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Rocker geometry


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Hi

I'm setting up my rocker geometry on (what will hopefully be) a fast TR4 rally car (10.4 compression ratio, RTR rally cam, light flywheel, webers etc) . I am using standard rockers with a tuftrided shaft and spacers not springs; where do I set the Rocker shaft height? 

Some say at exactly 50% of valve lift, another I’ve read suggests 40 to 50% down from the valve tip (is this allowing for the almost 5mm of wipe area we have and the rocker arm effectively increasing in length as it increases valve lift? – so increasing the rocker ratio) , and others comment on only adjusting the pedestal height to get the correct swipe pattern on the valve tip (to avoid valve guide wear).

I calculate my valve movement is 11.46mm, so 5.73mm rocker height (down from the valve tip) might be ideal geometry?  (assuming a rocker ratio of 1.55 to 1, and cam lift is 0.291”)

Aiming for this, I have shaved/filed the base of the pedestals down so the rocker shaft is now 4.7mm down from the top of the valve (within the 40 to 50% -2nd theory!).  However, looking at the rocker now touching the valve with the valve in the closed positon - the rocker’s wiping part (register) is close to the rocker side of the valve stem - see attached photo. I think if I shave the extra 1mm from the pedestals for a 50/50 rocker height, my rockers will be right on the edge of the valve; so is an option to grind and so round the inside part of the new rocker register to move the contact point away from the valve edge? 

My plan is to get the rocker geometry set up as best I can, then get a set of pushrods made the correct length for this particular engine – perhaps we can discuss pushrods later too; but I need a bit more understanding of the correct geometry and best TR practice please

Any help or discussion would be appreciated


rocker jan19.jpg

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If a standard head has 100 thou taken off, then shims of 100 thou are needed under the rocker pedestals to maintain the geometry, provided Triumph got it right in the first place, or the push rods need to be shortened by the same amount.

            Cheers Richard

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That's a good link Ed - thanks -not seen that before.  The pic shows why midlift is ideal, and also why that will give me the minimum side movement of the pushrods - if I get the length right. The pic shows the wipe action going out and then back across the valve stem; with a 4.5 to 5mm wipe area visible on a the vavle stem and rocker register, am I right in thinking makes the rocker arm actually increaes in length by 4.5 to 5mm so it will wipe one way opening the valve and then wipe back as it closes -is that right?

Richard - I can understand the idea of putting shims in will maintain the same pushrod length, but the rockershaft - I think - will then be 2.54mm higher than standard, so will probably put the rocker geometry out further from my ideal.  The valves are still in roughly the same place, and nothing has been changed on top of the cylinder head, so rocker geometry would be close to how Triumph designed it - wouldn't it?  Which in those days I think the rule of thumb was rockershaft 1/3rd down and 2/3rds up (compared to the valve movement)

Thank you guys - my apologies for not signing off earlier!

Len

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Len--

The differences are small, but in theory, a mid-lift geometry maximizes valve lift.  On the other hand, it moves the valve stem contact patch off center.  Some consider this a negative.  It also increases the sliding (as opposed to rolling) of the contact surfaces.  I suppose with roller rockers, this would be a non issue.

Shimming rocker pedestals to compensate for a shaved head does restore geometry on the pushrod side with stock rods, but it changes the valve side geometry.  Moving to shorter pushrods (shorter by the same amount the head was shaved) maintains stock geometry on both sides.

Changing rocker geometry is not necessarily a bad thing, but its good to know the effects and side effects.

 

Ed

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Kastner's invaluable "Triumph Preparation Handbook" covers this in considerable detail, including the method of shortening of pushrods (with photos).

I had earlier copies of Kas's Handbook some 40 years ago, but bought my most up-to-date copy from the TR Register's website (book sales).

Ian Cornish

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Thanks Ian - Got Kastner's books too, he does concentrate on the pushrods more than the rocker geometry

Can anyone explain how do the geometry on the Rocker axis and the shoe radius stuff on Ed's link? 

My maths is good - but 'O' levels were a long time ago to work out that geometry!  The result might give an axis point a little above the 'mid-lift point' - which I would feel happier with, as I always felt 50/50 or mid-lift was too low a position for the start of the rocker register wiping action

Thanks Len

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there is another way of get,n  least wear an max lift,

an its some thing that  as far as im aware of, no many folk doo it,

 

but, it gives moer lift wid less wear,

think aboot it, see if ye can sus it oot, its quite simple really

 

M

Edited by GT6M
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For my TR2 I used a thicker (and fairly expensive) copper head gasket to compensate for a shaved head, with the benefit of not raising compression, and not having to change rods or use shims to get the rocker geometry within the adjustment range.  For the Doretti, I am using shims under the pedestals instead, but I haven't gotten to the point of firing it up to see how it works.  I am curious to follow up on Ed H's response and learn how the geometry will be affected on the pushrod side as opposed to the valve side.  There is a good company here in the States, British Frame and Engine, that does shorter pushrods in a variety of lengths.

Cheers

Dan

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26 minutes ago, GT6M said:

there is another way of get,n  least wear an max lift,

an its some thing that  as far as im aware of, no many folk doo it,

 

but, it gives moer lift wid less wear,

think aboot it, see if ye can sus it oot, its quite simple really

 

M

Firstly I am not really clued up on valve timing etc but the mechanical situation looks not too difficult.

bear with me -

The valve has a certain amount of depression (it is not lift) - lets say it is 0.400"

You need the rocker arm horizontal when the valve is depressed 50% ( 0.200")

This allows the arm to be equally displaced at fully open and fully closed - this maximises the mechanical advantage of the lever (rocker arm)

As the rocker arm tip approached the 12 or 6-o-clock position  any radial movement does not give the full vertical movement - as per finding TDC on the piston.

 

I may be completely wrong - usually am. But food for thought.

 

Roger 

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Roger--

What you've described is a "mid-lift" geometry.  Is that the way TR4s were delivered from the factory? 

TR6s did not use mid lift geometry (at least not in '74, and I think all TR6 used similar rocker gear).

Ed

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I am sure that the length of the pushrod has very little effect on the rocker geometry.  The rocker arm rotates around the rocker shaft, which is inserted/fixed to the pedestals, which are then bolted to the head, and so the rocker shaft is in a ‘fixed’ position on top of the head; and I think, at what height it is bolted - this sets the rocker geometry.  We can change the height of the rocker shaft by removing metal from the base of the pedestals, or raising the pedestal height by putting shims under the pedestal. 

Skimming the head does not effect the rocker geometry as nothing changes on the topside of the head where the rockers are bolted down, it’s just that after skimming the head, the top of the head is now closer to the cam shaft, and so the old pushrods are too long

Interestingly, I’ve measured the distance between the rocker shaft axis and the adjustment ball, and with the ball adjuster fully adjusted up or in (most threads showing above the rocker), this distance is the least (and so the best rocker ratio); and adjusted out the 7 full threads, this distance increases by 33 thou.  Not much but it does alter the rocker ratio from 1.55 down to 1.50 if the adjuster is fully out (just enough thread left for the nut). So presumably the rocker adjustment screws need to have as many threads as possible showing where the nut is to have the best rocker ratio, I think this is where you would also get the least side to side movement of the pushrods in their vertical motion

Still not decided what height to put my rocker shaft axis!

Len
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