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MG MGA - Installing new County Pistons in 1500 MGA engine

Hi, Got new County pistons CK306 0.02" for a 1500 MGA. I don;t have the final hone size of the piston. In other words what should the final size of the cylinder be after hone. The cylinders are 73.51 mm.
I've tried to find the manufacturer, but could not find anything. Does someone have experience, or can someone point me to the manufacturer?

-- Paul
PvA Paul

According to the workshop manual, you need a clearance of .0017 - .0023 inches at the skirt bottom. This should correspond to .0035 - .0042 inches at the top. You may be surprised to know your new pistons are oval and tapered. The ovality gives more clearance in the fore and aft position, so the skirt clearance is measured to the left and right. So, measure your pistons at the skirt bottom, at right angles to the gudgeon pin, add .002" and hone the bore parallel to that dimension. If your previous pistons / bore were standard,, you will need a rebore before honing. Are you doing this yourself?
The County pistons should have come with these sort of instructions.
Art Pearse

Piston clearances are determined by the piston design and material, and are not necessarily the same as OE. Clearance specs should come from the piston manufacturer. You might contact Chris Betson (UK), a frequent poster on the MGB board, and good supplier of parts including the County pistons I think.

FR Millmore

I have always thought that if a cylinder bore is slightly oval from use that honing will only increase the ovality. To get rid of ovality you should bore the block. Is this the case? I have also gotten new pistons with no instructions at all. I have always given the information that Art listed to the engine rebuilder for cylinder boring. What are the alternatives to buying County pistons. I have used them with no ill affects but I do not hear many good things about their products.

John Progess

IF your rebuilder knows what he's doing, he will use the WSM data as a guide, modified by what the piston manufacturer and his own experience says. The engine may or may not wind up with the clearances you supply. The usual ranges are such that "about right" will almost always work, but will certainly not be optimum for either wear/durability or performance. Piston designs change over time, and metallurgy has also moved forward a bunch over 50 years. These differences may not be visible. A split skirt taper/oval ground low expansion piston might be best at .0005 bottom of skirt clearance, while a forged solid skirt race piston might need .007 or so, for the same engine model.
Lots of experience should let your builder get close, but be certain you tell him what the engine use is, and do not fall in the "race car design is good for my street engine" trap!
It has been the case since the beginning of engines that a good machine shop will NOT bore and hone cylinders until he has the actual pistons to be fitted in his hand - each piston is measured and fitted to a particular bore.

Contact Chris for free advice here:

John -
The usual "honing" as practiced at home is really a glaze breaking process. If done with the floppy ball brushes it ONLY breaks the glaze; if with a three stone hone it tends to straighten out the bore, not make it worse. True honing is done with rigid mounted adjustable stones, and is used to make the bore MORE accurate than boring can, to precise size, and desired finish. If the bore is worn enough that ovality is an issue, then it will also be oversized, especially at the top, leading to out of spec in both size and taper. In that case it needs to be bored and finish honed.
My understanding is that County parts are about as good as you will get now that parts supply has shifted from old "real" companies to the third world. They supply a lot of parts under the "County" name, and likely each different item comes from a different factory in a different country, so you can expect variations all over the map. Some third world stuff is state of the art and far better than the OE were, others are absolute rubbish. It is and was ALWAYS the responsibility of the rebuilder to check EVERY part for suitability and size.

FR Millmore

Indeed the main issue is that the pistons as I got them did not come with the instructures as Art indicated. My supplier is unable to give that. So if there is someone that can give me a copy of his County piston instructions, I'd feel a lot safer. The person who is doing all the machining on the block rather work off of the instructions, because this shape of the piston is quite a bit different compared with the old one: This one has a groove in the skirt. If nothing else he could indeed take the 0.002" mentioned earlier. Afterall I'll not be racing this thing (spent too much time in it to do that....). Just for the fun added a picture of the old piston..

-- Paul

PvA Paul

Paul, I'll see if I can find the instructions that came with mine.
Art Pearse

New pistons +040
Top Skirt
1 Fore/aft 3.0230 3.0241
Lateral 3.0356 3.0372

2 Fore/aft 3.0232 3.0240
Lateral 3.0355 3.0372

3 Fore/aft 3.0233 3.0233
Lateral 3.0353 3.0371

4 Fore/aft 3.0233 3.0238
Lateral 3.0353 3.0372

3.0293 3.0305

Avg fore aft 3.0232 3.0238

Avg lateral 3.0354 3.0372

These are my measurements of my new County pistons, from Moss, for a 1622 size. Nominal bore is 3.000 + .040
You can see the ovality and taper.
Still looking for the clearances recommended, which is what I asked the shop to do.
Art Pearse

OK Paul, found the paperwork.
I bought Moss 420-658, County CP397K040, or Aerolite MM6-040, its not clear which was supplied, but the instruction sheet covered both with the same instructions.
"All pistons should be measured to determine the biggest dia. at 90 deg to the pin axis. This will normally be just below the top oil ring."
In my case, only one oil ring and the biggest dia was at the skirt. I used a mike to .0001
the recommended clearance at this point was .002-.0025 for County and .0025 - .003 for Aerolite. I asked for this. I have not yet run the engine but I can sure feel the compression by hand.
For the CP306K pistons, it lists .002 - .0025" for the County.
Hope this helps!
Art Pearse

This thread was discussed between 05/02/2011 and 06/02/2011

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