Fixing 4th gear, what else should i check/replace while im in there?

Well 4th gear is skipping on my 2003 450f so im gonna pull the motor today and have my neighbor take it all down and go over everything. Im thinking its either a shifter fork or the dogs... anyway is there anything else that goes bad in these motors? I was thinking it would be smart to buy rings and crank bearings while its open....


Also my neighbor wanted to port the head and "lap" the valves for me. He is a 4 stroke guru and has a COMPLETE machine shop in his pole barn. My only concern is how likely could it be that he maybe ports too much or leaves to many variences in the head and it becomes worse then stock? Maybe im over thinking it? I would like a little feedback on the porting idea, not sure if it would be best just to leave it or have him do it...

One thing that goes out more often of the '03's than in later ones is the rod bearing.  the cage breaks, letting the rollers skew off line, and lock the engine up.  The trouble is that there's really no way to see it coming (if it even is, it's not THAT common), so you kind of have to evaluate it on everything else and take your best guess.  New OEM cranks, or rebuilding with OEM parts are your best bets.  Hot Rods cranks are OK for non-full time race bikes. 


Watch the condition of the left side main shaft bearing, too, and don't overlook the balancer shaft bearings. 

don't think you should lap the valves in these bikes unless they were replaced with stainless valves at some point.Once the titanium coating is gone the valves wear quickly so lapping them will shorten their life bigtime.

I missed that. But the fact is that YZ400F's and the 2000 model YZ426 were built with stainless valves.


Even so, ABSOLUTELY NEVER lap titanium valves for any reason.  Ever.  Titanium valves depend on hard coated valve faces (the area that contacts the seat) for their ability to resist wear.  Not just so they last longer, so that they last at all.  Without that, they wouldn't last more than a few hours.  "Lapping" uses an abrasive paste placed between the valve and seat, using one to grind the other, presumably into a matching contour so they'll seal better.  The problem is that the coating on the valves is extremely thin because of manufacturing restraints (it's too hard to machine), and any damage to it can seriously shorten the service life of the valve.


Yamaha's manual contains a reference to the procedure, even in recent model year manuals for bikes that use Ti valves.  It should have been removed long ago, but it goes along as a part of the legacy information that gets lifted from one year to the next without review.  Ignore it.

Okay I'll let him know to check the mentioned bearing and not to lap the valves, how offen do valves need to be replaced? Hopefully their still in good shape...

When there is wear that requires a shim size change greater than  - .10mm from the original shim on any valve.

Thanks for all the knowledge so far, what's the main difference between steel, SS, and ti valves?

Even though I hate be a smart ass, the answer is the material they're made of.


That's about it from a truly practical standpoint.  Ti valves are 60% as heavy as stainless, which does mean that the bike can run lighter valve springs, which in turn means benefits in high RPM valve control and stress on the valve train in general, but for most people, the valves could as well be stainless and they would never know the difference. 


Converting to stainless can seem attractive, because the valves are often cheaper, but they require heavier springs to avoid valve float, and in the case of many of the aftermarket sets, the spring sets are so expensive that they make up for the savings on valves, and you come out about the same.  In the long run, you're just better off staying with OEM.

Okay so we got the engine opened up yesterday, found some good news and some bad news, so i would like some feedback on this....


Well for the good news, the head is already ported, and looks to be recently rebuilt, it also has stage 1 hot cams and a weisco 12.5:1 piston in it....


Now for the "bad", after taking measurements on the cylinder he told me its at the end of its "spec" and he wouldnt feel comfterble honing it one more time, he recommends i replace the cylinder and get new rings....


Rod bearing seems to be worn, as well as the timing chain gear on the crank, he recommends getting a whole new crank kit to save time and hassle, as well as a timing chain....


Oil pump has some scoring on the shaft, he recommended replacing it or else i will have low oil pressure


As for the transmission, both shifter forks seem to be bent, each one has marks that look like a gear has grinded on them before. Obviously he recommends replacing these, my question about that is buying used but in good shape shifter forks safe? Or is it just prone to doing the same thing faster then new oem ones? Id like to save some money but i dont want to compromise reliablity.... Also the dogs on the gears especially 4th seem worn, he recommended sending out the trans to get undercut instead of replacing any gears, thoughts? Ideas? Sorry about the super long post....

Having the gears recut is an option, as long as it doesn't require too deep a cut.  Go through the hardened surface layer and there will be no point in reinstalling.  Then consider the price of a brand new gear vs. the cost of the work.  Shift forks are too cheap to look for used ones unless you run across a complete trans the was pulled from a bike getting a 5 speed conversion in good condition.

What would you say is too deep of a cut?

More than .004" would be worriesome to me. 

Okay I'll let him know about that, are aftermarket cylinders trustable?

Having the gears recut is an option, as long as it doesn't require too deep a cut.  Go through the hardened surface layer and there will be no point in reinstalling.  Then consider the price of a brand new gear vs. the cost of the work.  Shift forks are too cheap to look for used ones unless you run across a complete trans the was pulled from a bike getting a 5 speed conversion in good condition.


How hard is this 5 speed swap you talk about? Is it as simple as buying a 5 speed trans from a newer bike?

It's not hard, it's expensive, and the only 5 speed that will direct fit the '03-'05 is the complete trans from an '03-'06 WR450F, a wide ratio off-road gear set.  I know of at least one person who adapted the close ratio 5 speed from a 426 into one some years ago, but it was a major problem involving quite a bit of custom machine work.  The WR trans just bolts in.

I think I'll stay with the 4 speed haha, what's your opinion on sleeving the cylinder? Would that be any less reliable then buying a complete aftermarket cylinder?

Sleeving? No.  Replating? Maybe. 


An steel or iron sleeve creates two problems.  One is a thermal barrier that naturally forms between two layers of metal.  Heat from the sleeve is not conducted from the sleeve to the barrel efficiently unless the liner is cast in place using fluxing/bonding compounds.  The sleeve itself also naturally conducts heat rather poorly compared with the plated aluminum of the stock bore, which leads to the second problem:


The sleeve will expand neither as rapidly nor as much as the aluminum cylinder or the piston.  This means that either a piston expressly made for use in a steel liner has to be used, or the more commonly available replacements, both aftermarket and OEM, have to be fit looser so that they expand into a correct fit at operating temperatures.  That in turn accelerates wear on the piston skirts during warm up, and raises the possibility of damage occurring if the engine is not warmed up more carefully.


Replating works, but how well depends on the care and precision used by the shop doing the work.  Done right, it works as well as OEM, but it takes down time, and often doesn't save all that much money vs. simple replacement.

Okay I'll just get a replacement cylinder, I want this bike to last after going through the entire motor...

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