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4mm Stroker Crank

This is a discussion on 4mm Stroker Crank within the Yamaha YFZ350 Banshee (1987-Present) forums, part of the Performance /Technical category; Just wondering what types of riding are the 4mm stroker cranks made for? My stock crank just went out and Im thinking about bidding on ...


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Old 06-18-2004, 04:30 PM   #1
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Default 4mm Stroker Crank

Just wondering what types of riding are the 4mm stroker cranks made for? My stock crank just went out and Im thinking about bidding on this ..

http://cgi.ebay.com/ebaymotors/ws/eB...category=43977
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Old 06-18-2004, 04:31 PM   #2
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You can use it for many types of riding. I know some guys that use them for just trail riding, etc. A 4mm setup can be pretty reliable.

Craig
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Old 06-18-2004, 10:08 PM   #3
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the auction says i will need a "4mm spacer plate" what exactly is that and where does it go and where can i get one ?
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Old 06-19-2004, 04:29 PM   #4
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the guy i know has one in a play banshee for sand, it hauls ass, he also has an 8 mil but you have to trench the case when you go that big, another friend has a ten next spring im going to put stroker in mine :D
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Old 06-20-2004, 08:32 PM   #5
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jim--- the 4mm spacer plate is a aluminum plate that goes inbetween the case and the jugs to give u that extra 4mm of displacement/space

if i were you i would contact the manufacturer of the crank and ask to buy one
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Old 06-20-2004, 10:18 PM   #6
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One more question... is the 4mm spacer plate the only thing that is needed to run a 4mm stroker crank? Any special pistons, porting etc?
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Old 06-21-2004, 09:47 AM   #7
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Quote: Originally Posted by JiM417
One more question... is the 4mm spacer plate the only thing that is needed to run a 4mm stroker crank? Any special pistons, porting etc?
only 4 hours left on the auction.... plz help
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Old 06-21-2004, 11:16 AM   #8
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I'll admit, I've never been inside the banshee engine, but I DO know my two-stroke engine theory, and the 4mm spacer plate doesn't make any sense to me.

First off, as far as I knew, a 4mm stroker moved the pin out 2mm on the crank. So you get an extra 2mm at TDC, and 2mm at BDC, which gives you your 4mm longer stroke. So you would only need a 2mm spacer to keep the piston from hitting the head. If you used a 4mm plate, but the piston is only 2mm higher at TDC, you'll lose lots of compression and thus you would lose power. The only way around this is to also increase the rod length by 2mm, which may be necessary to keep the piston from hitting the crank.

Or do they move the rod out 4mm, adding a total of 8mm to your stroke? I'm pretty sure it's only 2mm, giving you an extra 4mm stroke.

At the same time, the 4mm spacer would throw all of your port timing off. By increasing the height, your ports open sooner, and close later, much like a high-rev port job would do, but far less effective.

So, somebody knowledgable on the banshee motor clue me in. How does it all work together?
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Old 06-21-2004, 02:36 PM   #9
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Quote: Originally Posted by wilkin250r
I'll admit, I've never been inside the banshee engine, but I DO know my two-stroke engine theory, and the 4mm spacer plate doesn't make any sense to me.

First off, as far as I knew, a 4mm stroker moved the pin out 2mm on the crank. So you get an extra 2mm at TDC, and 2mm at BDC, which gives you your 4mm longer stroke. So you would only need a 2mm spacer to keep the piston from hitting the head. If you used a 4mm plate, but the piston is only 2mm higher at TDC, you'll lose lots of compression and thus you would lose power. The only way around this is to also increase the rod length by 2mm, which may be necessary to keep the piston from hitting the crank.

Or do they move the rod out 4mm, adding a total of 8mm to your stroke? I'm pretty sure it's only 2mm, giving you an extra 4mm stroke.

At the same time, the 4mm spacer would throw all of your port timing off. By increasing the height, your ports open sooner, and close later, much like a high-rev port job would do, but far less effective.

So, somebody knowledgable on the banshee motor clue me in. How does it all work together?

interesting! ive always just assumed that it just ment the rods were 4mm longer
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Old 06-21-2004, 03:31 PM   #10
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Quote: Originally Posted by lilsherm89
interesting! ive always just assumed that it just ment the rods were 4mm longer
Not at all. A longer rod will change your power curve a little due to the angles between the piston, rod, and crank (Honda did this exact change on the 250r in 1987) but it will not increase your stroke length. The stroke length is determined by the distance between the center of the crank and the rod location. If you move the rod out further on the crank, you get a longer stroke.

Displacement is a volume measurement. Imagine you have a pint of beer. Now raise that pint 4mm off the bar. Do you have more beer? No, it's just in a different location.

Now, many engines us a long rod (heh, heh, he said...) to decrease the rod angles and increase piston and rod life. When you do a stroker crank, let's just use 4mm as an example, they move the rod OUT on the crank by 2mm. But this then means that the piston will be 2mm higher at TDC, which would most likely hit the head. So they get a connecting rod that is 2mm shorter, so the piston is at the same place at TDC.

This is all well and fine, unless your connecting rod is short to begin with. Let's paint two scenarios: Long rod and short rod. Picture the crank, rod, and piston in your mind. As the piston goes up to TDC, it is less than 1mm from the head, so if we make the stroke taller by 2mm, it will hit the head, unless we do something about it. Now imagine as the piston travels downwards to BDC. Obviously the rod must be longer than the diameter of the crank, or else the piston would hit the crank.

Let's imagine that a quad with a 60mm stroke has a rod that is 72mm long. The piston requires 4mm to clear the crank at BDC. Now do a 4mm stroker on it, which means you move the rod out 2mm, and then decrease the rod length by 2mm so the piston doesn't hit the head. Now you have a stroke of 64mm, and a rod of 70mm, which still gives you the 4mm you need for the piston to clear the crank.

Now, imagine the same scenario, but the rod is only 66mm to start with. When you shorten the rod, it's now only 64mm, the same as your stroke, and you don't have the 4mm clearance for the piston.
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Old 06-23-2004, 08:06 PM   #11
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Very good expaination
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Old 06-28-2004, 03:06 AM   #12
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If you up stroke you get harder crank to rod to piston angles, and wear out the engine more quickly, but it revs faster.

Most people go with a long rod stroker to alleviate some of the increased rod angle. Some do go with a short rod stroker to get that rev though, mainly in drag bikes and such

Some of those guys running 10mil strokers got some fast running engines
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Old 06-28-2004, 11:33 AM   #13
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I still have the same question. Why the 4mm spacer? What exactly do they do to the banshee crank that requires the spacer?

Yeah, it's obvious they move the rod out on the crank, I'm looking for a little more detail than that. Do they move the rod out 2mm AND get a connecting rod that is 2mm longer?

If so, why? Is the rod too short at BDC, or is it just to prolong engine life. It would drastically affect the port timing, is it really an acceptable trade-off?
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Old 06-28-2004, 04:15 PM   #14
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Quote: Originally Posted by wilkin250r
I still have the same question. Why the 4mm spacer? What exactly do they do to the banshee crank that requires the spacer?

Yeah, it's obvious they move the rod out on the crank, I'm looking for a little more detail than that. Do they move the rod out 2mm AND get a connecting rod that is 2mm longer?

If so, why? Is the rod too short at BDC, or is it just to prolong engine life. It would drastically affect the port timing, is it really an acceptable trade-off?
The 4mm spacer plate is really 2mm thick, since that is only as high as the piston goes extra into the head, the head can also be cut to gice the clearance. The Long rod versions have a 5mm longer rod and use the wiseco 795 series pistons which have the wrist pin hole 5mm closer to the top on the piston. this off sets the length off the longer rod so no port timing is changed and to head relief is need for this. Blaster pistons were used before the 795s came out for the long rods, because the wristpin location was 5mm closer than the shee, but the smallest size they were was 68mm...hence the 370 long rod engines. The Long rod is only use for reliabilty and some say it gives alittlew more torque. Crank clearance at BDC is not an issue on the +4mm. The added stroke does change the port timing of course, and the timing does need to be altered for the stroker setup to run adaquately.

Hope I answered all your questions if i didnt id be glad to explain more
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Old 06-28-2004, 04:42 PM   #15
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That answers everything perfectly. Interesting bit of trivia, with the blaster pistons.
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Old 06-28-2004, 04:45 PM   #16
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Good explainations Wilkin and Rebel!

Another thing to note is that if your cylinder has been ported based on your stock crank, going to a 4 mil requires different porting due to the change in port height. Typical (not sure of the exeptions) once jugs hava been ported to be utilized for a stock crank, you can't use those jugs on a 4 mill stroker (won't run good).
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Old 06-28-2004, 06:39 PM   #17
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with some porting you can re-port for a +4. Mine were previously ported and jim at passion was able to readjust them for my +4, but they were right at the limit. I believe with a spacer you can have more radical porting and still re-port as opposed to a cut head, since the porting isnt being raised with a cut head.
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Old 06-28-2004, 07:35 PM   #18
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Certainly not. The cut head would have the effect of LOWERING the ports, since the piston is now sitting higher. To get the same port timing, you would have to cut more material out, and there is obviously a limit to how far you can go before you run out of material or hit a water passage. The spacer plate would obviously be preferrable.
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