head replacement?

[Dj]

I put the bike in to have the clutch looked at on Wednesday, ended up replacing the friction plates, apparently the steels were alright, just needed a lil roughin up or something along those lines, the guy I took it too also replaced the plugs, cleaned and balanced the carbs, stuck in a new set of needles, fitted new chain and sprockets and set the valve clearances

obviously, this has made a somewhat dramatic difference to the bike, all for a fraction over $400, which i thought ended up being reasonable

When i went in to pick it up today, he told me that the valves are actually seating further into the head, than they should be and that after the ride day thingo Im doing on sunday, the head will more than likely be in much the same shape as it was before he redid it.

Now, the questions I have are.
Would it just be cheaper/easier to replace the whole engine, or if its only necessary to do the head, to go with that option?
These motors to the best of my knowledge, are exactly the same as the GPX/Z???? 750's, are, or are they not? And can anyone give me a ballpark figure of what I might be looking at spending, given the two options above?

The bike in question is a ZXR750H1.

Thanks
Dj

[Barrabob]

I asume this is valve seat regression he is talking about where the valves literally bang themselves into the head.

Heard about this on some dunnydore taxis and its why they used to put lead in petrol to cushion valve seats.

fitting hardened valve seats fixes it but at what price??

maybe run some of that flash lube stuff.

[Lainie]

I know nothing bout anything bike related but had teh head done on the GPX250 a few months back and was under $300 if that's any help.... I guess the best way is to ask round

[jpbikeboy]

A second hand head will set you back about $300-$500. Problem being that they are also likely to have higher km's on them due to age. You could have your head rebuilt but that's gonna cost heaps. If it were me i'd search around and find a head that is in reasonable condition. remember there's also going to be a head gasket, shims, oil, etc etc. so it might cost ya.

[I-K]

When i went in to pick it up today, he told me that the valves are actually seating further into the head than they should be

...and how does he know this without taking the head off?
No mech takes the head off a bike as part of a valve-clearance and clutch service and charges only $400 for the job; pulling the head off, checking it and replacing it alone will cost about that.

after the ride day thingo Im doing on sunday, the head will more than likely be in much the same shape as it was before he redid it.

Before he redid what? He set the valve clearances, which, chances are, haven't been touched since Bob Hawke was Prime Minister.

It's not inconcievable that your valve seats have started receding into the head, but it's pretty damn unlikely.

These motors to the best of my knowledge, are exactly the same as the GPX/Z???? 750's, are, or are they not?

The ZXR750H engine was *based* on the GPX750. The heads may well be externally identical (same outer casting), and it should be easy enough to dig up the valve diameters, to see if they're the same (I imagine they won't be, though), but the porting and combustion chamber shape are virtually guaranteed to be way different.

Would it just be cheaper/easier to replace the whole engine, or if its only necessary to do the head, to go with that option?

Straight-swapping whole motors or major subassemblies like heads and gearboxes with parts sourced from wreckers or Ebay should really only be done on recent-model bikes which have sustained some kind of major mechanical failure; stripped dogs on the gearbox, dropped valves, etc. In that situation, the wrecked bike being cannibalised is going to be new enough, with few enough km on it, for the parts coming off it to be trusted. Just bolt them in and go.

An older bike which can be found at a wreckers these days is likely to have ended up there fairly recently, after a lot of use and a lot of km. Parts coming off it are unlikely to be in much better condition than the parts they're intended to replace.

Essentially, with an older bike, you'd go to a wrecker for engine parts only if your engine has done something catastrophic, like put a rod through the cases, shattered the clutch basket, or snapped the camchain and made a mess of the top end, and, before using them, you'd get the replacement parts checked over and reconditioned. Conversely, if your old motor is tired, get it rebuilt; you might stumble across a secondhand motor which came out of a bike 15 years ago and has been wrapped in plastic ever since, but the chances of that are low.

This is one reason why I concentrate on newer stuff, mid-90's and up, with my resurrections; used parts in good condition are easy to come across, and the motors aren't overly likely to be worn out, meaning I don't have to shell out a fortune on specialist tools and develop a level of mech skill far beyond what I have now.

I leave that to Gos, for now...

SHort answer to your question, though... I still doubt your head is dying, but, if it is, it'll be more worthwhile having yours rebuilt than trying to find a good used one. It costs money, but not so much money to make it really scary.

[I-K]

fitting hardened valve seats fixes it but at what price??

No need. Kawasaki have been using hardened valve seats since the Z1 in 1972. Japan went to unleaded fuel many years before Europe, US and Aussie.

In fact, did Kwak build a four-stroke of their own design before the Z1? If they hadn't, it means every single four-stroke Kawasaki ever built has been unleaded-fuel compatible.

[Gosling1]

....No need. Kawasaki have been using hardened valve seats since the Z1 in 1972. Japan went to unleaded fuel many years before Europe, US and Aussie.

In fact, did Kwak build a four-stroke of their own design before the Z1? If they hadn't, it means every single four-stroke Kawasaki ever built has been unleaded-fuel compatible.

That is indeed correct, the big old Zeds were compatible with unleaded fuel since conception......

The 900 was not the first kwakka 4-stroke, that honour goes to the Meguro-based 500cc K1 of 1959 - heavily based upon a BSA A7 of the same 'era'. I would hazard a guess that these bikes did not have hardened valve seats.....

The W1 of 1965 was the first 'recognised' Kawasaki 4-stroke. I can find no reference to hardened valve seats for this model.

...and how does he know this without taking the head off...

easy - there is a limit to the valve clearances for any 4-stroke, and if you have a range of shim adjustments (as this model kwakka does), then its very easy to determine that, for example, #3 exhaust clearance is now less than the accepted minimum, the only conclusion you can make from that is that the valve has dropped into the seat.....this is really apparent when the clearance is reduced so much that the valve in fact no longer closes, and you start to burn bits of valve down the exhaust. A compression test on all cylinders will also show *possible* valve issues due to a drop in compression on the applicable cyclinder.......(you know all this mate, I feel like I am preaching to the converted.... ).


without exception, every 900/1000 head I have serviced will always reveal problems with valve seats once the clearances start dropping right down to the acceptable minimums.......its London to a brick that the valve seat is cactus, and will need to be replaced.

Even with hardened valve seats, they do need replacement over time....

[I-K]

...and how does he know this without taking the head off...

easy - there is a limit to the valve clearances for any 4-stroke, and if you have a range of shim adjustments, then its very easy to determine that, for example, #3 exhaust clearance is now less than the accepted minimum, the only conclusion you can make from that is that the valve has dropped into the seat...

Oh, fershur, but what's the relative likelihood of the valve clearances closing up due to valve seat wear/recession, as opposed to the valve stem stretch?

[Gosling1]

relative likelihood is about 10,000/1. Valve stem 'stretch' is just not an issue in real terms, the materials used to make valves are just so much better these days ( and I mean since about 1970), then they were before then.

You can of course have the odd exception.......and if valve stem stretch does occur, then you can always 'tip' the end of the valve on a valve-grinding machine......but it is just very uncommon to need to do this ( in bike motors anyway.........car motors are another kettle of fish altogether...) - and in most cases you can only tip the valves by a very small amount, equal to about 3 or 4 shim sizes only.....

I would bet my house that in 999 cases out of 1000, if the clearances have reduced considerably to below service limits, then its because the valve seats are cactus, not the valves stretching.

[Dj]

according to this mechanic, the valve clearences were nonexistent, hence the trouble with it idling

someone had been in there previously and made a right mess of the job they were supposed to undertake

The thing i noticed when i rode it home, was that despite the efforts put in on his behalf, and from what he said, it would feel like a completely different bike....the clutch does, no two doubts, it idles higher than it did at about 1100rpm, but theres no noticable difference in the way the power rolls on with it.
Or am I expecting/looking for the wrong kind of improvement/s considering what was done to it?

[Gosling1]

...Or am I expecting/looking for the wrong kind of improvement/s considering what was done to it?

well, once the clearances are done, you should expect an improvement in starting at the very least, as the compression is back to where it *should* be. I don't think you will notice a *huge* improvement in roll-on, once again, there should be a little improvement, as the cylinders are sealing correctly (corect compression), but it does depend on the condition of the rest of the motor (ie rings, etc)

You need to do a proper compression test on the motor, and compare the results to the standard factory 'settings' - if the previous person stuffed up with valve clearances, then what sort of nick is the rest of the motor in ??

If you do need to do a 'hone and re-ring', then that is the best opportunity to replace any dud valve seats - as IK noted in a previous post, it is usually better to repair what you already know to be stuffed, than to take the plunge with another whole motor and/or head, which may need just as much work (or even more) than what you already have.

It's not *that* expensive to replace a couple of stuffed seats and/or valves, its just a bit of buggerising about. (well, thats what I call it, but other people call it a huge pain in the coit.... )

Good luck mate

[I-K]

I would bet my house that in 999 cases out of 1000, if the clearances have reduced considerably to below service limits, then its because the valve seats are cactus, not the valves stretching.

Nodz. This is what I said in the earlier post that I leave this sort of thing up to you...

Mind you, here's something else I'm wondering about... When valve seat recession occurs, is that because of seat wear, or because the action of the valves smacking into the seat serves to hammer them into the alloy of the head?

If the latter, how the phuq are you supposed to fix that with new valve seats?

And here's another one. Since the seats are flush with the wall of the port, how are you supposed to knock them out to replace them? Do you dremel a little notch in them, to give a screwdriver some purchase, or what?

[balanse]

Hey I-K this issue is huge in dirt bike circles at the moment. Honda have brought out their CRF 450 four stroke dirty with titanium valves and they have a achieved a reputation for valve failure (gained especially when not one of the brand new models finished the 2004 Aust 4 day enduro).

The problem is reputedly that Honda have made the valves so light that they are being pulled into the head (slowly crushing the head of the valve) by the valve spring.

Honda are saying it's air cleaner maintenance - of course.

[Gosling1]

...Mind you, here's something else I'm wondering about... When valve seat recession occurs, is that because of seat wear, or because the action of the valves smacking into the seat serves to hammer them into the alloy of the head?

If the latter, how the phuq are you supposed to fix that with new valve seats?

And here's another one. Since the seats are flush with the wall of the port, how are you supposed to knock them out to replace them? Do you dremel a little notch in them, to give a screwdriver some purchase, or what?

#1 - the action of the valves hammering the seats, is what eventually drives the seats into the alloy head. Valve seat *wear* is one thing, and can be sorted out when you do a '3-angle cut' on the valve seat, (generally done when you fit new valves) but when the seat itself starts to drop into the head, then its time to remove the seat, and replace....

the biggest problem is that people just bung heave-duty springs into a head , which immediately starts to accelerate valve-seat wear, and pull the valve into the head ( and in extreme cases, cause the valve-stem itself to stretch.....). It is the greatest urban myth that street motors need h/d valve springs..........its a load of cobblers. If you install a cam with higher lift, then what you actually want are lighter springs......

The old valve-seats, when they have dropped into the head, are 'machined' out, and a new valve seat is then installed. The new seats are an 'intereference' fit. Once the new seats are installed, then you need to do the standard 3-angle cut, and then 'lap' the valves into the seats, to obtain the correct seating width on the valve crown, then you set the valve clearances........it sounds a lot more complex than it is in reality (remember though that I only do 2-valve stuff, that 4-valve shit is for Japanese mechanics with tiny Japanese fingers.... )

Balanse - sounds like typical Team Soichiro bullshit to me......they want the reciprocating weight of the valve-train so light (because of the hi-lift long-duration cams being used), that service-life is no longer a consideration........They should have stuck with s/steel valves. You can't beat good 'ole s/steel valves in 9 out of 10 engines.

[HemiDuty]

If your springs are too light with high-lift cams, won't the buggers 'float' at high revs? As in not close fast enough after being bashed open by the faster opening speed of the cam profile? I have had valve float in cars before, but usually only from over revving them when they had standard springs with big cams etc.