Replacement Cylinder head

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Peter Cresswell
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Re: Replacement Cylinder head

Post by Peter Cresswell » Sun Jul 26, 2020 2:13 pm

Ian,
It is always worth checking the rocker angle if material has been skimmed of the cylinder head face and if necessary the angle should be corrected. This only helps the valve train to work correctly but is likely to minimise wear as well.
Like many people I hadn't thought of this too much but the XPEG racing engine I bought for my TC had been professionally built and that had 1/8in spacers under the rocker shaft pillars. From then on I have checked this on all the engines I've built over years. As I mentioned above, on my B engine the day was saved by using Mini Cooper S cam followers, but they may vary, so don't assume this will always work.

Something else to think about which is relevant here and I haven't seen talked about on this forum, is valve spring binding. This can happen with high lift cams, especially where high lift rockers are used. When this happens the valve spring coils touch each other and the valve spring becomes a solid tube, which rapidly wears the camshaft or worse the push rods are bent and lift is lost. If double (or triple) valve springs are used each spring needs to checked that it doesn't bind. I am using Piper single valve springs which have a higher 'poundage' than their double springs and they were checked (and found to be just ok) by the people who cut the 3 angle seats as they assembled the head for me.

You are right that there is no mention of rocker angles in the Huffaker manual, but this doesn't really surprise me! There is a lot of mystery surrounding racing engines and they wouldn't want to pass on easily something that might give them a reliability edge. I joined in with this when I was racing the TC and all the front running cars had their 'magic' which some people never knew or found out about!!
Pete
1969 MGB Roadster
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Ian Fozzard
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Re: Replacement Cylinder head

Post by Ian Fozzard » Sun Jul 26, 2020 2:31 pm

Thanks Peter. I will check my engines at some point. I guess I may be drawn into adjustment of pushrod length as well - another thing I don't regularly do.
I do always check valve springs for binding, but never had a problem there. My guess is that my engines are generally in a mild state of tune and can get away without doing most of this esoteric stuff!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Re: Replacement Cylinder head

Post by Ian Fozzard » Sun Jul 26, 2020 2:40 pm

Just an afterthought which probably explains my ignoring rocker arm angles!

When a head is skimmed it does not alter the relationship between the end of the valve stem and the tip of the rocker arm - so this angle will be unaffected.

What is changed is that in effect, the pushrods are lengthened? I have always been able to correct this slight "lengthening" by adjustment of the adjusting *. Taking care of course that the ball of the rocker arm always has freedom of movement through the whole arc.

So, difficult to see how valve opening is affected here?

Is there a flaw in this argument?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Re: Replacement Cylinder head

Post by Peter Cresswell » Sun Jul 26, 2020 9:20 pm

Hi Ian
Your thinking is spot on!

The question though is what to do when there is no more adjustment available using the tappet adjustment sccrews? Peter Burgess suggests machining the pad on the bottom of the push rods so they are shorter by the amount to bring the rocker arm back into the realms of the adjustment sccrews. I don't really like this idea and I would need evidence that the push rods will not wear quickly. You can use adjustable push rods which are really a 3 part kit - the ball on the bottom, a tube and the cup that adjusters sit in. The tube is cut to the length required. Then finally you can shim the rocker shaft pedestals to regain the adjustment.

If there is still some adjustment thread left after setting the tappet clearance correctly then fine, but if not you have to do something to regain the adjustment.
Cheers
Pete
Pete
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Re: Replacement Cylinder head

Post by Ian Fozzard » Mon Jul 27, 2020 10:15 am

Peter,
I must have been cogitating overnight and have some further thoughts, if you have the time and inclination for further discussion!

1. I agree that the rocker angles need to be optimal, and that this may entail checking to ensure optimal valve operation.

2. If we can assume that a cylinder head left Abingdon with correct rocker angles (perhaps a bit optimistic?) then any skimming of the head will not per se change that angle. But adding shims under the rocker pedestals will change the angle and possibly move the angle away from optimal? Changes due to skimming the head will probably be dealt with adequately by the rocker adjusting scrww, or in extereme cases by shortening the pushrods. But definitely NOT adding shims under the rocker pedestals?

3. What will change the rocker angle from the original (optimal?) setting will be any changes to rocker, valve (especially valve stem length) or valve seat area (for example valve seat recession due to wear or valve seat inserts to allow use of unleaded fuel). Changes here cannot be accomodated by adjusting the pushrod length or adjusting the rocker adjusting scrww, but will require fitting of shims under the pedestals.

So, for me, the takeaway message here is do not assume that just because you have skimmed the head that you will need shims under the rocker pedestals. Always worthwhile to check the rocker angle, but any action required will depend on what is causing the issue that you identify

All sound reasonable?

Ian F.
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Re: Replacement Cylinder head

Post by Peter Cresswell » Mon Jul 27, 2020 2:15 pm

Hi Ian
You are right that in thinking that skimming the head doesn’t change the relationship between the Rocker Shaft, the Rocker Arm and the top of the valve. Also the Rocker Shaft Pedestals don’t change. However I think you are missing an important point! See the attached schematic diagram of the MGB Valve Train, hopefully the explanation underneath will make it clear.
Valve train schematic.jpg
All of the parts inside the red box are part of the cylinder head. All of the parts within the blue box are within the cylinder block. The connection between the two is the Push Rod which actuates the Rocker Arm to open the valve, but includes the Sccrew adjustment on the Rocker Arm and the cam follower. The distance A is the height of the Rocker Shaft above the face of the Cylinder Head, and distance B is the Height between the top surface of the Block and the centreline of the Camshaft.

When the Cylinder Head is skimmed distance A is reduced by that amount and similarly when the Top of the Block is skimmed Distance B is reduced. This means the length of the Pushrod, plus the Cam Follower, plus the adjustment sccrew has to be reduced. Up to a certain amount, this can be catered for by the adjustment sccrew on the Rocker Arm, but as more metal is skimmed off the Cylinder Head Face and the top of the block then all the adjustment on the sccrew in the Rocker Arm can be used up. Then none of the other components that operate valve are adjustable so the problem arises.

Adjustable push rods are available but they are quite expensive. Removing metal from the ball on the bottom of the Push Rod is possible but as I said before I am not convinced this can be done within the limits of the surface hardening. So we are left with the cheapest and most practical solution, of putting shims under the rocker shaft pedestals to raise the height of the centreline of the Rocker Shaft and so restore height A and B back to where it was.
Cheers
Pete
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Re: Replacement Cylinder head

Post by Ian Fozzard » Mon Jul 27, 2020 2:35 pm

Thanks for this reply Pete, and your patience!

I am still not convinced (or missing the point?).

However much you reduce A or B, the rocker arm and the valve tip do not "notice" and their angle does not change?

By putting shims under the pedestals you are definitely increasing that critical angle. It seems to me that you should only be inserting shims if you have reduced the angle by altering the valve, valve seat, or rocker arm.

As you have illustrated, the sole connection between the cylinder head and the cylinder block is the pushrod. This is the only item which "notices" any skimming of block or head, and is the item which should receive attention after any skimming.

Ian F
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Re: Replacement Cylinder head

Post by Peter Cresswell » Mon Jul 27, 2020 2:59 pm

Try this!
The point you are missing is that as the rocker gear is separate to the cylinder head and as it is tightened down on the 4 studs through the pedestals, the push rod will effectively be too long because the distance A+B has reduced. This forces the adjuster end of the rocker arm up and the valve end down. In extreme cases this might be enough to open the valve slightly. The push rod is fixed in length, an so is the cam follower, so the adjustment Sccrew is provided to effectively make the push rod adjustable, and to allow the working clearance between the rocker arm and the tip of the valve (the tappet clearance). If this can't be achieved with the adjusting sccrew, then a shim under the pedestals raises the centreline of the rocker shaft independent of the cylinder head, so that the adjustment on the sccrew is restored.
Pete
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Re: Replacement Cylinder head

Post by Ian Fozzard » Mon Jul 27, 2020 3:29 pm

Peter, let me know what you think to this:

Let's say that the rocker angle with the valve tip is 90 degrees when set correctly and originally.

With the head off the block we can insert shims under the pedestals. Let's say this increases that angle to 100 degrees - rocker tip now pointing downwards. I find it difficult to see how we can get the correct alignment (as described in the article you referred to earlier) without lengthening the valve stem by a similar amount.

I suppose I am saying that shortening pushrods is the ideal solution, shims may work but ideally the valve tip needs to be raised by a matching amount (to maintain that angle).

I understand if you want to draw a line under this debate, apologies if I am being obtuse!!

Ian F
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Re: Replacement Cylinder head

Post by Peter Cresswell » Mon Jul 27, 2020 6:44 pm

Hi Ian
Ah - I now see the point you are making about the valve height. Yes you are absolutely correct that a correction might be needed for the valves, as well as the pushrods.
The way to deal with this is to have the seats cut further into the head so the valves sit higher relative to the Rocker Shaft centre line. The instruction I gave the people who cut the valves seats for me was 'when fitted with springs all the valve stems must be at the same height'. It is fiddly for them to do this as there is a fair bit of trial and error during the machining process and the valves have to be fitted, a straight edge places along the stems and any gaps measured and then taken out again and the seats corrected. I have to say they did a fine job for me and all the valves were the same height. This obviously helped with the rocker angles, as did the use of the Iskenderian Cooper S cam followers which allowed the push rods to sit further into the cam followers than MGB 18V ones do. So despite the amount skimmed of my head I didn't need shims under the pedestals.

Sorry I missed the point you were making and hopefully all is clear now?
Sleep well tonight!!!
Pete
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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