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Big Bore 262 vs 280

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Was wondering if people who have tried going to an 80mm overbore (280 kit) have experienced any problems? Reading some of the posts, Rick Peterson seems emphatic that the bores will go out of round at this size after 20 hrs of use or so depending how hard pushed. Anybody experience this? How about rod/bearing failures from the increased piston weight? Overheating problems?

Also, when going overbore, can a cylinder from an '04 or newer be used? These have the cutouts.

I'm wondering if you can only go +2mm (262cc) on the cylinders with the cutouts or if the cylinders with the cutouts should not be bored at all and you need to get the older type cylinder if you want to bore it.

I'm guessing that when they design the cylinder it has enough margin that you can go 2mm over without a problem and thats why so many outfits offer the 262 kit. Going 3mm might be pushing it but it would be nice if you could get away with it. Both Vertex & Wiseco have pistons available. Cost would be the same between the two.

Also, what sort of jetting changes are required. I've had a few people tell me none but seems to me that if you up the displacement and make more power you need more fuel in there and a bigger main jet to deliver it. Wonder if this is why some are reporting loss of top end, if they are not jetted rich enough.

Thanks for any feedback. I'm going to try this over the winter and need to decide 262/280. I don't want to lose the topend rush my '05 yzf250 has but would be happy if it got stronger everywhere.

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I have the +2mm in my bike, there is a nice increase in power with no loss of revs. If I had to do it over I would do the +3mm which is actually 270cc. The +4mm piston does make the cylinder walls thin enough to cause problems.

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Also, what sort of jetting changes are required. I've had a few people tell me none but seems to me that if you up the displacement and make more power you need more fuel in there and a bigger main jet to deliver it. Wonder if this is why some are reporting loss of top end, if they are not jetted rich enough.

had a 262 on my 01 for 3.5 years. never a problem.

you go leaner as the bigger piston can pull more air (and therefore more fuel) with each stroke. just look at the jetting for a 450 as proof.

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I've been pondering the very same question, 79mm vs 80mm (btw- 80mm with a stock stroke = 270cc).

Here's some measurements of interest- The stock cylinder skirt on my scrappy 02 cyl measures 85mm outside diameter. Doing the math, that leaves a stock bored 77mm cylinder with 4mm cylinder skirt thickness, a 263cc has 2mm remaining, and a 80mm bore would have only 1.5mm (.060") of wall thickness left. 1.5mm sounds really skinny to me.

Also, I noticed this in the respective catalogs-

-Vertex makes a 79, 80, 83, & 84 mm piston, but there are no mentions of the cylinder mods required.

-JE sells a 79 & 80mm piston, and indicates the 79mm can be used with an overbore + replating, but it notes the 80mm should have a sleeve.

- Wiseco does not list a 80mm piston on their website listings. I know they make an 80mm for some aftermarket companies, but Wiseco doesn't sell it directly to the public.

Rick Peterson had indeed cautioned about the 80mm bores barreling in a stock YZ250F cyl casting, but, I don't recall whether he was referring to the newer (03+) cylinders with the side cutouts, or for all yrs including the full skirted 01-02.

Does anyone remember what Rick P had told us about the full skirted 01-02 cylinders? Whatever he says, IMO, his experience is certainly worth heeding.

There are sooo many bad & expensive possibilities should the cyl get barreled (ask the 04 CRF250 boys), so we want to be careful here. But, the extra CCs would sure be nice.

Hmmm,,,, maybe one of us needs to just call or email him. :thumbsup:

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As far as I know, the +3mm 270cc that Thumper Racing has been selling for quite a while is very reliable, the 4mm which doesnt seem to be around anymore is not, apparently.

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We ran the +2mm Wiseco race piston for the second half of the year in Chris's 06. He was very happy with the incease in pull off the bottom without hurting the over-rev.

You can send your cylinder to Mellenium Technologies and have your cylinder bored and get the 2mm over wiseco piston thru the TT Store and you are set.

Chris also liked the FMF mega bomb header best with this combination.

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does decreased cylinder wall thickness also apply to kits like the 290 athena?

The Athena kit comes complete with piston, rings and a new cylinder. No need overboring your stock cylinder.

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My 01 with the thumper kit was sold along time ago it's the 3mm kit. It's still running fine. The motor was awesome. I'd love to borrow it one day and see how it is against my ice cube. The guy and his son have constantly used it with no problems.

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you go leaner as the bigger piston can pull more air (and therefore more fuel) with each stroke. just look at the jetting for a 450 as proof.

Could you expand on this? Not intuitively obvius to me. If fuel/air is at stoichiometric optimum and then cylinder size is increased it needs to be filled with more fuel/air at the optimim ratio. If the flow through the main jet is choked it will need to get bigger to keep up with the airflow. What would explain why it needs to get leaner?? 262 kit is only ~5% bigger so maybe it doesn't matter

Also, I noticed this in the respective catalogs-

-Vertex makes a 79, 80, 83, & 84 mm piston, but there are no mentions of the cylinder mods required.

-JE sells a 79 & 80mm piston, and indicates the 79mm can be used with an overbore + replating, but it notes the 80mm should have a sleeve.

- Wiseco does not list a 80mm piston on their website listings. I know they make an 80mm for some aftermarket companies, but Wiseco doesn't sell it directly to the public

I also noticed that Wiseco specs different pistons for the '05/'06 application vs the '01-'04. Also some differences in compression ratio. Does anybody know what is different on the '05/'06 piston or is it just comp ratio diff between the years.

What are the cut-outs in the '03 and newer for? To reduce the amount of backpressure the piston has to fight on the downstroke? Do the '05/'06 run a slightly lower CR and this is why?

I've had an adult beverage and my math was never good....

I've been pondering the very same question, 79mm vs 80mm (btw- 80mm with a stock stroke = 270cc).

Here's some measurements of interest- The stock cylinder skirt on my scrappy 02 cyl measures 85mm outside diameter. Doing the math, that leaves a stock bored 77mm cylinder with 4mm cylinder skirt thickness, a 263cc has 2mm remaining, and a 80mm bore would have only 1.5mm (.060") of wall thickness left. 1.5mm sounds really skinny to me.

thanks for correcting me, I meant to ask if people had problems with the 270 kit (80mm), I screwed up my post title. I took down my '05 after ~50hrs and the piston/cyl all checked within spec. I wish I had measured the wall outside diam like you did on your scrap cylinder. If the OD on an '02 cylinder is 85mm and it gets bored 3mm over to 80mm (270cc) wouldn't the wall thickness be 2.5mm? To your original point, still a significant reduction from the OEM 4mm.

So far a few positive reports on the 3mm (270cc) overbore, no negative reports yet....the 262 kit sounds like a no-brainer, just have to figure if it isok to do on the cylinders with cutouts

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Could you expand on this? Not intuitively obvius to me. If fuel/air is at stoichiometric optimum and then cylinder size is increased it needs to be filled with more fuel/air at the optimim ratio. If the flow through the main jet is choked it will need to get bigger to keep up with the airflow. What would explain why it needs to get leaner?? 262 kit is only ~5% bigger so maybe it doesn't matter

when I put in my 262 kit, I had to go down a size on the main. The more displacement makes a slightly bigger "bang". That makes it suck more air through that smaller opening. It's like sucking through a straw and just barely trying, or trying to draw the whole thing through the straw in three seconds.

A 450 can REALLY suck the gas/air!

that's my understanding of it anyways..

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Wild Iguana,

Oops, yes, the wall thickness would be 2.5mm w/ the 80mm. My bad. Thanks for catching the math error.

The difference between the 05-06 piston & the 01-04 piston is this:

The comp ratio on all yrs is about the same. But, the 05-06 has a larger head volume and different valve angles (not sure what's with these diff angles though). In order to maintain the same comp ratio, the 05-06 piston has an almost flat top compared to the 01-04's concave top surface. It is possible to run the older Wiseco high comp pistons with a newer head, but the resulting compression ratio is about the same as stock. Bottom line, be sure to use the correct pistons with the respective head.

The side cutouts are to reduce the cranking pressures. The 03 has a fairly modest cutout and the 04+ have a pretty aggressive cutout. These cutouts are REALLY nice for installing the piston because the cutouts give you the room to install the piston pin when the piston is already in the cylinder. MUCH easier to fight those little piston rings on the workbench than in the bike.

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Well after calling several of the shops which sell/specialize in the big bores for the yz250f I decided to give an 80mm (270) a try. Peterson is the only one proclaiming doom, two other shops claim no probs, unable to find individual report of problems with the 80mm. Have confirmed that no-one stands behind the 81mm bore any longer. I now have the parts in my possesion, a vertex piston and Millenium plated cylinder with the cutouts. I'll be curious to see how the jetting pans out, all the shops say richer 1-2 sizes on the main, lots of posts say it will need to be leaner. I guess I'll find out, seems no two bikes like exactly the same thing anyways. For kicks I'm going to take some cylinder bore measurements new and then ~40-50 hrs assuming its running good enough to make it that far. Hope to have together next week and ready (jetted) for race in mid Feb.

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The bigger displacement will draw a higher velocity through the same bore carburetor. A higher velocity will cause a higher pressure drop through the carburetor resulting in more fuel being drawn in per unit amount of air. Therefore you need to put leaner jets in or a larger diameter bore carburetor to achieve stochiometric conditions. I hope this helps a little.

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The bigger displacement will draw a higher velocity through the same bore carburetor. A higher velocity will cause a higher pressure drop through the carburetor resulting in more fuel being drawn in per unit amount of air. Therefore you need to put leaner jets in or a larger diameter bore carburetor to achieve stochiometric conditions. I hope this helps a little.

:lol::confused: Correct-umundo. My person experience supports this as well.

The 270 bore in is a slam-dunk no-brainer. The 290 Athena/Thumper is more power, with the Vertex 298 being even more. These are all around the same price, and work awesome if you don't ride on the rev-limiter. If you ride on the bump box, you would want to stay at the 270 bore-in. The RPM Ice Cube kits work great, but they are BIG coin. I currently run the Vertex 298 and can't say enough about it. I am a Vet class C rider / weekend warrior. Good luck!:confused:

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search the internet! my first ice cube i got for 350 on ebay. the second one i found on this site for 400 for a friend. I got two ice cubes for $750!!!!!

I still don't thing anything can touch the value per dollar of the thumper racing 270 kit. pipes cost twice as much and don't give you nearly as much

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