Valve Clearance & Camshaft

[Mad Mart]

The engine I'm rebuilding was running Weber 48s and a modified cam. I haven't identified the cam yet but it looks aggressive compared to an STR91 (I meant to measure the cam lobes but forgot before I packed up for the evening). I did, however, measure the valve to head dimension when the inlet valves were at their fully open position, which was 8.5mm. I just wondered if somebody can measure a standard Sprint head in the same way for a comparison. The reason being is that there is evidence of the valves hitting the tops of the pistons at some point in time. The head is also fitted with double valve springs.

The cam is marked as 'Stanpart' and also 'STR 0516'. This is cast on the body of the cam not on the end. There is also an extra 'lump' between cylinder 4 lobes. I've not seen this before, anyone?

[xvivalve]

It is purported elsewhere that STR0516 refers to the cam blank, not any specific profile...

Where are you measuring to on the valve?

[Mad Mart]

I'm measuring to the lowest point on the edge of the valve head, the bit that would hit the piston.

I also measured an exhaust valve which was 5.7mm.

[xvivalve]

Inlet = 5.5 mm

Exhaust = 3.0 mm

Measured using a steel rule at room temperature

[Mad Mart]

Thanks Alun. Seems to be about an extra 3mm. lift.

[Mad Mart]

I measured the cam lobes this morning and there is a cam lift of 0.433" (measured with a vernier). This looks like the cam is an STR139 (or similar) which is what I suspected with the double valve springs. and the nicks on the piston crowns. Stripping the head there are four helicoils needed. The bottom valve spring retainers (148885) are different than standard, possibly custom made. None of the standard valve stem oil seals are fitted on the exhausts but some did have 'O' rings, I say some, I think about half of them, although I can't see what use they would be. I tried a standard seal but the inner spring wouldn't fit over it.

[tony g]

Mart if you measure the cam lift by total lobe height and take away the width of the base circle then its not the way to do it accurately. When you measure the narrow bit its actually on the start of the ramps, so the actual full lift will be more than you suspect. if you want me to explain a bit more just say as its hard to explain in type in one sentence.
Cheers

Tony

[Magenta Auto Sprint]

these folk suggest that it is lobe height minus base circle but also total valve lift is dependant on rocker ratio.

http://www.lunatipower.com/Tech/Cams/CamSpecTerms.aspx

as we set tappet clearance on the base circle I would have thought that this was the datum with which to measure valve lift.

Not saying you're wrong but this seesm to be common practice.

malcolm

[tony g]

Yes lift is lobe height minus base circle, trouble is with larger cams, measuring the cam at 90 degrees from the lobe height is NoT the base circle. The base circle is the difference between the cam journal (of the cam) and the cam where the feeler blade goes. So if theoretically the cam journal is 20mm and the "height" of the base of the cam is 4mm taller (as viewed looking at cam base- lobe downwards), this is the RADIUS difference so x2 gives the diameter difference in this case 8mm so 20mm + 8mm is 28mm. Thats the base circle. Deduct this number from total lobe height to give cam lift. Minus the valve clearance to give valve lift. On the inlets with cam over bucket theres no rocker ratio to worry about, on the exhausts its a factor of course.. In this example of sizes you would find in most cases the "width" of the base circle as measured with a vernier would be wider than the actual base circle, so theoretical lift will be less. Said its hard to explain didnt I lol. Need Jeroen here with one of his sketches

Tony

[tony g]

Of course, with a built head its easy to open the valve and measure how far it opened then thats it

Tony

[Mad Mart]

I've been mulling this over but I can't work out what you mean Tony. I'm not saying you're right or wrong but if you are setting the valve clearance from the heel of the cam then surely the cam lift must be the overall height of the lobe minus the base circle?

I know I was only after a rough dimension for comparison but I'm interested to know how to measure more accurately, if possible.

[tony g]

Yes Mart its sort of right as you see it. If you draw a circle lets say 20mm diameter. Now draw a lobe on there. The "flanks" or ramps of the lobe will actually cover part of your circle beyond the diameter line of that circle-Yes?. In this example your base circle is the 20mm you drew before adding a lobe. When you measure the "width" of the lobe where you expect it to be the base circle diameter, its actually fatter there than the 20mm diameter.
Now if you draw 2 circles, the 20mm one as before and the cam journal in the head lets say 18mm. They have the same centre but the cam circle is 1mm below it (and 1mm above it to make up the 2mm difference).
To measure the true base circle diameter we need to put a straight edge across the "heel" of the cam lobes and measure the distance down to the cam bearing surface, in this case it will be 1mm (its the radius difference of the two circles). We know the cam bearing is 18mm diameter (radius of 9mm) and we have 1mm MORE radius on the heel of the cam. So the diameter of the base circle is 20mm (radius of 10mm). 99% of cams will measure wider than the actual base circle.
Hope thats clearer I'll try and find some online pics it'll be easier to see I think rather than me waffling on lol

Tony

[tony g]

In reality the cam journal size is the larger of the 2 .
I just measured a stock Sprint cam and the base circle IS the width as measured with a vernier so my explanation is not needed. On aggressive profiles we would see its wider at the width.
Mart can you measure the distance from the cam journal surface down to the heel of your cam?

Tony