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Michael Cahill

Basic engine rebuild checklist...

16 posts in this topic

My yz400f is in need of a freshening up.

Tons of hours, weak compression #'s etc.

Id like to put in new rings, hone the cylinder, and either replace, or somehow save my exsisting valves. (is that even possible? lapping, etc.)

What is the best and most cost effective way to go about doing this?

Ive already priced out the OEM parts online.

but dont have any machining capabilities.

so does anyone have suggestions for a good place to send my head/cylinder for honing, and if needed recut valve seats?

Thanks :thumbsup:

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A new cam chain is a must!!!

The valves need inspected and most likey you should just replace them if it has "tons" of hours. That is pretty heavy on the hours.....haha pun

I would also replace the piston along with the rings. Check your connecting rod for side to side clearance that the manual calls for, and there should be no up and down movement on the rod. New exhaust gasket should be on your list as well, the one that is in the exhaust port. I'm sure there is more but I can't think of them right now.....sleepy.

More people should chime in

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Any local engine shop can hone the cylinder for you, however, with Tons of hours, you may be looking at a replate - try Millenium Tech if you end up needing it.

Crankshaft - inspect the timing chain gear - if it's worn you can't rebuild the crank but if it's still good, you can have the crank rebuilt and save yourself a lot of dollars there.

Main bearings - replace

Piston, definitely replace it.

Valves - I would be replacing them - you are running SS valves in that bike so they will be a lot cheaper and last a lot longer - check your springs too. Definitely would get the seats cut, and for what it's worth - new seals and guides.

While you are in there, check all your gears and bearings in the tranny and replace as necessary - gears are cheap.

Check the shift forks for uneven wear, check your oil pump internals for wear.

Cam chain replacement is a definite too.

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Well if you do end up rebuilding your crank. You might as well put a 426 rod and piston on it and get a cyl to fit that. Check out threads on here for the conversion and also look at Lukesracing.com as they sell a kit for that I believe. You might as well make it a 426 or a 444 if you are going to end up doing lots of work to it

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Thanks for the input.

so far looks like im gunna need:

new valves, and valve seats, allong with seals.

cam chain.

piston/rings (what is the reason for needing a new piston?)

main bearing(s) (can i save them if there in no noticable slack?)

What do you mean by "rebuilt crank"?

anything else?

I'd like to do most of the work with the engine in the bike, as my swing-arm bolt is seized. :thumbsup:

that even possible? (i would only be able to get to the cylinder and head tho)

and do any of you have a ballpark cost estimate on just the above parts?

plus the head machining work?

Thanks again :ride:

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worked on a buddy of mines atc250r a couple of years ago with a stuck swing arm bolt.... we laid it on it side, got a flat nosed punch the size of the shaft, i held the punch with a long pair of vise grips and he hit it with a sledge, numerous times, then it finally came out.. surprisingly it did not hurt the threads too bad... for sure a 2 man job....

sometimes you can put the nut back on loose and protect them that way for a little while..

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Let the machinist doing the head work tell you if you need valve seats or guides. It's very rarely required on YZF's.

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Let the machinist doing the head work tell you if you need valve seats or guides. It's very rarely required on YZF's.

Well if its rarely required, whats the point of sending it to a machinist?

can i get away with simply buying new valves, and lapping them to the original head then?

or am i missing a step here :thumbsup:

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I know this sounds cheap, but can i get away with simply getting new rings, honing the cylinder, and if needed, new valves?

(i'd put in new seals, and a cam chain as well.)

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Well if its rarely required, whats the point of sending it to a machinist?

can i get away with simply buying new valves, and lapping them to the original head then?

No, not usually. While it's true that stainless valves can be lapped, lapping is not an appropriate means of refinishing valve seats. The seats need to be reground, preferably, or recut to a precisely flat, precisely round, correctly angled finish when the valves are replaced, and lapping won't do that.

While the machinist can make the cut come out exactly as he wants it so that it perfectly fits the new valve, lapping uses your new valve as a cutting tool. An abrasive paste is applied to the valve face, and then the valve is rotated against the seat. The idea is that the abrasive will grind away the irregularities of the seat until it matches the valve, but the effect is that BOTH the valve and the seat are abraded; the flat surface of the new valve wear the irregularities of the seat down, and the irregularities of the seat grind an impression of themselves into the valve face. It's just not suitable for anything but correcting the slightest mismatches, and with modern machine equipment, it's both unnecessary and obsolete.

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ah i see,

i didn't know grinding and cutting new seats were two different things.

also, does the machine shop only need the head, or do they need the new valves as well?

Any suggestions for a shop?

perferably one that can do the machining, and install/adjust the valves.

and also hone the cylinder.

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Also, im hoping i can leave the bottom end alone. (main bearings/crank)

is there any way to tell if they are shot without splitting the cases?

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They'll need the valves, and you should let them install the springs.

Engine Dynamics in California is a good place to send out to. Don't know about anything in Washington.

The manual describes how to evaluate the rod. The mains are like any ball bearing. You can can check the clearance, but you can't tell how long 'til they fall apart.

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ok, thank you.

any idea on expected price for honing, gridning seats, and assembling head?

we talking $100, or $500?

just trying to get a general idea.

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