head replacement?

[Neka79]

wow..this is an interesting read....
for wot its worth.....i had my cb900 (81) rebuilt cos it dropped a big end bearing....i kno this isnt the same, but for wot its worth it cost me $1500 for a full engine rebuild....

[Dj]

Gos, wtf is a 3 angle cut?????

....pardon my ignorance......


everytime i read this thread, it makes more and more sense, i can visualise whats being said in a better context.....

this is the only thing that leaves me puzzled however....

[krusty]

Could also be that the valves have burned up a little. I have a set from the old head of my gpz250 that have worn below the minimum spec size.

[aardvark]

Stop it you guys. You're making my head hurt.

[Gosling1]

Gos, wtf is a 3 angle cut?????

....pardon my ignorance......


everytime i read this thread, it makes more and more sense, i can visualise whats being said in a better context.....

this is the only thing that leaves me puzzled however....

I have attached a scan of the relevant page from my well-used Z1000 Factory manual. It shows a '3-angle cut'......the 3 angles being 45deg, 30deg and 60deg, in that order. These are the 3 main angles used in valve seat 'cutters' (the machining tool that actually does the cutting) - this stuff I leave to the machine shop.....

You need the 3 angles in order to ensure that the valve itself is properly sealed against the valve seat ( these are the diagrams above - they show the correct thickness of the valve seal........you use *engineers blue* to determine the thickness (just an ink applied to the back of the valve, rotate the valve a few times, and heh presto, you have an immediate indicator of the thickness...)

[Gosling1]

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.

If the springs are too light, yes, you will get float at high revs. Its always a bit of a 'balancing' act with valve springs/valves etc. Most street cams for bikes *do not* require anything other than standard springs. Drag racing engines absolutely need h/d springs, due to the *huge* lift and duration compared to street, and the rev range in which the engine is designed to operate.

The cam willl not open any quicker at all with a higher lift, it just pushes the valve in by the additional lift amount (eg 1.5mm). The duration is what determines the speed of valve opening......then you have the additional issues of using 'asymetrical' or 'symetrical' cam profiles........Jason, your head will *really* start hurting if we get right into this topic mate

...could also be that the valves have burnt up a little...

spot on - once the clearances have been left too long, the exhaust valves (and they are always the first to tighten up due to the heat they deal with), will not seat properly, they are now sitting 'proud' of the valve seat, the valve then starts to 'burn' around the edge with prolonged operation at incorrect clearance, and you start to lose bits of the valve out the exhaust.........then this affects the compression (a compression test can help identify this), and you have a shitty running motor, that is hard to start, uses more juice than it should, blows weed etc etc .....or worse.....

Just for background, nearly all the stuff I have learnt about motors was learnt from my bro-in-law, he is a good mate of >20 years, and was the Senior Fitter/Machinist at a local engine rebuilders workshop here for almost 15 years........he builds shit-hot *red* motors, and also helps with the engine builds on a local drag car...........he knows his stuff I am really only an apprentice .......