head work

(1999 yz400)ok so Im going to be diving into the top end this weekend, first thing Im doing is putting in a new piston. Ive never done this before however I have had the head off and replaced the timing chain and checked valves before by myself, I had no problems following the manual with this and everything was fine when I put it back together phew. Ive actually had no probablems with compression or blow by before now I just want to replace now before any proglems as its a 1999 with original everything but cam chain.

just wondering if there is any tips or anything I should know before I get in there and start doing stuff. Ive got a new piston and ring kit, came with full top end gasket kit and new pin as well. Is there anything else I need? Is there any specials tools I need? I read the dissasembly and reassembly instruction and didn't seem too hard, I was going to take it a shop but after reading the instruction I thought screw that I'll do it myself. Im a firefighter and we work 24hour shifts so as long as we are not busy Ive got lots of time and space in the truck bays and guys to help if need be.

the guys already think im crazy as last shift i brought my carb in to install the boyenseed quickshot AP and to clean/inspect it. haha wait til i bring my entire engine in saturday....

well wish me luck....

Tammie - Assemble the piston/rings into the cylinder while both are off the bike and on your workbench. This will save you a lot of swearing and cussing later on. It's a lot easier doing this job on a work bench at a comfortable height. Leave just enough of the piston protruding out from under the cylinder so that all the rings are safely/properly compressed and in the cylinder and enough of the piston is sticking out so you can insert the pin. Put the new gasket in that goes between the case and cylinder. Then position the piston/cylinder assembly to align it with the small end of the con rod and push the pin through. The pin just slides in with finger pressure - no tool required. Don't forget the clips at each end of the pin. Stuff a lot of rags around any opening so you don't drop something into the case. Good luck.

thanx sxp i will for sure have the entire engine out, im taking my whole bike apart for the second time in 4 years...so i will definately have the engine on a bench at work........


"Don't forget the clips at each end of the pin."

And be certain these clips are fully seated in the grooves in the piston before reassembly. i would share more on how important this detail is because of the damage a loose pin-clip can cause,but it's too embarassing...

You don't need to take the engine completely out of the frame for a top-end job. But regardless here's a couple more tips; make sure you use new cir-clips and make sure the cir-clip openings on the pin face 90deg to the travel direction of the piston. Also re-torque all your head bolts after a few hours ride on your new piston.

thanx for the tips guys....i take it all apart just for fun and to learn and clean it all inspect.....I don't mind i have lots of time at work and lots of space. And I do have a torque wrench so I will be using if for all bolts.


Some other items I like to use are an assembly lube like Mellings for the wrist pin, cams and the cam journals, a good anti-seize paste which is great for the exhaust header collar bolts (high heat can seize them as well as the gunk thrown up on them by the front tire,) an assortment of Loc-tite formulas for the nuts and bolts needing this on re-assembly of the bike (the manual will tell where to use these and where not to use them) and some zip-loc baggies to keep the nuts, bolts, screws washers and assorted clips/keepers together and organized.

Man TT sure has some guys with great advice for these situations. Thanks Gray Racer and all the others here.


hey thanx florida, I actually have all that done, the baggies etc, and thankfully I had put anti-seize on the head bolts last time i took it apart and they came off relatively easy.....the manual just says to use engine oil to lubricate everything before re assembly so is that ok?

I did the top end on my '99 YZF400 last winter. Some things I ran into and did to make it easier.

The cam caps on the earlier YZF's tend to "squish" when they are torqued (after adjusting valve lash) When this happens and you torque cam caps the cams bind. I sent my head to a place in California called Engine Dynamics. They guy there is super friendly and very familiar with this problem. He line bore it (cam caps and head) and re-polished the surfaces (I also had him do the falve seals and guides along with a valve seats). The cams spin like the head is brand new. I set the valve lash with the head off the engine, on my work bench (cams installed and torqued) This makes it easier than when the head is on the engine, you don't have to manuever around the frame. Before honing the cylinder, using my dremel, I ground reliefs about the size of a nickle cut in half on the bottom of the cylinder in line with where the wrist pin will go. I rounded the edges of the reliefs and polished them. This makes it easier to put the wrist pin and clips on. Most of the newer bikes come with reliefs already cut. I also compressed the rings and slid the piston in the cylinder before putting the piston on the rod. Put one wrist pin clip in the psiton, slide the piston (with rings in the cylinder. Use a piece of duct tape or have a buddy help hold the piston in place. Line the wrist pin hole up with the hole on the rod and slide the wrist pin through, then have your buddy hold everything in place while you put the second clip in place. This ALOT easier than trying to slide the piston in the cylinder with the piston already on the rod. I did it alone going slow and using a lot of patience. Make sure all the internals (pistons, cams, cylinder, etc) are coated with oil.

I have about 30 hours of HARD riding (after break in) and my bike is running stronger than my partners low hour 426. It's all in attention to detail.

Other mods:

Aluminum 426 subframe, about 4 pounds lighter

04 450 radiators, bigger, better cooling

02 426 carb, more snap, better suited for a dirt bike

race tech built/tuned suspension

All this and my 06 450 is still faster and smoother.


hey eric does the 02 426 carb just bolt right up no mods?? thanx for the all the info......i got it all back together easy last night at work......everything worked great no problems.......id had the cams out before so i was familiar with most stuff in the head...........ive got to open it up again though when i get the auto decomp exhuast cam......

I had no problems with the carb. I used a JD jet kit for the 400 and fine tuned it. The 98 - 00carb is a street bike carb adapted to a dirt bike. The next generations Keihins were built for dirt bikes.


Don't forget to check the end play in the rod at the crank end. Hate to see you do all that work and have a bearing go out later. You got the rest in the bag. Good luck.

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