Wiseco problems??

My riding buddy has a 2003 Yz450f that he got used, that the previous owner raced for one season, and said if he were going to race the bike he would recommend a new top end. Since my buddy doesn't race, the old owner said it would be fine for another year of woods riding and occasional MX track riding.

My friend is thinking of going with a Wiseco 13.5:1 new piston etc.

When I made an inquiry at the local store/shop that I frequent about the wiseco rebuild; he commented that wiseco is the last choice he would use for the rebuild. He stated that wiseco has it's name because of big advertising dollars, and that he has seen a fair share of problems with wiseco. He said they tend to change some things about the piston design that deviates from OEM, generally not informing their customers of the changes, that results in using race gas when the buyer didn't think he needed it, and other problems.

He also stated that forged piston's in these bikes tend to create more problems than cast pistons due to thermal differences from the cylinder cooling at one rate, versus the forged piston cooling at another rate.

Is this guy full of it?? or is he giving me sound advise?? He recommended another piston manufacturer (the name escapes me right now) and I don't know if he's yanking my chain to sell his item, or he's telling the truth.

Let me know your thoughts.


First off you need to ask specifics. What bike, what piston did they use, what tolerances was the bore set to, etc. Someone just saying "yeah I have had problems with those" is just a line of BS until there are facts to prove it.

That being said, you need to order the piston you want. There is stock, 2 ring vs the stock 3ring (and single ring for 2smokes), hi compression, big bore, big bore hi compression, and ultra light wiseco pistons.

If you are using a stock replacement (or another other piston), you need to read the directions on the clearances these pistons require. If your machinist/mechanic does not set up the tolerances properly, yes you will have problems. You cannot just expect to drop a new piston and rings in an old bore and expect things to be perfect. So measure, measure and measure again.

Wisecos will require you to warm up the motor a bit more than a stock piston, but nothing else will be required from the rider.

Yes the piston has a good chance of becoming damaged without proper warm up. Ive seen it first hand but I belive this is a problem with any piston. I have also heard of a few mechanics that do not like wiseco.

I would recommend an OEM piston. Can't beat OEM!

Thanks for the replies...... I have a 2004 Yz 450f and basically I'm using my buddy as a "guinea pig" to see how the top end rebuild on his bike will work out.

I purchased my bike with about 50 hours on it (trail ridden only, no racing) and I've put about 38 hours on it (4 hours MX, the rest trail riding). Since I don't race it, and I usually short-shift the bike (I don't ever think I've hit the limiter) how long do you think I can go without doing the top-end?? I change oil and filter every 10 hours, clean the air filter after every two rides, and keep everything well lubed. As far as I can tell, I haven't lost any performance whatsoever, so what kind of life should I expect before doing a new piston???

All bikes are different. I would measure your compression and do a leakdown test to see if it is still in specs. Probably the best way without tearing it down.

Listen what is said. You dont just replace a piston. You need to measure the bore and either get and oversized piston or new cylinder or sleeve the cylinder to keep things in spec and ROUND.

I've been running a Weisco 13.1 piston in my 2000 426 since last summer, it's been fine. I wouldn't hesitate! I love the power it has now! I've also used Weisco piston's in a few two-smokes I've had in the past and had no problem's either...like everyone is saying, just make sure you setup your cylinder clearance's to match your new piston and you'll be fine. :applause:

I posted a similar thread a while back. You should check your cylinder first before you order your piston. A stock size piston will do you know good if your cylinder is out of spec. It won't matter what piston you put in there if it's out of spec..........OEM, Vertex or Wiseco will create problems.

I always HATED Wiseco :ride: for my smokers. I was forced to use them twice in two different bikes because of availibility and time. I had Bad luck in both. Anyways after reading feedback here on Wiseco's for 4 strokes I thought I would give them a shot again. My bike has been running great with no problems. The Wiseco actually looks like its made better than the OEM when you compare them side by side. I will use Wiseco again for a thumper, I won't if I get another 2 stroke. No matter what you can't go wrong with OEM maybe more $$$$ but thats it.

I have an 04 and I am the original owner. I am glad that I pulled my top end apart when I did because my piston was looking pretty beat up. Its easy to pull it apart and check it. :applause:

I've got a 2mm+ Wiseco (stock compression)with about 20-25 hrs on it with no problems. I'd use again without question.

I happen to be an authorized Wiseco dealer and ride both YZ 400 and 426 Fs in the woods and on the track. I have also rebuilt countless dirt bike, ATV, PWC, and snowmobile engines and sold a lot of Wiseco piston kits. I have NEVER had any negative experience, but have seen others, and it has either been due to 1. lack of warming up (like a guy on a PWC that gets it off the trailer fires it up and wails to show off). 2. assuming that a machine shop got all the grit out from a bore/hone/re-nikasil job, only to put the engine together and sieze it up fast, or on 4 strokes, immediately ruin the babbit on the cam journal bearings. These 2 failure modes can happen with any piston, OEM or Wiseco. You should note that the other replies are correct in saying to check for true cylinders. There are many variables that affect reliability (not using assembly lube, no warm up, not checking ring gap, not checking other tolerances, not using a torque wrench, etc). It is way to ez to point the finger at the piston. The pistons are forged, and will show a different rate of thermal expansion compared to OEM. Just treat it with respect and like the complicated machine it is and you should not have any problems.

I also sell Wiseco piston kits, Wiseco cams, Cometic gaskets, Wiseco clutch baskets and plates, and Thumper racing parts at super prices. If anyone needs some, just PM me.

A lot of this has already been covered, so forgive any redundancy. Most of the "problems" with Wiseco, or for that matter, forged pistons in general, are a result of people failing to realize that things change, and what was true 30-40 years ago isn't true anymore.

One of the things that used to be a big problem was that cylinders were iron, and pistons were aluminum, and the two metals expand at radically different rates and amounts when exposed to heat. That meant the piston had to have a large amount of clearance when fit to the bore in order to have any left at operating temperatures, and hopefully it wouldn't get too much hotter than it was supposed to, or it would grow to an interference fit, and seize itself.

Metallurgists had already learned by that time that they could add silicon to the aluminum and cut down the thermal expansion of the alloy by an enormous amount, which meant that cast pistons could be safely fit at less than .002" cold. But the use of silicon made the alloy unsuitable for forging, as it was done then, so forged pistons still had to be fit looser. That lead to problems like cold piston slap, and sometimes "cold seizures", where a piston that was warmed up too quickly could seize before it fit the cylinder well enough to lube and transfer heat correctly. With that came a reputation.

But time goes on, and engineers find solutions for problems along the way. About half of the current negativity toward forged pistons is directly due to remembering stuff that went on a long time ago. But, the same rules no longer apply for a number of reasons. For one thing, the cylinder and piston are now usually both aluminum in modern MX bikes, so their expansion rates are more similar. For another, a great deal has been accomplished in controlling aluminum's thermal expansion while leaving it forgable, and about forging hard-to-forge materials as well. A forged piston for your dirt bike no longer needs to be fit with significantly more clearance than your OEM cast unit did, but some still think it does, and that leads to trouble.

Matt made a point that the bore needs to be round enough to use with a new piston, and that failing that, a bore job is required. That's true. But it isn't uncommon to find a nikasil cylinder in near perfect condition, either. You shouldn't assume you need a bore/plate/diamond hone job, but the bore does need to be checked, and the tolerances need to be adhered to.

One thing that comes up is that the engine will get assembled with an excess of clearance, either because the machinist failed to read the manufacturer's spec's, or thought he knew better, or the piston was made slightly small and assembled without checking the fit. Manufacturing errors are one thing, but just sticking a piston in an engine without measuring anything is like just grabbing valve shims at random when assembling your cams. Sure, it's supposed to fit, but if it doesn't, and you put it together anyway, whose fault is it?

Wiseco is a big company. They've been around for a long time, and a lot of people use their stuff and are happy with it. Most people who aren't have themselves or their machine shop to blame, not the product.

Thanks everyone for all the info. It looks like "operator error" is more the cause of failures, than just the name/type of the piston.

I was suprised because the owner of the business I purchase from, seemed to really have an axe to grind with wiseco, and I know a lot of people use their products.

My bike runs perfect (so far) but I will set aside some time to recheck the valve clearances, and maybe do a compression test.


The OEM piston is also forged ,is it not ? I remember looking at one and could have sworn it was forged . Does Yamaha claim it has forged ?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now