Thinner than usual valve pads

[Philippe-J.]

Next, and hopefully last, episode of the story:

I ended up ordering a new set of valves to Rob Beere Racing, I received them, I installed them, I measured the clearance between valve tips and cams, I inferred the needed new shims, and they all, but one, fall within the commercial available range
The thinner one is just 3 thou lower than the thinnest commercial one. Rob Beere advised me to grind or rub off a few thou.

This story confirmed that not only do Rob Beere provide the right stuff, they also are very helpfull

[Gfhug]

That’s good to hear Philippe. Keep us aware of how you progress

Geoff

[Stevemorris]

Sounds like you are getting there Phillippe!

So you are just replacing the valves? If it were me, and the head is clearly off at the moment, I would be inclined to fit new seats. If the new valves are giving you a 'close to the lowest' shim requirement, I would be much happier for long term fix if the shim requirements were either in the middle of the shim range or at the higher end? It's so so easy to do the seats now, so why wouldn't you?

Just to clarify, when Rob Beere says 'grind' the shim he doesn't mean on a grinding machine, this is a job best done by hand. It takes a long time but doing it 'cold' means the shim will retain its hardening. The shims loose this when they get hot on a grinding wheel..

Also, when you say 'commercially available' shims, assuming you mean from SNGB? They can supply shims down to 0.075" now (previously only available down to 0.085"). If you are having to take one of yours below the smallest available shim from SNGB I would say you definitely need to do the seats?

Sorry to appear negative but just trying to help!

Steve.

[Philippe-J.]

Hi Steve!

Thank you for your remarks & advise. I think I can confirm your points:
- according to the local workshop the original seats needed not to be renewed, just reground. They were not that worn. The issue I had appeared to be due to a wrong model for the valves. It was cleared off with the ones Rob Beere provided, except for one: it requires a 0.072" shim instead of 0.075" which is the lowest available at SNG. The others go up to 0.095". So for budget sake, I elected to keep my original seats.
- There was another reason. The above local workshop failed in the middle of their work for my engine. So they were out for a valve renewing job. The second workshop did an excellent job in surfacing the head, honing the cylinders and so on, but proved reluctant to renew the valves. I wouldn't want to force them in a job they were not volunteers in, would I?
- Yes Rob Beere advised to grind this shim by hand with sandpaper. I asked him if I should polish it afterwards, he said "No just rub it and hold in a valve spring retainer cap ". Your explanation is welcome, however.
- Yes I ordered my shims to SNG. They sell an extended range, even if not obvious to be found on their site (guess why I'm saying that). Rob (again) told me you can thin a shim down to 0.060". You can shorten the valve stem too, but it is a bit more difficult to stay perpendicular to the stem axis.
- I have to add that I change the guides, springs and tappets too.

Philippe

[Series1 Stu]

It takes a long time but doing it 'cold' means the shim will retain its hardening. The shims loose this when they get hot on a grinding wheel.

Absolutely not the case. The only way to properly grind shims is on a surface grinder. Otherwise, you just won't get accurate and parallel thickness of the shims. The process of surface grinding removes a very small amount of material at a time and doesn't generate any amount of heat that would affect the temper of the material. There is also the option of using coolant to minimise the temperature of the material.

Unfortunately, most of us don't have access to a surface grinder so have to resort to emery paper on a sheet of glass.

Also, there seems to be a misconception that shims are case hardened. They are actually through hardened, so don't worry about spoiling the temper.

Regards

[Philippe-J.]

Stuart,

is the "surface grinder" associated with some sort of "magnetic vice" I heard about? I understood that a "magnetic vice" is necessary to hold the small shims while grinding them. This equipment is rare for individuals like us.

[Series1 Stu]

Yes, Philippe, that's correct.

They are high precision machines that generally incorporate a magnetic bed. When items are quite small you need to make sure they are positioned on the bed so they get maximum magnetic field to avoid them being thrown off when the wheel hits them. It's a skilled process.

Obviously, they won't work with non-magnetic materials.

Regards

[MSM]

Stuart

That is very interesting. I had always understood that the shims were case hardened and thus not really adjustable but learning that they are through hardened is good news for future reference.

Mike

[angelw]

Stuart Wrote:

Also, there seems to be a misconception that shims are case hardened. They are actually through hardened, so don't worry about spoiling the temper.

Hello Stuart,
How have you determined that the original, or after market shims are through hardened?

For quite awhile, I've been considering manufacturing tappet shims, but haven't got around to determining the material to use. A hardness test indicates a surface hardness of circa 60 RHC, with my first thought that a through hardened piece of the typical thickness of these shims would be too brittle for the cyclic pounding they are subjected to. My next step in the dance, which is the one I've not got around to, is to definitely determine if the shims are case hardened, by a process called macro-etching or metallographic cross-sectioning. This can be done by cutting a shim circa in half along a chord of its circular shape, not in half thickness wise, then polishing the cut face. After polishing, apply Nitric acid to the cut face to etch the surface. In case-hardened carburized steels, the core is lower in carbon and will appear dark, while the high-carbon case will appear lighter.

Stuart Wrote:

They are high precision machines that generally incorporate a magnetic bed. When items are quite small you need to make sure they are positioned on the bed so they get maximum magnetic field to avoid them being thrown off when the wheel hits them. It's a skilled process.

It's not just a case of having a magnetic bed (magnetic chuck) and positioning, the magnetic chuck needs to be fine-pole (or micro-pitch) magnetic chuck, which is engineered specifically to prevent the magnetic field from passing completely through thin material. Unless a machine shop does thin thickness work piece surface grinding, its unlikely they will have the required magnetic chuck to do the work. Accordingly, you won't be able to take shims along to any machine shop to have them ground to size.

One other method of work holding thin material for surface grinding, and it works for non magnetic material, is to use a "freeze chuck" or "ice chuck". It uses water as a bonding agent, rapidly freezing it into a thin, solid layer of ice that firmly holds parts in place without applying clamping pressure. This is very useful where the surface of a thin part that would interface with a magnetic chuck is not perfectly flat, and due to its thinness, the not so flat surface is pulled down flat onto the surface of the chuck. When the magnetic clamping force is released, the part will generally revert back to its original state, and the surface that has just been surface ground flat, will no longer be flat. A Freeze Chuck will normally only be found in shops that do a lot of thin, or non-magnetic material grinding.

Regards,
Bill

[rfs1957]

Based on a lifetime of Ducati cylinder head work, I couldn’t count the number of times when I’ve either shortened valve-tips, or come across engines where someone else has done it.

I’m not a metallurgist, and don’t know much is « safe », but I wouldn’t think twice about grinding 1mm off if it took me into an easier shim range.

Chesmans of Coventry properly butchered a couple of new valves when they rebuilt my new-seats and « ready to fit » head, albeit with no oil ANYWHERE and a whole host of other botches, keen students of the genre may remember, but only because the ends were free-hand ground and nowhere near square.

Which makes me think that if it’s done properly there’s nothing wrong with it.

A whole lot easier than grinding shim after shim.

Bill ?

PS Hailwood’s Ducati, in either 1978 or 1979, at the Isle of Man TT, saw the Ducati factory mechanic Pedretti and Steve Wynne of Sports Motorcycles grinding shims on the stone of a Douglas hotel steps.

[Philippe-J.]

Rory,

In 1978 or 1979 I would have been too young to appreciate the scene of Ducati factory mechanic grinding shims on the stone of a Douglas hotel steps, but today I can figure out the image !

Your sad experience of Chesmans of Coventry with valve-tip grinding pushes me to have a try on grinding the only lower than out of range shim I need. It is the advise of Rob Beere, too.
I'll keep you informed.