23 replies to this topic

[PandO]

Hi Carbon, the tilt is when the valves are closed - when open (full lift) the tilt is more extreme. The head I've used was 2'nd hand bought off eBay - it was ported and skimmed previously - I now assume it was prepared for use on a non 1275 engine and the skimming was done to increase the CR for that application. I measured the head thickness and 80 thou (almost a tenth of an inch) had been removed. I had to grind the chambers to the profile to the maximum show in Vizzards guide - almost to the line of the head gasket to get the CR to an acceptable value. I estimate that I'm loosing several thou of valve lift - this will also have some effect on duration. With my engine spec I would expect to have approx 70bhp (the MG Metro is quoted at 72 bhp). The only sure way to find out would be a before and after shimming rolling road setup - planned at some time after doing the shims - I'm hoping to get a noticeable improvement before I book her in - paying for a test before then will take funds that could be better spent so I was just wondering if anyone had first hand experience or was able to say with some certainty that the benefit would be measurable. I've drawn an example using a clock face as a base - a tangent between the hour hand at 2:45 and 3:15 is the same length as at 3:45 and 4:15 - but if 4 parallel iines were drawn across the face one pair would be closer than the other - this demonstrates to me that the lift will be different. Has anyone added shims in a similar situation and proved my theory. Maybe AC Dodds would have an answer.

[Ethel]

You could mock it up  on the actual head,

 

Release an end rocker, measure the lift on the push-rod and see what valve compression it would give at the various angles with rocker slid clear of the valve. You'd probably have to rig up an extension for the shaft, or slacken of the lot and slide it clear, but that'd be necessary anyway, if you want to try fitting shims without unseating the gasket.

 

A simple bit of trig will also give you an idea. 80thou divided by adjuster ball to rocker axis distance is the tangent of extra tilt the skim is giving. At a guess it's only 3 or 4 degrees.

[absx2]

Crikey fellas, you have me worried enough to pile in with a small hijack.

I have a 12g295 with 0.0935" 2.38mm milled off the face. I have 1275 valves that are 3.26mm longer so I intended to fit 3.26mm rocker and spring shims to nearly correct the geometry. Now i`m thinking 2.38mm post shims to get the pushrod closer to the rocker arm      

[PandO]

Hi Noobies - I'm worried that I need to put shims under the rocker shaft pedestals to correct the height of the rockers in relation to the standard valve lengths - your longer valves are over compensating for the amount of skimming of your head and you have approximately half of the misalignment I have - but this will cause your rockers to be tilted in the opposite direction to mine. You would need to have a small amount milled from the bottom of your pedestals as I see it - adding shims will add to your misalignment. Oh what fun these A series engines can be!!

[tiger99]

If you have longer valves you need shims, and may then need longer adjusters or pusrods too.

But in every case the geometry is defined by the valve seat position and valve length, neither of which change just by skimming the head, even by a large amount. That only affects the adjuster and push rod length.

If you have deepened the combustion chambers and had the seats recur so the valves remain flush, then, and only then, have you created the situation where a shim is necessary, although a shorter valve would be preferable if spring free travel was ok. Nothing else, apart from combustion chamber deepening, or a seemingly pointless head top side skim, can affect the valve to rocker geometry.

Special high ratio rockers which move the pivot point should not need shims as the new rocker shaft pillars should be the correct height. But perhaps not...

[ACDodd]

I have a jig for setting up the geometry for each engine I build (those that want to pay my time to do it). The pushrod and rocker shim thing is a valuable excercise for those that have the time to do it properly. Best results are obtained almost always with the rockers too high and the pushrod longer Than standard. Of course different rockers require different setups. I have used setups with as much as 6mm of shimming under the posts. These are derived by testing rather than calculation. As much as 0.020" of lift can be on offer on certain combinations. I can't offer any hard and fast rules here, other than to say that road engines benefit just as much as race variants. There is more to it than is written in any book. Experimenting is your best guide here. I built a jig to teach myself. Do the same if you want to get the best from your engines.

Ac

Edited by ACDodd, 06 June 2016 - 05:50 PM.

[PandO]

Thankyou AC for your words of wisdom.I will shim by 80 thou to compensate for the previous machining of my head. If the performance is the same as now I will not have lost by trying, if it is worst I can remove the shims - but if I gain anything it will be a bonus in two ways - better go and peace of mind. On many A series engines I've had over the years the rockers have never tilted to the degree I now have - so I'm sure it will be worthwhile exercise. Cheers.

[PandO]

Hi - sorry for the delay in replying. Tilt is evident at no lift. I'm aware that they tilt with lift and will tilt more so with a higher lift cam. I'm running an MG Metro cam which has the same lift as standard GT cams.

[carbon]

In your original post you mention that you adjusted the tappets but the rockers seem higher at the push rod end than they should be.

 

Putting shims under the rocker pedestals would make them even higher at the push rod end.