23 replies to this topic

[PandO]

I've fitted a ported and large exhaust valved 12g940 to my (now) 1293) 1979 1275GT, I've had to enlarge the chambers as the CR was way too high. With an MG Metro cam and twin 1 1/4 su's the performance is much better than when it was factory standard. The head was skimmed by approx 80 thou and I fitted it and the rockers without shimming under the rocker shaft pedestals as I failed to realise they were needed. I've adjusted the tappets but the rockers seem higher at the push rod end than they should be. I'm thinking of fitting shims to get the geometry correct and as it's an 11 stud head I'm hoping to get away with doing the job without disturbing the head gasket. My question is - will I see a performance gain as the valve lifts will be slightly more or is the difference likely to be negligible? Any comments will be gratefully received. Cheers guys.

[carbon]

If you have refitted same rockers as before then should not need shims under the rocker pillars. The geometry of the rockers in relation to the valves is not affected by skimming the head, and fitting shims is likely to result in rocker tip pushing down on valve at too much of an angle at full lift.

 

What is different is height of the pushrod above head, this is now 80 thou further up than before and adjusters will be screwed out accordingly.

 

Are you using a modified or standard cam?

[PandO]

I'm using an MG Metro cam and 1.3 roller rockers. I'm of the opinion that as the arc of the rocking motion is not centred around the mid point the valve lift will be reduced. It will only be marginal bit it is going to be a good few thou - how many thou can I afford to lose without a reduction in noticeable power.

[carbon]

I would check how much valve lift you're getting. If it's 8.0mm or more then should work well.

 

There are likely to be other areas where attention to detail will give more useful torque gains, such as carbs, inlet manifold, exhaust, and distributor advance curve.

[Dusky]

Measure it and correct it if needed. You'll end up with correcting it anyway after a few weeks.

[PandO]

I will be fitting the shims soon as the uncertainty is playing on my mind. Carbs are working well - plugs are just the right colour and were set up by an experienced mechanic friend - as was the 123 dizzy with a programmable curve. I just don't feel I'm getting the best out of the cam - I had an MG Metro some years back and this had a noticeable increase in power as the rev's started to rise around 2,500 - 3,000 rpm. The cam is in good condition - I'm losing power somewhere. On the whole the car pulls well - instant throttle response means the car is a pleasure to drive - a couple of extra horses would be very welcome - especially if the twin box rc40 would emit that extra sound of urgency. I'll find out in the next couple of weeks but still wondering about the outcome.

[Ethel]

Skimming the head will make a difference as you're effectively lowering the rocker shaft relative to the push rods. But by the same means, especially if it's a regrind, the new cam could have done the opposite by about a 1/3rd or more.

 

The maximum lift won't matter as much as opening and closing rates. The geometry will give the best acceleration the closer the rod is to vertical and the closer to rockers are to perpendicular. Bear in mind the cam side of the rocker is effectively from the ball of the adjuster to the axis of the rocker shaft, so the further the adjuster is screwed out the less the rocker ratio will really be.

[Earwax]

Going along with the theme, that the rate of early lift is part of the new cam's advantage, if your valve seat area and guide have not been tidied up then much of the low lift advantage is lost.  It could well be that your earlier 12g940 head combo had a better port mod and was extracting more of the cams capacity. I can't comment on what the science/ art of success at head upgrading is, except to say that when you get a goodun, you really notice what was missing. Unfortunately not able to be explored without head off.- just food for thought - you may well choose to live with the subtle improvement as it stands.

[PandO]

Thanks for the comments guys. My cam is not a regrind - it's a genuine MG Cam that had no sign of wear and the head has had a lot of work done to the ports + all the seats were recut when unleaded exhaust seats were fitted. I've cleaned up the inlet manifold and also vizzarded the carb butterflies and the rc40 is fitted to a lcb manifold - all has been done to make the best of the set up except with this motor I did not go to the expense of fitting rim flow valves. A set of shims are of low cost so I must fit them. I don't want to chance fitting them at some time but with a planned trip to Cornwall in July and club runs through the summer I'll probably wait - I wish I'd thought of this when the rebuild took place.

[carbon]

PandO, from your description of motor not really coming to life at 2,500 to 3,000 rpm that does sound like not enough lift at the valves.

 

I run the Kent MD266 in a 1293 with similar spec motor (MG Metro cam and MD266 are very similar). Started with a set of A+ sintered rockers and the performance definitely lacked at mid-high rpm. Changing to different rockers with about 1.25 ratio gave 8.0mm measured lift at the valves and completely transformed the engine at mid-high rpm, much more responsive. It also needed a lot of adjustment to the distributor advance curve to get it running well at low rpm.

 

You mention the CR was too high to start with, what is it now?

[tiger99]

Be aware that rocker shims CANNOT correct the geometry after head skimming. In fact they make it wrong, guaranteed, because they move the pivot axis and effectively tilt the rocker. If the engine is running badly it will not be improved by shimming, and as has already been suggested, severe valve wear may result, and the intended camshaft profile is not what the valve will see. You can never, ever make things better that way.

Shimming is a desperate measure only needed if you run out of valve clearance adjustment at the adjusters. The correct fix is shortened push rods, which retain the correct geometry. IF YOU HAVE BEEN ABLE TO SET THE VALVE CLEARANCES CORRECTLY YOU DO NOT NEED SHIMS. Simple as that.

[Carlos W]

Be aware that rocker shims CANNOT correct the geometry after head skimming. In fact they make it wrong, guaranteed, because they move the pivot axis and effectively tilt the rocker. If the engine is running badly it will not be improved by shimming, and as has already been suggested, severe valve wear may result, and the intended camshaft profile is not what the valve will see. You can never, ever make things better that way.

Shimming is a desperate measure only needed if you run out of valve clearance adjustment at the adjusters. The correct fix is shortened push rods, which retain the correct geometry. IF YOU HAVE BEEN ABLE TO SET THE VALVE CLEARANCES CORRECTLY YOU DO NOT NEED SHIMS. Simple as that.

So, if there's no more adjustment on the tappet screws you need shims, if there is you don't?

 

Answers a question

 

thanks

[PandO]

Carbon,I'm not sure what the CR is exactly but I enlarged the chambers and measured with a burette to get a CR of approx 10.5:1. I've always run on BP Ultima or SHELL 100 octane fuel and played with my 123 distributor settings to give best results. Tiger99, My reason for thinking shims are needed is because: with the tappets adjusted to 14-15 thou the rockers are extremely tilted - if I add shims this will be corrected and allow the rockers to move in an arc centred about the horizontal position. Doing this should marginally increase the lift that the rockers will give. My question originally was: will the marginal increase be noticeable enough to warrant their addition. Also the expensive 1.3:1 roller rockers deserve to be installed correctly so I will at some point do the job properly. Cheers.

[Ethel]

You've lost me there a little Tiger,

 

If the shims are just restoring thickness taken from the head then the only other factor I can think of is that they'll  lower the valves relative to the rockers. I don't see that's any worse as you'd still end up taking in the slack with the tappet screws.*

Without knowing what work was done to the valve seats we can't really say if shims will exacerbate or ameliorate. (  must've slept on my dictionary last night)

 

*Thinking about that, no shims could be better as the adjustment would shorten the effective rocker length on the cam side so increasing the rocker ratio and valve lift.

 

Short of plotting vectors, I doubt there's enough difference to give the OP the improvement they're seeking. I'd be looking at ignition timing & fueling.

[carbon]

Carbon,I'm not sure what the CR is exactly but I enlarged the chambers and measured with a burette to get a CR of approx 10.5:1. I've always run on BP Ultima or SHELL 100 octane fuel and played with my 123 distributor settings to give best results. Tiger99, My reason for thinking shims are needed is because: with the tappets adjusted to 14-15 thou the rockers are extremely tilted - if I add shims this will be corrected and allow the rockers to move in an arc centred about the horizontal position. Doing this should marginally increase the lift that the rockers will give. My question originally was: will the marginal increase be noticeable enough to warrant their addition. Also the expensive 1.3:1 roller rockers deserve to be installed correctly so I will at some point do the job properly. Cheers.

PandO, when you say the rockers are extremely tilted, is this just at full lift?