Rocker Gap

[TIMS]

Guys

 

A little while ago, I think it was Neil, posted a thread on rocker gaps for a non standard cam.

 

There was a reply from one of my Leicestershire colleagues, Terry Pruce, who advised that the gap he is using, and was recommended to use, was, I recall, 0.028".

 

Others responded that the standard 0.010" seemed fine in all cases and that this figure of Terry's seemed very large.

 

My 6 is running a Kent Cams TH5 and I have been mulling over whether 0.010" is correct. Following a Leicestershire group meeting on Thursday last where I discussed this with a couple of people, thanks Reg & Andy M, I looked the matter up on the Kent Cams website.

 

http://www.kentcams.com/Downloads/Files/Ke...sBrochure07.pdf

 

At page 26 I found the Triumph cars, and the TH5 for the TR6.

 

Valve clearances - 0.022" inlet and 0.024" exhaust. Quite a surprise.

 

One query for the forum.

 

The gaps given are for "a theoretical Rocker Ratio of 1.52:1" But I'm sure I have read that the standard TR6 rocker ratio is 1.46:1.

 

1) Am I correct on the standard?

2) And if I am, does it make any difference to the gaps recommended by Kent?

 

Thanks in advance.

 

Regards

 

Tim

[jean]

Tim, did you check under the valve cover, when changing the cam to a non original, a good engine shop should stamp the required clearance in the head

[TIMS]

Jean

 

Nope, nothing there, and they had been set at 0.010", and that was done at my revisit to the builder after 500 miles for him to re-torque the head and reset tappets ect.

 

Now set at 0.022" and 0.024" and it started better and seems to run well. A little bit of top end noise when cold but I'm not unhappy with that.

 

One oddity I noticed. I have a CR car with a CR engine, original as per the Heritage Certificate, Car is CR 2823 O and engine block is CR2800HE. The oddity, the head is casting number 308351 and machined number 516816, My reference books tell me that is an early TR6 head? is that correct? and is it better or worse for me. NB I have a correct tabbed headgasket and later, 125, throttle bodies.

 

Regards and thanks for looking.

 

Tim

[jean]

Tim, sorry don't know the make numbers by head, just know of 2 different TR6 cyl.heads main difference is the spacing of inlet/outlet and internal flow caracteristics.

It is said that the US spc. heads had a far more efficient gas flow

Edited March 23, 2008 by jean

[TR674]

Tim

I have a non standard cam. My tappets run nicely at 0.018". Any bigger gap and I get a tapping noise. 0.010" is too tight and I get a bit of blow-by on the valves.

Regards

Craig

[Bald Rick]

Tim

 

There is a table of 6 cyl head numbers on the "Technicalities" CD. The infor on there is helpful. You can then trace valve sizes etc from various catalogues and parts lists. I don't think you need to worry about ports. I have an early "Narrow Port" head and it's number is 516323.

 

As far as I know, the main difference on USA heads is the thickness. But they can be machined down to give the required compression ratio.

 

Hope this helps.

 

Tony

[TRTerry]

Hi Tim,

 

My clearances are stated as .022/.024, not .028 as you recalled. I once accidentally had them set to .010 and the engine sounded most distressed, It chuffed and thumped on tickover as if it was very tight. I immediately returned them to the prior settings. So I am surprised that you are running better from changing the .010 to the wider gaps. From your cam I suspect somewhere in between the two extremes might be best. I don't get any tappet noise from mine unless I have my foot down (so that's most times then). As I understand it, it is better to hear them than not to.

[TR 2100]

I have been following this thread, hoping someone with a technical background would respond and confirm/refute

my understanding of camshaft design and rocker gaps (which really comes back to tappet clearances).

But – nothing – so here goes.

 

There are three basic design features of camshaft. Start out with the base diameter, add on a lobe to lift the cam

follower/tappet and whop the valve open and a slight build up between the round part and the lobe (and that’s

where clearances come in).

 

The point is that you need clearance between the minimum diameter of the cam and the tappet to allow the valve

to close tightly, also allowing for heat expansion. If the cam were to hit the cam follower at speed, the impact would

make too much noise, so the idea is that you take up the clearance between the tappet and the base diameter slowly

(well, relatively) and then whop the valve open.

 

If the clearance is too great, the build-up before the lobe doesn’t close the gap, the lobe hits the tappet at too high a

speed and you get tappet noise.

If the clearance is too small, the valve may not close properly and you risk burning out the valve.

 

When a camshaft is re-profiled, the base diameter is reduced (to allow the lobe to be greater, giving more valve lift

(and a different cam dwell/overlap etc, but let’s not get into that here) and the re-profiler can adjust the gradual

build-up between base diameter to whatever he wants.

 

Therefore you should be able to tighten the rockers safely (when hot) until tappet/rocker noise is just eliminated,

but no tighter.

 

AlanR

Edited March 28, 2008 by TR 2100

[TIMS]

Alan

 

A good summary I feel, from what I have found in research over the past few days, thanks. Only one comment, from all that you have written, your last paragraph needs one more phrase? "when hot"?

 

Terry

 

Memory not as good as.... oh I forget when. Yup it feels happier with the Kent Cams recommended gaps. Needs a good run now to get really warm and see what if any noise we get then.

 

Tony

 

Didn't think of looking there! Thanks. It seems to me that the head I have is the best Pi head going and mounted on the later, stronger, block, things are looking good. Famous last words?

 

 

The next point is, what effect will changing the gaps have on timing? Back to advancing and slowly retarding to get rid of the pinking methinks, the good old way.

 

Kind regards to all

 

Tim

[88V8]

The next point is, what effect will changing the gaps have on timing? Back to advancing and slowly retarding to get rid of the pinking methinks, the good old way.

 

Nil effect, afaik.

 

Plug gap has an effect, valve gap not.

 

Ivor

[Red 6]

For any given cam the effect of decreasing the clearance means that the lobe will open the valve earlier thus retarding the cam timing. (decreases torque and mid range performance.)

 

Increasing the clearance means the lobe will open the valve later thus advancing the cam timing (decreased bhp but increases torque and mid range)

 

This is why it is important to follow the manufacturers specification.

 

For example whoever built the engine would probably have timed the cam as per the manufacturers instructions therefore the clearance as specified will be correct.

 

Some one else when fitting the same cam might have advanced the cam timing i.e the inlet lobe might act upon the valve so that it opened later (closer to TDC) to give more torque and mid range. If that was the case decreasing the clearance (from say 24thou to 10 thou) would move the opening point away from tdc (reducing the advantage of advancing in the first place) and increasing the clearance would move the opening point closer to tdc probably undoing any power gain.

 

So as you see its not just about which clearance gives least noise, or what people think is best just, in this case its probably best just to do what the makers advise.

 

Hope this helps in some way

[marvmul]

"The point is that you need clearance between the minimum diameter of the cam and the tappet to allow the valve

to close tightly, also allowing for heat expansion."

 

I wonder if the gap has to be that big to achieve tight closing and allowing for heat expansion : the block is expanding also, maybe more and earlier then the push rods?

Perhaps the rather big gaps are necessary to allow for manufacturing tolerances and distortion of the camshaft at speed : these too factors can make that the center of the lobe base circle and the centre of the camshaft axis are not congruent at all times. Maybe, if the valve clearances would be much smaller, this clearance could disappear or even worse : maybe the valves could open a little when part of the base circle is in contact with the cam followers but the centre of the base circle is located higher in the block by bending of the camshaft or by manufacturing tolerances. I found more than once that the gap is wider just before the valve has to start to open than at the position of the cam were the gaps are to be set.