mk 2 square four- piston crown height

[Brian.Fosh]

Hi,
I went through similar this year using IMD's in a '51 mk1 with skimmed head and barrel. I'm really pleased with IMD.

I went through quite a bit of torture with the newly built engine missing after long gentle runs.

In the end, it was simply down to the plug temperature from running a much (much) higher compression with mk2 pistons plus all that skimming.

I raised the temperature ratings from the original spec lodge CN's, then to klg's equivalent 5, then to 6 & 7 but now run fully trouble-free on both 8's and 9's, these having a much shorter electrode nose to get the increased heat away.

Last month I clocked up around 600 miles with camping gear, visiting Squires biker cafe campsite from Milton Keynes and then touring around Yorkshire, coming back on the A1M at 70 all the way.. It never missed a beat.. and I never needed to take any oil. (it used half a pint).
The Otto gasket held well. I checked the cylinder head nuts today and they didn't really need tightening.

Brian.

[nevhunter]

Good to hear. It's better to run a plug a bit colder if there's any doubt rather than the other way around unless crawling along is your thing. The plug can become a glow plug at high output. I've had considerable trouble over the years with plugs too hot .. Nev

[john.bebb]

Thanks Nev for input; - definitely to bear in mind...
so Brian; - plz, did you actually measure the piston protrusion as did Bob? -
& have you had the pistons skimmed down to liner level...
or did you (more simply) leave them proud and just run with colder plugs?
Thanks, Bebbspoke.

[Brian.Fosh]

Hi, I never had any protrusion but I had used an extra thicker base gasket in expectation... I stuck one of Drags paper-thin gaskets on to a piece of stout gasket material and then cut out the shape.
Most metal had come off the head in my case, rather than barrel, because the toolmaker struggled at one corner of the head, front nearside, having to remove a lot more than he would have liked to get the full surface machined.. so much so he asked me if the engine had thrown a piston when I collected it... indeed it had.


I dug out this photo which may not be at TDC .

Regards, Brian

[nevhunter]

What's the purpose of the raised bits on the top of the pistons? Sharp edges are not a good idea, neither is increasing the surface area of the piston top. (More area more heat absorbed). Nice clean looking engine. Nev

[john.bebb]

Thanks Brian for the pic! - see you've got a Morgo - sensible man!
Nev - you having a funny turn, or something??? - the lumps on top of the pistons are those which raise the compression!
also (alegedly) the lumps will invoke better "squish" (i.e. distribution of combustion charge)...
therefore improves power of stroke & gives better cylinder scavengeing...
A higher compression engine (in theory) should run COOLER than "std" - providing of course; -
that the mixture is correct for the compression.
Cheers, John