standard. Had a much tighter control of water temps which should translate to more even wear
it's not a bad thing till you throw a KLR into the mix.
those cheap ass bitches can do anything with ductape.
(PostalDave on ADVrider)
Good link. I really like this guy. He nutted something out and went to the trouble of having a go. His data looks good but I'd still be worried about potential for boil-up at some point if not diverting max flow into rads. Doesn't seem to be any control or adjustability in his bypass. I like it though. Great food for thought. Cheers Pete.
Was talking to a mate and he argued that 2 stroke exhaust gas is cooler than 4 stroke. This could definitely be right. I can nearly put hand on my expansion chamber but you wouldn't try that on a hot 4 stroke header. Probably has something to do with higher compression ratios. I've seen a Holden V8 race car with red hot headers. Didn't take long to get hot either.
I seem to remember from somewhere, (Top Gear TV?) when they cold start an F1 car, oil is heated before adding to engine. I imagine they have very accurate control of engine clearances.
This is just a brain fart, but I wonder if you could run a cylinder heat up system using exhaust gases. Water jacket around header, closed loop with engine water jacket until operating temp?
I'm not worried about the KLR having a boil up - the radiator has way too much cooling capacity for the engine - I've blocked off the lower half of the rad with duct tape to try and get a bit more heat into the engine. I'd like to try the bypass next.
But you'd still be cold starting the engine in order to get the hot exhaust gas.
If you're really worried, put a small electric heating element into the block and warm the engine that way before starting. I believe this is done with fire appliances when parked in the station, so they are always ready to go.
And if you're REALLY paranoid, connect an external pump to the lube system to get oil pressure up before starting.
it's not a bad thing till you throw a KLR into the mix.
those cheap ass bitches can do anything with ductape.
(PostalDave on ADVrider)
Very interesting about the fire appliances Pete. Was thinking about a tire warmer type arrangement to wrap around barrell. Power off car and heat up while towing bike to destination. I can imagine the laughs I'd get as device unplugged, but who then would have the last laugh? I'd be worried about heat not transferring to piston properly though, as contact mainly via rings. I can imagine a problem if device not on for long enough - ie hot cylinder, cold piston.
Still keen to know clearances at operating temp. Someone around here must know. Bump.
I managed to talk to a company in england about the piston clearance of my big bore kit.Was told .003" and to warm it up gently before every ride. Seams the piston material is very strong but expands more then usual im told.
Reply With Quote
Originally Posted by jonbuoy
Bookmarks