on pipe

1. Are the knurled glow plugs on an .020 higher compression than the plain ones, just like an .049? 2. Also, has anyone ever tried to lap/surface the bottom of the glow plug, (where it contacts the gasket) to raise compression? I realize that at some point the lower fin on the plug will contact the top of the cylinder, so could you lap this as well, to make a worthwhile compression gain? Maybe enough to gain a little power, or to be able to use a lower nitro fuel? I remember doing this to an .049 when I was a kid, but don't remember if the motor ran any better or not. Any ideas? Ross

ffkiwi

The knurled and plain 020 heads are the same, and have been for many years-since around the late 70's early 80's from memory. The head profile may have changed-from the original classic trumpet shape to a lower compression style more hemispherical shape-but when this was done I am unsure. The original Pee Wee head of 1957 was a trumpet shape-and the TD (which appeared in 1961) simply carried this on.
Kustom Kraftsmanship sold a high compression TD 049 plug which had had the lower face turned down by about 0.005" to create a higher compression conical shape-and there is no reason why this couldn't be done with an 020 head-but with the scale effect you would have to do it about a thou at a time, and test back on the engine each time to see what the effect was-and then it would only likely be workable for one particular fuel.
Lapping by hand on a sheet of glass would achieve a similar effect-but with less control over the amount of material removed.
Workholding becomes the issue in both cases-to preserve the accuracy and alignment of the sealing face.

'ffkiwi'

David Ingham

The knurled ones look the same compression to me. Skaliwag said he thought it was the process they used early on. The engine I have with a knurled head also has a different shaped starter spring and otherwise appears very old.
Yes, I have successfully "milled" a head with wet and dry sandpaper. I did it to compensate for shimming up the cylinder to "hop it down" by eliminating the sub-piston sub-induction for a muffler. I did have to sand the top of the cylinder to clear the fin.

David Ingham

You are right, Ff, the knurled ones are conical and the un-knurled ones are hemispherical. But I can't see the difference in volume. The conical shape probably retards ignition, so it the compression isn't higher I suppose the old ones run leaner.
Anyway, it appears that the old knurled heads will give more power, adjusted for a given fuel. Very interesting.