Why are our pistons so tall?

I have been seeing many late model bikes being rebuilt etc. on youtube, and it seems like the piston skirts are really short. Why are our pistons so much taller than new bike pistons? It is because new bikes tend to be more oversquare and have not as much chance to wobble in the bore?

I don't know for sure, but I would guess it has to do with piston to cylinder clearance. With air cooled engines I would think we need to run more clearance to have room for the expansion of the pistons; with the water cooled bikes they can most likly run tigher tolerances.

I know for sure our ring gap is larger because of this. There are just sooo many benifits to water cooling.

I wish the POS bike I had bought years ago had been a watercooled zx750 not my kz650...( I am to damn stuborn to sell the 650 and buy something that actually makes sence) but such is life.

:S

Here's a shot of the new 2010 BMW R1200RT/GS piston. This is still air/oil cooled engine.



Very short pistons too???

Wow DUAL overhead cam too! How futuristic!

They simply keep making them ligther, reducing them to bare minimun to lower the reciprocating mass and increase RPMs. Have you seen how tall were the pistons used on flat head engines of the '40s?. In the '50s with the advent of short stroke OHV V-8, even the cutouts for the crank weights was a revolution.

those newer short skirted pistons are strictly a high rpm thing,very little reliability in that design.

I wonder what the chain guide/tensioner on that BMW engine is made of? It looks like it would need to be pretty tough not to wear out quickly with that sharp bend in the cam chain. Ed

PLUMMEN wrote:

those newer short skirted pistons are strictly a high rpm thing,very little reliability in that design.

I have a hard time believing that. Please tell my how many miles before these new bikes are wore out? My friends ride sport bikes, and while I don't think they will hit 100,000 miles on the factory cam chain like Ron's 650 did I am sure they will make it 30K before they need the pistons replaces. I have read more than one member recommend a full motor overhaul for KZ's with 30K miles or less.

compare what maintenance is required on a sportbike motor at 30,000 miles and what a typical 900-1000 motor needs at 30,000 miles! :laugh: ive gone through plenty of ancient dinosaur z1 motors that didnt need much more than a gasket set to keep em going another 30,000 miles.come back in another 10 years and show me how many late 80s-early 90s zx11s and cbr900rr's you see still running the street under their own power let alone on the origional engine! :laugh: short skirted pistons and flyweight valvetrains have been around in the automotive world since atleast the 60s,they originated in the world of drag engines that just had to make it a 1/4 mile at a time without blowing up. get a little bit of wear in the bore and watch them babies rock in those plated sportbike bores. and yes ive been a motor or 2,so ive got a little experiance.

Well I guess I will let my buddy know that his R6 is about to let go on him. He will have a laugh at that.


Tell you what, how about at 30K miles on the R6 I will have him take a leak down test... we will video the whole thing. He put 10k on the bike this year, so I am thinking middle of next season his 07 R6 will need a serious rebuild; that is if you actually know what you are talking about.


I love sport bikes, they are today what the z1 was. It just seems like the natural evolution.

But then some folks like going fast, and other folks talk a lot and post 30 year old photos.

Some thoughts. Pistons attempt to cant (rock) fore & aft wilst jumping up & down. Longer skirts/pistons restict this motion by providing a greater linear contact area and reducing wear... weight, inertia & friction are huge factors where pistons are concened. Advances in Metalurgy have allowed tighter clearances, thus shorter/lighter pistons. (often w/ only 2 rings) thus reducing the big 3. & allowing higher revs, As a Kid, I often questioned the Answers, If intrested, you might research Pistons/rods /thermal expansion/redlines... G

I dont say its just the pistons,i say the whole design is throw away!