CRF 250 цилиндр 78mm->80mm??

[Andrey]

Так как восстановление и покрытие цилиндра CRF 250 стоит на 20 долларов дешевле чем новый новый цилиндр)), возникла идея расточить его до 80мм (поршня есть в обилии)
Может есть кто поделится опытом такого рода эксперимента?

[Zippo]

кого расточить, алюминий???
Гильзовать надо. Все приемущества никасиля пропадут.

[Константин 1]

Так как восстановление и покрытие цилиндра CRF 250 стоит на 20 долларов дешевле чем новый новый цилиндр)), возникла идея расточить его до 80мм (поршня есть в обилии)
Может есть кто поделится опытом такого рода эксперимента?

Вроде как Ю.П. в Запорожье уже делал такое.

[Andrey]

кого расточить, алюминий???
Гильзовать надо. Все приемущества никасиля пропадут.

Не-не, гильза не вариант. Разумеется, что никасилить после расточки.

[Zippo]

полагаю заникасилить после расточки для Ю.П. не проблема

[Yurii_FOX]

А теперь рассчитайте затраты на расточку+никасиль и негарантированный позитивный результат+дополнительная нагрузка на коленвал из за увеличения кубатуры. Лучше тогда новый цилиндр.

[sahacr500]

есть уже такие 350 см3 не меняя коленвала.давно продается такой набор

[Yurii_FOX]

а что за набор если не секрет?

[sahacr500]

звоните в харьков к торгашам.мне расказал про это вадик.у него сайт на мотокросс гум

[sahacr500]

но правда цена дороговата

[Yurii_FOX]

правильно, а у парня все на бюджете завязано (насколько я понял).

[Andrey]

Нет, на бюджете не завязано, но и переплачивать за Биг Бор кит не хочется...)))
350сс - таких в природе нет!
Биг Боры есть: 80mm - 262cc, 82mm - 276cc, и 302сс - строукер (82мм но с увеличенным ходом поршня)
262сс и 276сс стоят в районе 400-450$
Расточивать ОЕМ - 180+150=330$

[fanatykCZ516]

А балансировка КШМ? Поршень шире, тяжелее, вибрация и все такое? нет?

[Andrey]

В общем, на ТТ один человек (как оказалось, разработчик ББ китов) поделился многолетним опытом экспериментирования с увеличением размера цилинров. Он не рекомендовал растачивать ОЕМ цилиндры, в основном из-за уменьшения толщины юбки. Во многих его испытаниях расточек происходило откалывание фрагмента юбки со всеми вытекающими последствиями... Вот оригинал поста:

I would strongly advise against boring the stock CRF250 cylinder at all.

I have tried 10 different ways to use the OEM cylinder in a big bore kit, and none of the cylinders even lasted as long as a piston should.

Here are the steps I took on the path to realizing that I had to design a completely new CRF250 cylinder from the ground up:

1+2) I first tried boring the stock 78mm cylinder to 80mm and replating it.
I ran this as a 262cc on one test bike and 286cc on another bike with a 5mm stroke.
Both cylinders broke the skirts off in under 5 hours. (When I was in 2nd and closing on the leader, no less) Sadly, Josh Grant test rode my CRF286 when the cylinder was on it's last legs and the bike was way down on power compared to just a few days before because the cylinder was getting out of round. Then the little sucker turns out to be one of the fastest riders in the country. Hate when that happens!

3) I then welded the 04 skirts to fill in the cutaways. This is very similar to what Honda did later on the 05 cylinder. This 262cc cylinder lasted about 10 hours before the front skirt cracked and it started smoking.

4) Then I tried offset-boring the cylinder toward the intake side so that the thrust face in the exhaust side would remain thick. This cylinder got out of round and lost about 3 HP on the dyno within a few hours. (We offset bore the newer CR125 and CR250 cylinders as well as the KTM125 cylinders towards the intake because they are weak above the exhaust ports, so it was a simple matter to test this n the CRF250R)

5) I then tried a cast iron sleeve. The skirts did not break, but the piston and cylinder were worn badly in under 20 hours. There are a number of reasons not to run a steel sleeve that have been documented over the past 25 years. On a recent YZ250F big bore thread, I've made posts related to this.

6) I then tried an aluminum sleeve that was a little thicker than the OEM skirts that hang down into the cases. This cylinder got out of round and the front skirt cracked slightly.

7) I then did the same thing with a forged aluminum sleeve. It too, got out of round. It did not break, but the power fell off within a few hours.

8) At this point I realized that the sleeve OD had to be increased. This means that the cases had to be ground to clear the thicker skirts. My men built a "split sleeve" when I was mountain-biking at Mammoth one week. It had a lip on the top like a traditional sleeve and then it got thinner as it went into the bore. The skirts that hung down into the cases were thicker than the part that went into the bore. This required splitting the sleeve, pressing it in and welding it in the center. This was an awful idea. It leaked water right away.

9) We welded a thicker aluminum "skirt ring" onto the bottom of a cylinder. This got out of round.

10) We tried it again using less heat in an attempt to avoid softening the cylinder too much. This didn't work either. You just can't weld a cylinder that much without softening it. Soft cylinders don't hold their shape.

I have a picture that flashes as a screen saver on my desktop of 9 blown up CRF250 cylinders (all tested on my 3 shop bikes. None were ever sold to customers) that I can post if one of you guys knows how? It's gruesome. I also have a digital picture of the cylinder, cases and crank of a stock 04 CRF250 that were all damaged when the stock cylinder skirts broke off.

It took me and 2 of my crew of 12 men over a year to develop the CRF276 and CRF302 ICE CUBE kits. We bought 3 new test bikes early in 2004 and at least one of my techs and I spent several hours every day trying to develop a Big Bore kit for the CRF250. I spent well over $100,000.00 in time and materials before giving up hope of developing a STEALTH CRF262 Big Bore kit.

Please take my hard-earned advice and do not bore the stock CRF250 cylinder.

Over the past dozen years I've bored and replated thousands of 2-stroke cylinders as much as 6mm oversize. The 2-stroke cylinders are reinforced by the transfer port ducts and are much stronger than modern 4-stroke cylinders.

The CRf250 cylinder should not be bored at all.
On all of the other 4-strokes I've worked with, we've found that the 2mm oversize is the way to go if you are planning to bore the stock cylinder.
Use the ICE CUBE or other reputable big bore such as the Athena kit if you plan to go larger than 262cc or 468cc on the bore alone.

Going larger than a 262 with a stock cylinder on the YZ250F or KX250F / RMZ250F will lead to a cylinder that gets out of round.

We've found the same thing with the YZ420F's (416cc) that we built for White Brothers, DRZ417's that we built for Yoshimura, and the CRF468's that we built for Pro-Circuit.

I think I went into this more on the YZ250F Big Bore thread a few days ago, but the bottom line is that the modern day 4-stroke OEM cylinders are built with very little meat in them.

The CRF276 and CRF302 ICE CUBES do require case machining or die-grinding, but that's what it takes to build a reliable CRF250 Big Bore.
(Note, The YZ276F, KX276F, RMZ276, CRF488, CRF498, YZ488F and YFZ488 ICE CUBES do not require any case work at all. They bolt on like stock and run on 91 octane under standard conditions. The STROKERs like the YZ315F, CRF302, KX315F, RMZ315, YZ523F and YFZ523F should be run on 100 octane)

I hope this helps save you from having a problem with the OEM cylinder.
If you have any tech questions related to Big Bore kits, I'll be happy to help you with the best advice I can give.

Rick Peterson
Product Designer
MaxPower-RPMs