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TOPIC: Why are our pistons so tall?

Why are our pistons so tall? 17 Dec 2009 06:52 #339385

  • 9am53
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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?

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Why are our pistons so tall? 17 Dec 2009 08:23 #339398

  • hardr0ck68
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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.

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1977 kz650 c1

bought it because I was told it would never run again...I like to prove people wrong.

Why are our pistons so tall? 17 Dec 2009 08:28 #339399

  • YUKABODOS
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:S

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Last Edit: by YUKABODOS.

Why are our pistons so tall? 17 Dec 2009 08:39 #339400

  • s3steve
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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!

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1973 Kawasaki Z1
1975 Kawasaki S3
2006 BMW R1200RT

Why are our pistons so tall? 17 Dec 2009 11:11 #339422

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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.

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Why are our pistons so tall? 17 Dec 2009 15:30 #339446

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

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Still recovering,some days are better than others.

Why are our pistons so tall? 17 Dec 2009 16:00 #339454

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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

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1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

Why are our pistons so tall? 17 Dec 2009 16:05 #339455

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PLUMMEN wrote:

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


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.

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1977 kz650 c1

bought it because I was told it would never run again...I like to prove people wrong.
Last Edit: by hardr0ck68.

Why are our pistons so tall? 17 Dec 2009 16:22 #339461

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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. B) 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. B)

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Still recovering,some days are better than others.

Why are our pistons so tall? 17 Dec 2009 16:59 #339470

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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.

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1977 kz650 c1

bought it because I was told it would never run again...I like to prove people wrong.

Why are our pistons so tall? 17 Dec 2009 17:02 #339471

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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

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[img] 1977 KZ1000A1

Why are our pistons so tall? 17 Dec 2009 18:18 #339506

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

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Still recovering,some days are better than others.
Last Edit: by KZQ.

Why are our pistons so tall? 17 Dec 2009 18:23 #339509

  • roy-b-boy-b
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Dear Hardrock, As you may have noticed this is a community.

We all don't agree.

You seem to disagree with everyone.

Could you please state you opion and let it alone?

I personally think the short skirts are the results of much testing.

There have been vast improvments in materials that make it possible for radical designs that would not have worked years ago to be possible today.

I like the idea of a big bore and a short stroke.

By the way, What would be the piston speed be on a modern bike with a over square bore?

I don't know "nutin" compared to some of these members here but the good thing about that is if i read what they post i then have a insight to what they know and i can sit in my shop and basically build what they have because i kept my mouth shut and didn't question them and will have thier support.

You can only drag something through a fire so many times and then it is going to flame.

These posts are a good example. They take away from the technical and become a flame war.

We don't need no stinkin flame war.

May you have a wonderful day and be blessed. :) Roy

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1979 LTD Street Fighter.1977 KZ1000

Why are our pistons so tall? 17 Dec 2009 18:28 #339511

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i got nothing against sportbikes, they go fast and handle well etc. but I can't imagine them going for 30+ years like these old bikes do. Like was said, in another 10 years I can't imagine too many early 90's sportbikes around, and sure they got beat up, but these old bikes got beat up just as much.

Riding around on the street on a flashy coloured plastic covered new sportbike is the equivalent of pimping it up in a souped up civic with an h22a and a spoiler. (kind of juvenile) Sure they are efficient, and fast, and handle well, but a 1967 merc cougar gt will get chicks horney too, and still be around when fast and the furious dvd's are in the bargain bin.

And not that I know anything, but after watching the youtube clips I mentioned in the OP I recall a few talking about some nikasil plated bores that need to be replated or something, I don't know, and these are NEW bikes...

skip to the end




I just figure with a taller piston like we have, we can run the "stroker" cranks that we basically all do, and although the con rods are moving in bigger circles the piston height keeps them running true, whereas with the short stroke new bikes they can run shorter pistons because the lateral movement will be less because the crank is the size of a pencil...but I guess like in the video even that logic fails

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Last Edit: by 9am53.

Why are our pistons so tall? 17 Dec 2009 18:29 #339512

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roy-b-boy-b wrote:

Dear Hardrock, As you may have noticed this is a community.

We all don't agree.

You seem to disagree with everyone.

Could you please state you opion and let it alone?

I personally think the short skirts are the results of much testing.

There have been vast improvments in materials that make it possible for radical designs that would not have worked years ago to be possible today.

I like the idea of a big bore and a short stroke.

By the way, What would be the piston speed be on a modern bike with a over square bore?

I don't know "nutin" compared to some of these members here but the good thing about that is if i read what they post i then have a insight to what they know and i can sit in my shop and basically build what they have because i kept my mouth shut and didn't question them and will have thier support.

You can only drag something through a fire so many times and then it is going to flame.

These posts are a good example. They take away from the technical and become a flame war.

We don't need no stinkin flame war.

May you have a wonderful day and be blessed. :) Roy




Very well put Roy, I could not have said it better.

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73 Kawasaki Z1
07 HD CVO Ultra Classic
82 Suzuki GS 1100
74 Yamaha RD 350 (My two stroke toy)
77 Kawasaki KZ 650B-1 (My putt around bike)
80 Indian Moped (My American Iron)
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Long Gone
75 Suzuki GT550
74 GT 380
79 RD 400 Daytona Special
72 Honda CL 175
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Why are our pistons so tall? 17 Dec 2009 18:33 #339515

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9am53 wrote:

i got nothing against sportbikes, they go fast and handle well etc. but I can't imagine them going for 30+ years like these old bikes do. Like was said, in another 10 years I can't imagine too many early 90's sportbikes around, and sure they got beat up, but these old bikes got beat up just as much.

Riding around on the street on a flashy coloured plastic covered new sportbike is the equivalent of pimping it up in a souped up civic with an h22a and a spoiler. (kind of juvenile) Sure they are efficient, and fast, and handle well, but a 1967 merc cougar gt will get chicks horney too, and still be around when fast and the furious dvd's are in the bargain bin.

And not that I know anything, but after watching the youtube clips I mentioned in the OP I recall a few talking about some nikasil plated bores that need to be replated or something, I don't know, and these are NEW bikes...

I agree..If the sportbikes are such long lasting things, then where are all these parts for the USD front ends and fat tired KZ conversions coming from?? :dry: Alot of guys putting 5 - 10 yr old wheels etc on 30 yr old bikes that STILL run...:)

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...1980 LTD 1000...
Last Edit: by will61310.

Why are our pistons so tall? 17 Dec 2009 18:37 #339516

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Fair enough I am going to step away. But I will promise when my buddies bikes turn 30K I will post leak down results .

Plumen for calling you, what would I have to say? I don't work at a bike shop, building my lil 650 is a hobby. I just don't believe you know what you are talking about when it comes to things from 1986 to present and you have never shown any proof to back your claims. But you do have some nice pictures...that you post repeatedly in every thread even slightly relevant.


Sorry, I don't mean to escalate... I will leave the thread now.

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1977 kz650 c1

bought it because I was told it would never run again...I like to prove people wrong.

Why are our pistons so tall? 17 Dec 2009 18:38 #339517

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Where's the love machine.....

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Why are our pistons so tall? 17 Dec 2009 18:39 #339518

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hardr0ck68 wrote:

For the most part these bikes are not collectors items(the z1's and some other odd exceptions excluded), they are junk... the real issue is no one makes parts for junk.

I still dont understand why you come to this site.

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Why are our pistons so tall? 17 Dec 2009 18:40 #339520

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I feel its a compliment that the sportbikes of today evolved from the mighty Z1. I remember when the 83 Katana hit the streets with its space age design, then the sportbikes evolved into what they are today
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73 Kawasaki Z1
07 HD CVO Ultra Classic
82 Suzuki GS 1100
74 Yamaha RD 350 (My two stroke toy)
77 Kawasaki KZ 650B-1 (My putt around bike)
80 Indian Moped (My American Iron)
1
Long Gone
75 Suzuki GT550
74 GT 380
79 RD 400 Daytona Special
72 Honda CL 175
74 Honda QA 50
Tampa FL