Sharp Combustion Chamber Edges

[Mr. V]

I've had my 940 head skimmed which has left sharp edges to the combustion chamber. Do these need rounding off?

I'm confused about the standard pressed steel rocker ratio. Is it 1.25:1? The reason I ask is because I've just acquired a Roebuck 'Micro' tappet adjuster and with that ratio the figures for 1.25 don't add up. I intend to calibrate it for 12 thou and 15 thou.

[les]

Just remove the residue from the skimming with a scraper. I don’t know about the rocker ratio.

[philthehill]

The standard pressed steel rocker ratio is 1.25.
1275cc pressed steel rockers are 1.27.
It is much easier to set the rocker clearances with a feeler gauge unless you have worn rocker pads - then a micro tappet adjuster is of benefit.
That said - if you have worn rocker pads and need to use a micro tappet adjust then it is time to replace or recondition the rockers.
As regards the sharp edge - use a scraper as suggested by Les above and/or just lightly sand with some coarse production paper making sure that you remove all detritus from the head and clean the head spotlessly before fitting. Make sure that you do not leave any marks on the head faces as that could lead to problems.
Here is a good read:-
https://www.calverst.com/technical-info ... ompendium/
The ex factory setting for rocker clearance is 0.012" but if using unleaded fuel the rocker clearance can be set to 0.015" because with unleaded fuel the engine tends to run hotter. The wider rocker clearance offsets the increase in cylinder head temperature. The rocker noise may be a bit louder but in my experience the engine(s) runs better for having the wider clearance gap.

[Mr. V]

I presume the 15 thou gives a slightly longer time for the valve to be in contact with the seat, thereby dissipating more heat. Is this also your recommendation for the inlet valves, which will be cooled by the incoming mixture and, in your experience, I assume this has no effect on performance?

[philthehill]

The recommendation is to set both inlet and exhaust valve clearance gaps to 0.015".
It is not so much the transfer of heat from the valve to the valve seat but more like the hotter valve stem gets longer through expansion so a cold set of valve clearances at 0.012" could be down to 0.010" when up to working temp.
It has to be remembered that when the 'A' Series was first produced there was no such thing as unleaded petrol (though saying that there was a type of low octane not fully unleaded fuel called Pool Petrol and it was awful). Therefore to compensate for using unleaded fuel various engine settings have to be adjusted from those specified in the BMC workshop manual and in my experience valve clearances are one of them.

[Mr. V]

OK 15 thou all round it is then. About the combustion chamber/ valve pocket edge, I don't quite understand how a scraper (and by that I understand an engineer's bearing scraper/surface scraper) will be able to remove the hard skin that is inevitable on the surface of cast iron. I was thinking of using a grinding head on a Dremel tool to round the corners, or is that unwise?

Please forgive my ignorance.

[philthehill]

A quick pass with a scraper is all you need no need for a full Dremel rounding of the edges.

[les]

That’s thrown me,( hard skin on the surface of cast iron) Sorry, maybe I misunderstood the original query. Post a picture ?

[Mr. V]

Thanks Phil. I presume a carbide scraper is then in order.

Les, what I'm talking about is the sharp edge between the bare cast surface of the combustion chamber where the valves sit and the machined surface created by the skimming process.

Thank you both for your contributions.

Steven.

[Michiel]

How can any engine with a thermostat run hotter with unleaded

[philthehill]

The thermostat controls overall engine temperature but it does not control or effect localised hot spots.
The actual combustion is hotter with unleaded fuel and can cause localised hot spots especially around the combustion chamber and especially around the exhaust valves. Unleaded fuel has no lubrication qualities and does not contain tetraethyl lead which was used way back to protect the valve seats and lubricate the valve stems. That is why I always recommend that valve seals are not fitted to the exhaust valves/guides. The exhaust valve stems need all the oil they can get. The exhaust valve guides need to be reamed to give a minimum of 0.003" clearance between valve guide and valve otherwise there is the possibility that the valve will seize in the guide. The inlet valves run a lot cooler because of the incoming fuel/air charge and valve seals can be fitted.