Z900 A4 Cylinder head refurb (advice needed please)

[Gray17]

Good afternoon to all,
As you may have seen I'm in the process of carrying out a gull refurb of my Z900, whilst i have worked and restored both cars and bikes before, not specifically the Z900 hence my call for help from more knowledgeable people.
I recently fully stripped my engine as part of the resto, i have had all of the cases, cylinder head, barrells, sump etc vapour blasted and rhey have come up like new. There were several issues i have found collapsed cam chain rollers etc but thats another story.
Before stripping the engine ran fine and didnt smoke at all, i also did a compression test on all 4 cylinders which were all very similar readings.
The head has come back from blasting and ot has obviously been apart before and had new hardened valve seats fitted because of carbon you couldnt tell before but now you can see slight cracks in the plug hole threads in 2 of the cylinders. I have no idea if when the seats were fitted they were too tight and caused this, or whether this is normal wear and tear? It could have been like this for years, also theres no compression leaks currently and cracks are only at last 15mm or so of plug thread remainder is all intact.
Whats your thoughts guys, can it be left as is or does it need urgent sorting, could it spread up the hole? if so who and ruf cost.
Many

Thanks

[warren3200gt]

Yours and just about every head out there Graham.
I wouldn't worry about it, it'll run fine., if your really worried you can get them welded.
What compression readings did you get?

[zed1015]

It'll be fine.
Most heads have those surface expansion cracks.
How do you know it's had new seats fitted in the past ??
The stock seats are hardened from the factory.

[Gray17]

Thanks for replies guys, i wrote the 4 cylinder pressures down, but cant find the paper now, its in garage somewhere, but from memory they were within a few psi of each other which usually means little or no wear or leaks.
Good to know the cracks are pretty standard as not seen that before, but then ive never stripped a Z900 before so nothing to go by.
As for Hardened seats i assumed these had been done by eng co. As i recently had a MGB head upgraded to hardened seats and looked very much like that.
Again nothing to go by but if you say thats how they came from factory then thats fine. Its def been apart before as had sticky glue stiff on all gaskets, nightmare to remove, oh and theres a helicoil in 1 of the plug holes, thats been peened over.
I will be re lapping the valves in the same place they were removed, just to ensure a gas tight seal, then do the petrol in chamber with plugs in place to check for seepage.
Its a lot of work, but hopefully be rewarding with a reliable engine.
I am still debating whether to replace the rings, even though good compression, as no idea how long theyv been in, bikes done 22k ish so may have been refreshed at some point but who knows?

[moizeau]

Use diesel not petrol to check for seepage, maybe wrong......

[Gray17]

Sorry excuse my ignorance but why diesel? I know diesels good for freeing up seized engines and bolts etc by soaking wasn't aware you use it for seepage. Do have both diesel and petrol so not an issue just wondering if theres a particular reason its better to use in this test.

[Ultim8pc]

Use diesel not petrol to check for seepage, maybe wrong......

I use paraffin.

[Gray17]

Any particular reason for parrafin, rather then the other 2 forementioned liquids.
Im guessing you could use anything really even water at a pinch, anything will tell you if you have a leak, guess petrol does evap quickly whilst diesel and parrafin dont, just thinking out loud now

[moizeau]

Petrol evaporates, diesel and paraffin don't, they are basically the same

[Gray17]

Yes realised afterwards, ill use diesel as got some in a fuel container.
Thanks

[Ultim8pc]

Petrol evaporates, diesel and paraffin don't, they are basically the same

[Gray17]

Afternoon Gentlemen, further to my initial query i now have yet another, and with the old saying 2 (or more) heads are better then 1 again seek your opinions.
As stated head has been fully stripped and vapour blasted, now before all this the bike was running with no smoke,approx 22500 miles ( but cant be verified due to age) as part of the gasket set i am fitting 8 new valve stem seals and all valves have been fully cleaned and ready for lapping in their original positions.
As part of checks all inlets are fine with very little play in their guides as you would expect.
The exhaust valves 2 are reasonable but the last 2 have what i call excessive. So firstly does anyone have a new exhaust valve that can measure the stem diameter please. Why well i could buy 4 brand new valves which may take up most of the wear but need to compare to mine now. I could replace the valve guides, but this would mean sending the head out for repair and reaming, my funds at moment are limited if they wernt id replace all 8 guides, springs and valves together as a set. Ive heard you can knurl the guides then ream them, or get oversize valves, but again need guides reaming.
Anyway any thoughts guys and wheres best and most cost effective way to resolve and buy parts if needed? Know i could leave it and see but dont want to get it all back together and have smoky issues!
Many thanks

[gray]

I suppose you could measure the affected valves at various point on the stem. From the factory I would have thought they would be the same width all the way up. So measure the stem width just around the collets groove area( That'll be unworn as it doesn't go into the guide) , then midway down the stem, then at the lower end of the area covered by the guide. If it varies that's bad. The workshop manual will tell you how much is acceptable.

[Gray17]

Hi Gray, thanks for reply, yes hadnt thought of that, i have a set of digital callipers so I'll measure the top of vavle and compare with further down. I have tried all 4 valves in the affected 2 and they all seem similarly loose. My guess its the guides, rather then the valve as made of softer metal. Suppose i could also try an inlet valve as they dont run as hot or wear as much.
Which leaves me several choices.
1. Do nothing put it back and see how it runs with new valve seals - cheapest option but may have to be re stripped if not right.
2. Have the 4 exhaust guides replaced and reamed to suit valves, however they aren't cheap to buy, then theres the cost of fitment and reaming, plus long lead timescale wise near me. More worryingly may aggrevate the small stress cracks in plug thread of head (see 1st photo)
3. Have all done at same time incl new valves and recut seats - most expensive option but back to factory spec, still got that stress fracture worry when their pressing them or tapping them in and out. Which may mean having them alloy welded then re tapped to plug size
This is option I'd like and prefer, but with everything else i'm doing to bike need to be realistic cost and timescale wise.

[Ultim8pc]

I'd just get new guides fitted and seats recut by a professional. That way you know everything is spot on.
I use Roger Upperton near Leeds and can recommend him.