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Brawg

big bore and stroke

10 posts in this topic

I am thinking of putting a cylinder works big bore coupled with a hotrods +3mm stroke (-3mm rod) on my 2008YZ450F. The motor started making a knocking sound mid power while riding last weekend. I am curious how the two parts will work together. I ride/race harescrambles/offroad and plan to put a heavier (heaviest i can find) flywheel weight. Please offer any and all insight. I would prefer to hear from folks who have done this, but will bs with anyone. i realize the implications to the power and how this "should" effect it, but would like to know how the compatability will work.

Cyl works offers a +3mm cylinder kit with the big bore. Does this compensate for the need for the shorter rod? or do i still need the -3mm rod that comes with the hot rodz crank?

The way i understand this scenario, i will be able to use the cyl/piston/rings/gaskets from cyl works with the hot rods +3mm crank. help please so i can order the parts.

thanks,

josh

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You will want to have the entire assembly balanced before you install it. The crank will have been balanced assuming the weight of the stock piston. The overbore will throw that off.

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You will want to have the entire assembly balanced before you install it. The crank will have been balanced assuming the weight of the stock piston. The overbore will throw that off.

Not only that, but if you have ever ran one of these cranks with a stock piston it will still rattle your fillings out. I highly recommend balancing these cranks no matter what.

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You'll need to find a shop specializing in small engine balancing. Engine Dynamics may be one source.

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The guys at cylinder works are saying that the crank and the cylinder kit are made to work together. I'm waiting on reply from CW to see if there is any additional issues with the head or anything else. If not, i'ma pull the trigger.

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I put an Athena BB kit and a Hot Rods +3 stroker crank in my YZ250F. Although it gave the bike a healthy dose of torque, I don't know if it was really worth it overall. I was hoping that the BB and stroker would keep it from vibrating, but not so. It became a paint shaker. It rumbled my hands and feet like a KX500. I didn't notice it so much when I was riding but I sure could feel it just sitting there warming the bike up. It also took a bit of jetting to get it to run right. It was a lot harder to kick over and required more technique to start. After 25 hrs the crank split in half on one of the crank cheeks. Luckily, it didn't damage the rest of the motor. Hot Rods gave me a new crank under warranty. I put the bike all back to stock and sold it.

On the 250F +3mm crank, they had to machine off part of the radius of the crank cheeks (opposite the crank pin) so that the piston wouldn't hit the cheeks at BDC because of the shorter rod and offset crank pin. The cheeks aren't circular in shape. Hot Rods just did what they had to to make it work without spacers, longer cam chain and guides, etc. I doubt they even took balance into consideration. It resembles a stock crank with the cheeks mowed down to clear the piston. Also, this crank pulls the piston 3mm farther out of the bottom of the cylinder at BDC (so that at TDC it is same height as original). Not good! The piston can rock more and the rings are dangerously close to being pulled out into the side cutaway area of the cylinder. Sure the BB kit and crank work together but it's not a perfect marriage. They were both designed to be used by themselves as well. They could have made their BB cylinder taller instead of shortening the rod and possibly still used the stock chain and guides. Yeah, it all works but it wasn't really the right way to do it, just the easy/cheap way. RPM's BB/stroker kits are much more thought out and are the right way to do it. But, they are substantially more expensive.

I know that was a 250 and not a 450 but I'm sure you will have similar results unless you get everything balanced. I'd hate to see you build a finicky, vibrating time bomb. If you like to tinker and are just one of those guys that likes to try stuff because you can, then go for it. (I did). If you want to bolt something on and go with no problems then I think you'll be dissappointed. I would look into other ways of making power. I agree that nothing fits and works as good as OEM.

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You will increase engine vibration doing any of this. But you increase torque and power also. it is a trade off. Factor increased cost and zero resale value and make your decision.

After doing it to my 03, I would probably keep my bikes stock. It is a blast to ride but it will wear you out much faster

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I personally have one of these cranks in my own bike balanced to a 97mm bore piston on the stock cylinder, and have installed countless others in various motors. Hot rod claims the cranks are balanced to stock pistons, but they aren't even close. This is why people have problems with them. However, if you pull the crank apart, balance it then true and weld it they are a great choice. They will make more torque then a stock engine, and run smoother as well.

One of the downsides to a single cylinder crank is you can't have it perfectly balanced at all rpms. You balance it for a target rpm. So on a big bore and stroker engine they will likely be a little rough at an idle, but if balanced properly at rpm will run much smoother than a stock engine.

However if you slap that crank in there as it comes from hot rod. it doesn't matter what piston you run, it will vibrate and vibrate excessively. This excessive vibration is the cause for the engine/crank failures. Now i'm not saying that hot rod cranks are perfect, but they are a pretty good alternative to a stock crank. heck if you want a stock 450 crank i have one sitting at home i will make someone a great deal on.

I prefer crank works in Arizona for all my crank service work. Shoot me a pm and i can hook you up with them and give you some more details. I'm sure other shops can balance it for you but from the hundreds of cranks i have had crankworks do for me they have all been top notch.

Oh and if you add more cc's and high compression you can expect an engine that is harder to turn over. However on my 450 with 13.5:1 compression at 490cc's i don't' think it is much harder to turn over then a stock engine as the decompression release works very well on these engines.

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