In the latest 'project small bore' article there's quite a bit of discussion around the setting up of the valve/rocker gear. GR's concentration was on getting the duration the same across all like valves, rather than getting even lift.

Any thoughts, experiences, issues?

As far as I know, both are equaly important. I think what Graham is implying is that many people check the valve lift which is relatively easy to do but duration is not checked and this is what causes problems and loss of torque through the rev range, when duration is not the same for all 8 valves. Although I must add that I don't understand how duration can be out if lifts are the same. I guess so long as all the like valves are the same.

When Graham and I did my head he measured lift and duration on all 8 valves at a standard setting of 18 thou tappet clearance, the lift of the inlets were all within a thou of eachother at 397 thou. The duration measurement (from memory) was 270 duration at 18 thou for all inlets, we then closed this to 16 though and it was 272 duration. Close it further to 14 thou and the duration was 274. Since the RE13 is 276 duration, the closest possible to this would be ideal (that's at least I think what graham said! )

Either way, after reading Graham's article again, i realised that the toyota rockers are much more similar to eachother than S ones are as I am going to set all of my tappets the same clearance whilste to get his S ones to be the same he needed to set the tappet clearances to radically different setting depending on lift.

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Lillee - 1969 Morris Mini K

This btw is the limit of my understanding of valve lift and cam duration

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Lillee - 1969 Morris Mini K

Lillee your bit about durations being the same but lift different is also what I'm struggling with. If the ratios of the rockers are different (which I assume is the issue here) then I can see how each valve could have different lift unless the clearances are changed. However duration is more a product of the lobe shape (I thought?). If two valves are displaying different durations then this would indicate irregular cam profile??

This would be true if duration was being measured from first opening to last closing movement - but is it in fact measured from X lift to X lift? If THIS is the case then I can see both durations and lifts being different because the different ratios will throw both out.

A series rockers have a reputation for lift ratio variation, the forged Cooper S ones are the worst for this.
Any slack in the rocker bushes just makes it worse.
Chong's Toyota Corolla ones I made up only have .0005" (1/2 a thou) clearance on the shaft. Which is what they should have, according to Vizard and KC.

Most hi-lift roller tip or forged rocker sets eg MiniSport or MiniSpares that you buy have lots more shaft to bush clearance than this when new.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yes, i did say I struggled to understand how duration can be different it lift is the same. But definately, each rocker can be different from each other by heaps which will mean the duration can be different because the rocker ratio will be different. This can apparently be quite a big variance.

The lobes on the cam of course are the same, it will be the case of rocker ratio variance. Also tappet clearance is the only way to make up for it unless you modify toe bush as kev says, as the valve will get on the cam quicker with a smaller setting versus bigger settings. As in the article you can see that he had to adjust the clearances so that the durations matched closer and make up for the variance.

As I said, most important is that the durations and valve lifts are the same.

I actually asked graham is there a big difference to performance? i mean surely 2 thou is not going to be a big deal right??. To which he replied It most certainly does! 2 thou can mean the difference between up to 4-6ft/lbs all the way up the rev range. So there you go

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Lillee - 1969 Morris Mini K

Ya I've had 4 hours of driving since I last posted, so plenty of thinking time while wifie slept! I drew a bit of a graph of the lift in my head - plot the lobe height vertically against rotation on the horizontal basically as a sine wave more or less and it starts to make sense. The shift in clearance means a positive or negative shift in the whole curve on the X axis only. No shift in the curve shape - above the Y axis is valve movement - below the Y is movement of the rocker but not the valve, due to clearance. So changing clearance obviously effects apparent lift (at the valve) but also duration (which can be deemed to be the two points the line crosses the Y axis). Cool.

Makes sense to me anyway

yep thats it

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the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates

Glad it's all clear now

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Lillee - 1969 Morris Mini K