need suggestions- 426 smokes on cold start after new piston

I put a new Wiseco in my 426 last week, used the 13.5:1. I've ridden it several times and it runs great, better than stock even. I was running some bean oil in the fuel the first couple tanks so I thought that was causing the smoke, but today I used straight 93 and on a cold start it smokes. It doesn't smoke right at start up but after maybe 30 seconds, then for maybe a minute, then everything is fine (I experimented off and on all day). I can re-start after it has set for 30 minutes or so or less, no smoke. But let it sit for more than an hour and it smokes like a Banshee hehe. One thing to note, the oil rings seemed very tight when I put them in. The other rings were fine, and the cylinder had slight vertical lines but very slight. I have maybe 5 hours on it so far and I have checked the oil religiously, no oil usage that I can tell(one oile change in there). I promised my son a trail ride tomorrow but after that it's coming back apart and I would appreciate any suggestions you might have. Thanks in advance...


you better dump that 93 octane and go buy some 108.your not supposed to run pump gas with 13:5:1 piston.super unleaded is 92octane.i suggest the $8.50gl 104 oct. purple juice.espically when breaking in new top end.

Typically when a 4-stroke smokes on start up and then only for a short period it is due to the valve stem seal(s) being bad. When the engine sits some oil seems through and onto the valves. When you start the motor it makes its way to the cylinder or hot exhaust header causing the smoke. I can't promise that's whats happening here, but that would be my first guess. Especially if you didn't use assembly lube on the valve stems when you reassembled the head. If the end gap on your rings was tighter than spec then you should definitely take care of that but I would think that any damage caused by this would cause smoke in more situations than only when the motor is cold. This is just my non-professional opinion, hope it helps.

Also ditto the recommendation for high octane fuel. I don't remember seeing any 13.5:1 engine being OK'd for pump fuel. Knock and ping can definitely put a hole in the top of your piston, not to mention other fun side effects.

valve guides can also cause the can usually tell if you pull the valve cover and get a flashlight..shine the light at the bottom of srings..usually see a little bit of metal dust at base of springs if some wear or possible gualding...and yep..i need better petro than 92 octane

I got a couple of questions for you.

1. What was the recommended piston to cylinder clearance from Wiseco ?

2. What did the piston mike across the skirt at the largest point ?

3. What did the cylinder mike, how much taper did it have and how far out of round was it ?

4. What color was the smoke ?

5. Why would you use a used cylinder with a new piston ?

6. How much ring gap did you have on each of your rings ?

7. Did you deburr all of your sharp corners and and deburr all oil holes on the piston ?

8. What was your total piston to cylinder clearance before assembly ?

9. And the most important question. Why would you run any oil in your fuel ?

This can cause severe cylinder and ring damage because of the added load on the rings at there seating points.

After you awnser these questions I can give you a good assesment of your problem, but here is a early hint.

If you have to much piston to cylinder clearance the piston will rock slightly until warm depending on the severity of the clearance, which in turn allows oil from under the piston to get by.

This is what we see on the CRF's because of the skirtless piston (rocking in the cylinder) all CRF's I've seen smoke until warm and the clearances close up allowing a good firm seal.

One more question.......Did you take your valves out and see how far they had sunk into the seat ?

Checking for a ridge on the sealing side of the valve.

Later, Jason

P.S. These guy's are right about the race better get some.

[ January 20, 2002: Message edited by: jaybird67k ]

Wiseco pistons are smaller that OEM pistons when cold because they expand more when hot. Assuming everything was put together well with good tolerances and good valve seals this could be considered normal when cold. Just warm it up slowly and everything should be fine>

Your ridin' bud, Kurt

Thanks giys for the quick response. My first thought would have been valve seats, but why would that just start, it ran fine before just seemed like it had lost a little power, and the valves have never had anything done to them, not even needed adjusting.

I was afraid one of you guru's would ask about the cylinder/piston tolerances. This I didn't check. I did check the top 2 ring gaps and they were good. I was told not to worry about the oil ring(s) was told hmmmm if I can remember... "just poke 'em in there". So I did. I am thinking along the lines of the piston being too loose in the cylinder, but I could be wrong. I have had valve seals go bad in Honda's and they behaved a little different, the bike tended to smoke immediately on start, not delay like mine is doing. The only thing that makes this such a mystery is the power, it is so much better than it was stock.

As for race gas, I was told before I did this by some of the guru's over at DRN (I know, drn is a cussword sorry) that I wouldn't have to run it since 12.5:1 is stock. I also had a report from another who was running this compression on pump gas. Anyway, that is no big deal, I will run race gas just don't have any on hand.

I might end up doing the Eric Gorr 97mm 440 kit thing, just thought I would try a short cut since I have a dealer who likes me and gets me parts cheap. Not that I have the money to throw around but this was a chance I thought I could take. Thanks again guys, I'll try my best to help you when I can...

kurt- I have read this before and was hoping someone would tell me this is the case, since it runs so well. And it would explain my symptoms. I just hate starting it in the pits and watching the gawks from my fellow riders lol. I guess we wouldn't have that mosquito problem anymore at my local track!

jaybird- also, the smoke is gray, it is oil smoke. I did the bean oil thing at the recommendation of my dealer who is an old time mechanic who splits yam cases with a rubber hammer, presses them back together with a press and gets more time from them than anyone else I know. Actually he gave me a bottle of bendzall and said to try it for a tank or two.

I can live with this smoke for a minute or so, I just don't want this thing to sieze on the face of a jump. I'm still healing from a bad wreck at the dunes two weeks ago and I'm too old for another any time soon...

by the way, the smoke is whitish-gray, not blue.


Mike, I rebuilt my '99 into a 420 last year. It took lots of patience to get the piston and rings into the cylinder properly. I double checked end gap like you did but it took too many tries to mention until it went in fine. My problem was with the oil ring spacer ends crossing over. According to the Wiseco assembly diagram this could cause smoking if not properly done. Just thought I'd mention it. Good luck and work patiently.

Hugh- good to see you are still around, I haven't seen you post in awhile, but these days the posts go away so fast (lots of new members, some good ones too). Anyway, I took it apart this evening and found that my oil rings were about 20o apart instead of the 180o I left them at (at least that's my story hehehe). I also checked the oil ring gap and compared them to stock (these I didn't check earlier). The new wiseco's are double the limit. I measured the stockers and they were still in spec so I think I will go ahead and re-use them. As far as I can tell they are exactly the same size except for the end gaps. I'm waiting on a new gasket now and I'll report again for the archives what I find. Thanks...


Blue or gray smoke is obviously the cause of oil burning in the combustion chamber.

It takes a very small amount of oil to result in what seems like a lot of smoke.

In most cases, as SireThump as noted, such symptoms would conclude one to assume the valve seals.

This could be a coincidence, unless one of the valves got bent before, during or after your piston install.

However, I am more suspicious that your problem is with your oil ring. Mainly in that you had to force the ring into place.

Piston ring clearance is a critical spec in engine building. Your third ring, which controls oil flow, should freely move in and out of the groove. If not, excess oil would flow by.

Perhaps, due to heat, the ring is freeing itself after the engine heats up. Before then, obviously, too much flow.

Proper piston install also involves a precise match to bore and piston size. Simply purchasing a piston to size and installing it is sure trouble.

In all cases, the cylinder should be bored to match the new piston. Therefore a new piston into a stock cylinder should be .040 to .060 over.

Many pistons and cylinders wear together. After so many hours of use, the piston and cylinder usually get a bit on the oval side.

Therefore the new piston is forced into a hole that is tight on two sides, and lose on the other two.

Lastly, all new pistons are not made to the same exact size. Usually, good bore jobs are not spec’d to anything but a measurement of the new piston itself.

In multi cylinder engines, this means matching exact piston size to each cylinder bore.

At this stage, you may now have a damaged piston and cylinder. If you put a factory size piston into a worn cylinder, I would toss the piston, go with an oversized one, and re-bore. And of course, machine the piston to the necessary ring clearance spec. Not to say you can’t save this one, but it's a matter of doing a job correctly.

I don't think most piston manufactures ever ship rings that are matched perfectly for each piston. If anything, they provide you with extra material to remove for a proper match.

Just my two bits, so I hope there are some other engine builders on this sight with a little more of a modern view on this process.


[ January 22, 2002: Message edited by: DaveJ ]

Dave, if I understand correctly this would mean :

a) having the cylinder bored / plated using the new piston for dimensional reference

:D checking end gap by inserting ring( s ) alone into cylinder and filing / deburring ends

c) putting same rings into their respective grooves and again verifying end gap? ( If inserted into cylinder that I guess a wire guage would have to be used, If no room that eyeballing? )

Steps a and b are obvious but never even considered step c if that is what you imply.

Please share more with us. We ( myself actually )obviously need your help!

Mike O.K. Thanks for the head's up! Hope your rebuild goes well. As usual my advise is : Please work patiently! ( This is advise I like dispence but sometimes have trouble putting into practice myself ! ) :)

This was my first 4 stroke re-build, so I am learning on this one. Before I did this one I was seriously thinking about going to the Eric Gorr 444cc kit, so if I can't iron this out all I'm really out is the cost of this piston, and if I do get it ironed out I have another season of use before I do go to the oversize... Anyway, I have re-built the top end on several 2 smokers, and with the new cylinder plating I have always been under the impression that all you do is check your ring end gap, slap everything together and go ride. Maybe the tolerances are not as critical but that's how everyone I know does their 2 strokers.

Also the main reason I re-built anyway was the fact that my hole-shots were getting worse and worse, and it felt to me like my bike wasn't making near as much power as when it was new. I can tell you that even with the smoking problem this bike has considerable more power now than before the re-build, probably more than when it was new but that's really hard to say since so much time has past. Thanks again guys and I'm out to the shop in a but to put it back together for one more try. Working 10 hour days this week has sure put a damper on my fun. And Hugh- patience is something I wasn't born with but as I get older it seems to get easier, and I really enjoy all this tinkering anyway...


Yes to A. Good machine shops will always ask for the piston in which the bore job is being done for. And yes, they should match one to the other.

A bore job, or at least a measurement for size and shape followed by a hone, should always be done before a new piston is installed, (unless of course there are ceramics involved).

In some cases, a process called "decking" is also performed, so to assure that any post assembly distortion is considered in the bore set.

This means applying the same pressure to the cylinder as applied when the engine is fully assembled before the bore job is performed. This is done because some engine designs will distort a perfect bore when assembled.

I doubt many shops would do this on such an engine, nor is is really that necessary in single cylinder apps.

As for B and C, it's a little more complicated.

Most piston designs have a spec for two ring settings. The first is the gap between the ring and the piston. In other words, the excess space between ring grove size and width of the ring. This means the ring moves up and down in the grove when the piston goes up and down, and also assurs that the ring can move freely.

The other setting is as you mentioned, assuring that the ring ends have a proper gap in accordance to cylinder diameter (I kind of left that out). This is where most people will push the ring in the cylinder using the head of the piston.

Pistons and rings come from the manufactures as do all engine componets - in need of special attention in order for the job to be done correctly.

Based on what Mike reported during his assembly process and testing, I really think he needs to start over when the time allows.

Unless, of course, he simply still has some of that bean oil in there.


Thanks for the input Dave! One more question if you have time Guys : How do you go about getting piston / rings into cylinder? I use the slip cylinder on piston method and it took me sevaral time consuming tries untils I got it right. I would like to use piston into cylinder first method than install wrist pin with one clip already in, but feel that the piston and rings may pop out with the slightest rocking ( because of short skirt design )

Hugh - thanks for keeping the topic going.

You can purchase or sometimes rent a piston ring compressor. Well worth the money.

And I also forgot to mention that the gap between each ring is usually positioned at even increments. I think some pistons have positioning pins in them for this.

Talk with you guys later!


I gave up on making my own gasket, I did make a real good base gasket but when I tried to cheat and re-use my old head gasket it leaked (as I suspected it would) so I'll have to order a kit. It'll be a week or so now before I get to try again. I'll post back when I get another try.

Sorry I didn't get back to you on the private message you sent me sooner , been working some OT lately and haven't had the time.

Just some food for thought.

There is no magic to building these single cylinder engines as long as the necassary parts are all checked to be within tolerance.

I've built a bunch of engines in my short time on this earth and am just fortunate enough to be a Machinist so all the sizing and stuff comes second nature to me.

I never trust any part that is sent to me and I check it regardless if its OEM or whoever.

As you have probably found out it is a no no to put a new piston in a used bore, I do all my on boring because I trust myself more than I trust other people to do it.

I use a place called US Chrome to do all my plating for me after I bore my cylinders.

The standard practice for boring a plated cylinder is this......Piston size at its largest point(across the skirt) + Reccomended clearance + .007 thousands = bore size.

So lets say for grins and giggles your piston was 4.000" and the reccomended piston to cylinder clearance was .002", then you would have 4.002", then add .007 to that you would have to bore your cylinder to 4.009".

This is the same for all bore sizes that we deal with on these bikes.

If you look at the numbers you will notice that the cylinder has been bored .009 thousands bigger than the piston but only .007 thousands bigger than the finish bore which would end up at 4.002"

This is because the guys doing the plating are looking to get .0035" thousands a side for plating thickness.

It is cheaper to buy a OEM cylinder as it is to

have someone bore and plate one for you, an OEM cylinder is about $200 and to have one plated is around $160 plus $50 for boring.

You can have a stock worn cylinder with a new piston sent to US Chrome and they will chemically strip the plating and replate and diamond hone it to fit the new piston.

I personally see no reason to go oversize on the bore with the OEM ones being so cheap, unless you just need more power or you have nothing else to do.

I have my old cylinder from my 01 426 that I will bore and plate to fit an oversize piston and keep just incase I want to dump it in a year from now just for fun. Or sell it to someone looking for one.

I hope this helps you understand the boring and plating process a little better, If not let me know and I'll try to help.

Later, Jason

US Chrome

Do get a ring compressor, Sears has them and they make life much easier. Especially for pistons with multiple rings.

As far as pins in the piston to keep the rings in specific places, I've only seen this on 2-strokes to keep the ends of the rings out of the ports.

Thanks of the great explination on boreing, that was very informative!! I always thought that with the plated cylinders that the piston would wear out much faster than the cylinder and therefore the cylinder was frequently reuseable with new pistons because it wasn't yet out of spec. Does this not happen very often in real life?

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