Help Lapping Valves - ZXR250C

That valve is serviceable. Contact patch needs to be moved closer to the centre, so the seat needs a crown and face job if you can.

 

If you zoom in and look at the left in that pic you can see the seat surface is not 45 degrees anymore, it's gone concave with two different angles there. Needs a new valve and cutting the seat. Becomes problematic when the seat has recessed and you run out of shim sizes to use.

Half the problem is neglect and lack of valve clearances getting done, the other half is the relatively weak cam chain and tensioners.

I'd bet money all your valve springs are bowed out as well.

Do you know what your valve clearances were and the shim size for each valve before you pulled it apart?

New valves and springs all around would be a start, whether it is worth doing on your head comes down to the seats and if they are recessed, and if you will have sufficient valve clearance. New valves will help there.

If you can find another cylinder head or even complete engine truly in better condition that would be the most economical option.

Otherwise you'd be looking at new valves plus new seats, as usual the small size of the seats makes finding cutters and replacement seats difficult

 

OK, so for your first engine in the bike, sounds like it needs valve clearances done. That will help with the compression, especially if they are below minimum clearance when you measure them (at room temp). Set the clearances to the maximum value or as close as possible. If you are swapping shims around, try to set the inlets per cylinder to the same value, and just get the exhausts above minimum.

Measure the diameter of the shims against the valve/spring retainer, some engines have 7.48mm shims and some use 7.9/8mm shims. The part number for the valve retainer has gone through several revisions. Order shims in the diameter that matches your valve retainers on your engine and not what anyone else assures you is correct. My Malaysian C model had 8mm retainers and there were some 7.48mm shims in there. Not good.

After clearances are done, balance the carbs and set the mixture screws

Also, compression check needs to be done with throttle wide open or carbs removed. Use a gauge with a steel pipe and rubber tip instead of a rubber hose, the combustion volume on these engines is so small that having the rubber hose attached to the gauge will give you false readings.

 

If you lined up the ring gaps together that'll do it

 

Because for compression to be measured accurately there needs to be no restriction on the intake.

Realistically when the throttle is closed the engine is expending much of it's power drawing intake breath through a small restriction - that's where engine braking occurs.

Think of breathing through a straw.

Also be aware that with most compression testers with average size hoses to the gauge, the volume of that hose seriously affects the readings, HOWEVER for comparison between cylinders high/low readings it really doesn't matter all that much.

Ideally the tube between the compression tester and the gauge would be tiny so as not in add too much to the combustion chamber volume.

 

ahhhh, mr sparkle!


nitriding is a metallurgical surface hardening treatment, using a special alloy of steel and ammonia gas at relatively low temperatures. unlike carbon steels, performed at high, above "curie point" temperatures. nitriding is done at 200C or so. it doesnt "temper" or "draw". mostly used as it causes minimal distortion.

it is not a coating and no type of washing detergent will "strip it off". unless you define "coating" as a few thousands of an inch thick layer of the metal itself.

nor will any nitrided steel be suitable for valves. exhaust valves can glow red hot, well above the tempering point of any carbon steel alloy. theyre generally a type of stainless as its the only steel alloy that remains "austenitic". it doesnt harden with heat-treating like traditional carbon steels. a carbon steel subject to the thermal conditions of a valve will suffer unwanted distortions and grain growth, embrittlement, and finally failure. in a matter of minutes.

edit. ok, there ARE nitrided valves, but again... its a "case hardening" process. i didnt know there were nitridable stainless alloys? learn something new every day!

https://www.enginelabs.com/engine-t...s-of-the-nitriding-process-with-howards-cams/

still, point remains. it is NOT A COATING. its the actual "skin" of the metal itself, the "case". its relatively thick and does NOT "strip off".

a valve MAY have an oxidised surface that RESEMBLES some type of coating, but rest assured... they have no such thing as a nitride coating...

150 grit is more suitable for grinding a valve in, especially if its so far gone it needs serious work. and generally, as you grind, the grit gets finer anyway. a tube of "off the shelf" grinding paste is something like 60/120. its pretty coarse stuff! crunch crunch crunch...

you do a compression test with open throttle due to volumetric efficiency. at idle, closed throttle... the engine is sucking in, on average, 30% of the air the cylinder can hold. at idle, the compression ratio reduces to maybe 4:1... depends on the engine of course. you only get full compression with wide open throttle.

generally, as long as the compression tester has the one way valve in the threaded section that screws into the plug hole, it will read the pressure that builds up in the combustion chamber, regardless of the size (or squishiness) of the pipe attached. a large bore pipe, just like a large tank on an air compressor, just takes a lot more cranking to fill to final pressure. an engine is just an air compressor, after all.

to be honest, i dont own a compression tester and have never bothered performing a compression test outside of that one time at TAFE for the module... and that was twenty+ years ago. generally, you can HEAR if one or more cylinders are low just by the cranking sound. each to their own. ive never needed a compression tester.

in saying that, i just realised how to measure VE. ok, now i DO see a need for a compression tester, lol!

why? because the reading in "bar" is the compression ratio... 1 atmosphere being 1 bar... 8 bar is 9:1, 11 bar is 12:1, and so on... as most gauges read 0 bar or "gauge" at atmospheric pressure... so you gotta add 1. if its "absolute"? starts at 1? then its the same as the reading.

in psi, thats 14.7=1
kpa? 101.3=1
whatever... sidetracking...

only mention of what bike this is, is your profile... zxr250...

like all of these I4s... good luck finding ANY shop that can cut new seats, and will do it for a reasonable sum. without knowing what the actual dimensions on the zxr valves are... one can only assume theyre as ridiculously small as everything else... ruckusman probably got the closest with an "off the shelf solution" with the carbide cutters.

that is, the small QM holder...


the bottom blade i assume ruckusman has? (oi! you never actually elaborated!) for, as i already have shown... the two above are still too freaking big! whereas that long skinny one SHOULD work... i myself cannot vouch for that as i didnt get one... just rest assured, the upper blades minimum size is approx 22mm.


and as there only seems to be a 0.8mm insert with the 3-angle profile with that extended nose for doing pipsqueak valves, its probably better to get a bunch of the 90 degree cutters and grind them appropriately. 30/45/60. im yet to order my own set for doing this exact thing...


you will still need a 9mm arbor for the cutter holder, with a reduction to whatever the valve stem diameter is that you require.

at this point, a shop MAY cut the seats for you.
they may well tell you to bugger off, and then try to charge you a 5-10X markup on purchasing their own tooling, plus labour and setup fees... as they will never use that tool again except in a blue moon. and rest assured, they will charge the next guy just as much! gotta pay the rent and power bills somehow.

and doing it yourself? definitely NOT for the faint-hearted! you need a good machine, this isnt something you can do with a hand drill or drill press...

you "may" get away with a modified ""neway" cutter (is it linkin that did that for a cbr?), but again... single angles. they are extremely expensive. the smallest arbor they stock, iirc, is 4.5mm. my memory is hazy...

neways are the best for hand cutting seats but at that point, you may as well open your own shop? if you go that way, be prepared to cough up a fair bit of greenstuff... and, seeing you are in brissy... add shipping fees, conversion rates.... yeah... meh. 3 engines, thats 48 valve seats... your call!

the joys of pipsqueak I4 250s!

 

Our cutters at work were too large for the MC22 engine I was working on. Apart from replacing two bent inlet valves, the camshaft gear idler and doing a few helicoils, all I did was lap the valves in (thankfully they were in OK condition apart from the two bent ones), put a new head gasket in, set the clearances & time the engine, reassemble and compression test.

 

i know i saw SOMEONE with a cbr head and a modified neway cutter on here somewhere... meh.

iirc, the smallest body they have is 22mm. may have been 19?

i do love my (barely used!) neway, tossed up the idea of simply buying blades and making bodies, but yeah. even that was getting stupid expensive...

 

Okay, so with ring gaps on compression rings, you want them away from any thrust surface. So not pointing towards the inlet or exhaust side. The manual will usually say space the ring gaps 120 degrees apart. Too close together or lined up and you'll lose compression. On mine I believe I just faced them 180 degrees apart with the ring gaps in line with the wrist pin. Rings are directional, if they have a marking it faces up. The top compression ring is usually chromed and the second is not.

For the oil control rings, it's similar but you are dealing with 3 pieces, so just face them 120 degrees apart not facing any thrust surfaces.

 

@Gen has got the modified neway cutters I believe.

@gyro gearloose if you need a 7mm arbor with 3.5mm pilot for neway cutters I'd get the 3.5mm carbide rod and a C8 SLD3.175 tool holder extension bar - drill and ream to 3.500mm then use the carbide as a mandrel to turn the C8 bar down to 7mm, then - all available on aliexpress for not too many $$ all told - much less than the neway cutters at any rate.

I've been back on the lathe and will hopefully have a back side bearing support for the pilot tomorrow.

I managed to find 3.50mm I.D. bearings on aliexpress and have a way to hold 6 of them in a collet.

It slots into the where the buckets go (20.0mm as close as I have measured thus far) and goes right down to the top of the guide - that will give significant support and take the possible runout to micron level.

 

Yeah sorry about that - I need to finish my Quick change tool post fitment as this shimming cutters and parts to height is nucking futz.

I'm trying to do 'some' milling work on a lathe with all of this.

I figured another way around the cutter making it into the tiny exhaust throats that doesn't modify the tool holder but a small rebate cut into the guide pilot to allow the tool to go in far enough - the smallest carbide cutter won't make it otherwise.

So that's a milling end cutter in the lathe and the guide pilot holder in the tool holder to cut that small rebate - if you put a 9mm drill in yours and see where they come into contact you'll see where I mean

I've checked my C10 SDC04 3.5mm tool holder and collet for runout and concentricity, perfect in both respects - remember I commenced this BEFORE I bought a lathe - the SDC pull back collets are the business because even an ER8 collet nut won't go very far down the throat of an FZR250 intake - not far enough to not interfere with the cutter anyway.

hallelujah to that, everything is right at the limits or beyond.

 

i know i saw one somewhere... not important but is on topic vaguely.

no plans on neways, but someone might take that on board. just recall spending a few hours hunting through listings and options before i made those cutters. the 45 at least worked well enough to clean the seats up. now the diamond wheel i ordered way back has actually arrived i can revisit that when the time comes.

the "dreaded chug" is coming back. worth checking clearances now its done 2000km. strip it down sooner or later, and see how theyve held up. probably later...

at least we discovered with kiwimatts, that if you simply crank it over WITHOUT touching the throttle, it clears up in a few seconds. rather than sitting there with the fuel off, WOT, chug chug chug for five minutes...

in saying that... grabbed a bunch of "pneumatic manipulators"... 12 and 15mm suction caps. now theyre here, i gotta do the "twiddle stick". little pneumatic cylinder so i can REALLY pull some vacuum on the little suckers. try and do that before the weeks out...

like everything... "gunner"...

QCTP... 24 holders, and i STILL have to swap tools over! 250-111

got the smaller 250-000 NIB, too small for the main lathe but will probably eventually go on the baby lathe....