yz450f big bore + stroker help


15 replies to this topic
  • Lumberjack

Posted June 18, 2009 - 08:27 PM

#1

I ride a 2007 yz450 and blew my engine a few weeks ago via a valve head snapping off and basically destroying my whole engine, so I have ordered a hot rods 3mm stroker crank and Athena 480cc big bore kit since it is not much more expense or even cheaper than OEM.

My question is will the big bore and stroker affect the compression ratio or anything? Will this be a direct bolt on or will other modifications need to be done, I know both the stroker and BB kit are supposed to be straight bolt ons, but if I am doing both at the same time will it mess anything else up? If anyone can help that would be great.

  • davidl9999

Posted June 18, 2009 - 08:35 PM

#2

Lots of questions there.
I'll try to answer from the standpoint of doing any rebuild, rather than the specific product since I don't own either of those mods.

1) It seems like you haven't researched the product with the manufacturer (Athena), and I urge you to do so. I would expect them to be able to answer 100% of your questions, and I think that you will be happier overall with your decision to rebuild with their mods, or not.

2) I don't know if compression ratio is affected. I would expect that it will be about the same or slightly higher, or there's no real gain in HP from adding 30cc and dropping compression. You should expect more torque from a stroker crank - all things being equal - since the "lever" (rod journal offset) is longer than stock. That does not mean that it will rev as high (or should) since the piston speed increases and thus would mechanical stresses on the reciprocating parts and crank bearings/cases. Does that matter in this case? I don't know. Since the limiter seems to be set fairly conservatively on these bikes, I'd expect that you'd be okay.

3) Remember, if you add stroke, you need to shorten the rod or use a piston with the wristpin located closer to the piston crown. If you don't, expect piston crown-to-head and/or piston skirt-to-crankshaft interference. (Basically, make sure you have enough $$ to rebuild the engine again).

4) Read (1) again. Really. Call them, and good luck.

I'm sure that the TT community would be interested in what they say, so please post on this again. :worthy:

  • NEKOOHC

Posted June 18, 2009 - 08:42 PM

#3

you can fit a spacer plate or use a stroker piston ... the stroker piston has 2 piston rings .. or you can use the spacer plate and a 3 ring piston

  • davidl9999

Posted June 18, 2009 - 08:48 PM

#4

you can fit a spacer plate or use a stroker piston ... the stroker piston has 2 piston rings .. or you can use the spacer plate and a 3 ring piston

Good point; I forgot about that. Since the swept volume is the same (as with a shorter rod or narrower-decked piston), the compression ratio doesn't change. Other geometries do (slightly); usually no biggie.

  • Hensley419

Posted June 18, 2009 - 09:58 PM

#5

worked great on my honda 450 made it 511cc big power!! leaner pilot jet larger main.

  • grayracer513

Posted June 19, 2009 - 07:52 AM

#6

The manufacturer of the stroker kit is the best person to talk to with regard to its effect on compression ratio and they will also usually supply the necessary components, such as shorter rods or spacer plates, as part of the kit, or will direct you to use a stroker piston.

Bear in mind that the 3mm stroke means the piston travels 1.5 mm farther in both directions from the same center, so the amount that the head needs to be raised, or the piston or rod shortened, is only 1.5 mm.

Shortened rods are not the preferred method of meeting this need, however, unless the engine is being built for lower RPM use. Changing the length of the rod alters the piston speed curve, and shortening it creates a bias toward low speed power. It also increases the angular load on the crankshaft.

  • davidl9999

Posted June 19, 2009 - 10:54 PM

#7

Yep, much better to move the wrist pin and shorten the skirt. :worthy:
(still better than using a shorter rod for the above-mentioned reason)
No matter what you do, some geometry is going to change, almost always to some kind of mechanical detriment.

  • mx730md

Posted June 20, 2009 - 06:03 PM

#8

i dont know why you guys are talking about spacer plates and what not the hotrods are a direct bolt on kit. i have a 3mm and a ice cube 276cc jug on my yz250f . the crank is different from the oem one. i asked this exact question when i bought it all. i think they said the pin is 1.5mm lower in the crank halfs with the rod being 1.5mm longer . one thing i would say is if you want it be ready for some vibration in the bars or have send the crank to some one and have it balanced . mine feels like a 250 two stroke in the bars.

when i droped the crank in the cases i did take the piston and put it on the crank just to double check that it cleared the crank at bdc , if i where you i would do the same you wont have any problems but its just for that piece of mined

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  • husqy360

Posted June 20, 2009 - 07:34 PM

#9

:worthy: i want 1 two now :lol: .

what about reliability,? does it boil every sec?

till now i went oem for the piece of mind. but i do want some more power for the dunes:banana: .

how much gain in torque and hp you expect?

best luck.

  • grayracer513

Posted June 20, 2009 - 10:56 PM

#10

i dont know why you guys are talking about spacer plates and what not the hotrods are a direct bolt on kit.

If that is the case, then the rod has been shortened to compensate for the stroke so that off the shelf 450 parts can be used. Shortening the rod is, as I said, the least satisfactory way to approach the problem, but since it's only 1.5 mm, it won't be that important.

But you should plan on having the entire assembly balanced.

  • Lumberjack

Posted June 22, 2009 - 04:50 PM

#11

Thanks for all your help guys, I will call the manufacturer just to be sure and I will have the bike dyno'ed when it is all back together.

Before it blew up my bike had stock bore and stroke but had head work done, so it will be interesting to see how this feels compared to before.

  • Lumberjack

Posted July 28, 2009 - 10:57 AM

#12

I ended up going with a Cylinder Works big bore kit since they are the same manufacturer as Hot Rods and I figured that it should be alright. I spoke to the people over at Hot Rods and they recommended getting everything balanced, so that is where I am at. My crank is being balanced as we speak and I expect to have it back near the end of the week.

Also, I bought kibblewhite stainless valves and valve springs, retainers, etc...

  • Kyledvor61

Posted July 28, 2009 - 09:04 PM

#13

I am putting together the exact same setup as you. I've read that getting the carb bored on motors this big helps alot too.

  • mx730md

Posted July 29, 2009 - 04:10 AM

#14

I ended up going with a Cylinder Works big bore kit since they are the same manufacturer as Hot Rods and I figured that it should be alright. I spoke to the people over at Hot Rods and they recommended getting everything balanced, so that is where I am at. My crank is being balanced as we speak and I expect to have it back near the end of the week.

Also, I bought kibblewhite stainless valves and valve springs, retainers, etc...


why did you go with steel valves? i made that mistake too:bonk: :thumbsup: .

also how much does hot rod charge to balance the cranks? if you don't mind.

  • Lumberjack

Posted July 29, 2009 - 05:51 AM

#15

I bought the steel valves because I heard they last alot longer and a broken valve was what did my bike in, in the first place. Hot rods is not balancing the crank, I am having that done at a local shop up here in Ontario called www.bondiengines.com

What was your issue with the steel valves?

  • Lumberjack

Posted August 04, 2009 - 10:01 AM

#16

I just spoke to the people where I brought my crank and they said it was out of balance by a HUGE amount, apparently it was out by about 120 grams per side for or 240 grams in total, does this seem right to you guys? That seems like a huge amount, it will probably feel like my bike has a flywheel weight now.





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