940 head - exhaust valve measurements

[GeorgeHurst]

In a similar vain to the post by Declan_Burns I'd like to just check my measurements re sinking exhaust valves.

My calculations are based on those by Declan...

Valve to firing head face measurements:
Valve 1: 7.5mm
Valve 2: 7.9mm
Valve 3: 7.9mm
Valve 4: 7.4mm

Cam lift: 6.35mm
Rocker ratio: 1.27:1

So, 6.35x1.27= 8.0645mm

Less, 0.381mm (15 thou) exhaust valve gaps (8.0645-0.381) = 7.6835mm (travel of valve from closed to open)

Head gasket (compressed) = 0.8mm

So, clearance from valve face to block face (valve to face measurement + head gasket thickness (0.8mm), minus valve travel (7.6835mm) is:
Valve 1: 0.6165mm
Valve 2: 1.0165mm
Valve 3: 1.0165mm
Valve 4: 0.5165mm

There is then the debate about leaving 0.5mm for valve bounce, which would leave valves 1 and 4 close to hitting the block.

With these calculations in mind, what are peoples opinions about sinking the exhaust valves, necessary or not?

Thanks very much,
George

[linearaudio]

It's worth checking the actual valve lift, as apparently it is rarely as high as quoted! Mine turned out to be 7.8mm actual lift. Don't think you need worry too much about valve bounce with the standard cam etc! Ultimately, all the paper calculations don't seem to add up to the whole picture, hence I bolted everything down, (not torqued) and physically checked mine as my sums had given a theoretical 0.8mm on one valve, but when it was all in place there was plenty of clearance, checked at full lift with a prybar.

[bmcecosse]

It looks tight George on #1 and #4 - but in theorey - it should be (just) ok! Just don't bounce the valves.

[GeorgeHurst]

Thanks both for the replies.

Good to know I don't need worry about valve bounce. Can I just clarify exactly what you're saying LA...

How can one tell if there is plenty of clearance when the head is bolted down? The valves are not visible no?

And how does one use a pry bar to lift the valves? I guess between pushrod and rocker, but levered against what?

Perhaps the blutack approach is necessary here as it is a bit tight.

[bmcecosse]

You can still see the tips of the valve stems - I'm guessing LA just levers against the rocker shaft - and pushes each valve down slightly to prove it's not hard against the block. A fair enough test! But your measurements should be fine - trust them!
PS - Even though it's bluudy cold up here -I'm going to investigate the garage today George - mainly to check the Minor isn't frozen solid - but also will look out parts for you!

[GeorgeHurst]

Greats stuff, thanks

[cormorant]

On the subject of the 940 head what does the comp ratio go up to (approx) if you fit it to a 1098?

[bmcecosse]

Approx 10:1 ideal!

[GeorgeHurst]

Just had the cheapest quote back for sinking 4 valves, from Slark in Amesbury, and it is £65+vat for all four (this includes fabbing 4x 40 thou thick packing washers for the springs). This is definitely out of my budget unfortunately as I have other bits I really need to spend money on, so I am going to have to hope that the head is fine as is!

[bmcecosse]

Dear oh dear - that's a LOT! If your measurements are right - it will be fine. Ordinary washers are ok as 'packers' - they were just making an extra buck from you!

[GeorgeHurst]

Yes I thought so too! I'm fairly confident my measurements are accurate, this coupled with LA saying that the valve lift is rarely as high as it appears on paper and your assurances BM make me think its probably fine as is.

[bmcecosse]

I have the parts indoors George - PM me your email addy and i will send pics later - can't attach pics to PMs.

[GeorgeHurst]

Awesome, PM on its way...

[linearaudio]

Yes I thought so too! I'm fairly confident my measurements are accurate, this coupled with LA saying that the valve lift is rarely as high as it appears on paper and your assurances BM make me think its probably fine as is.

Yes, George. Use a large screwdriver as a prybar at a scew across the pushrod side of the rocker arm, it takes very little effort to further lift the valve. My "plenty" was visual could also be measured with a DTI on the top of the valve cap, or an engineers rule against the side of the valve. You can guesstimate 0.5mm by looking at a typical sparkplug gap. I find that easier than looking at the thickness of a feeler gauge.

[GeorgeHurst]

Brill, thanks for clarifying that LA

[Declan_Burns]

George,
You might be lucky with your head. Perhaps try the Blu-tack trick. You might also consider fabricating a different heater take-off as the 940 head take-off is at a slightly different angle. This is what I made up-test fitting before soldering and painting.[frame][/frame]

[bmcecosse]

Interesting - you have the so called 11 stud (actually 10 stud + 1 bolt) head there Declan!

[GeorgeHurst]

Thanks for that picture Declan, if I can avoid it I want to avoid fabricating new parts as I don't trust my welding/soldering skills for engine applications, but that pic will be really useful should I fin the different angle troublesome.

[Roni]

Using bits from "the box of things that might come in useful one day" and a trip to the local plumbing supply shop meant being able to make this adaptor to correct the angle of the valve. No soldering or welding which suited me fine.[frame][/frame]

[GeorgeHurst]

Very nice Roni. Is the different angle of the heater tap an issue then?