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MG MGA - MGA pistons

I've just finished a tear down of my engine for a rebuild. I also removed a crankshaft from a spare engine since the one from my driver is scrap (.050" under on both rod and main journals). The spare is an early production BP15GB engine. I've never seen pistons like the ones used in it. They have several wide slots (for expansion?) and 4 rings at the top, 1 ring at the bottom of the piston. The pistons from my first engine don't have any slots or holes, and only 4 rings, like most US engines I've seen. All are MOWOG pistons. Is there any advantage to using one or the other type?

Thanks, George.
G Goeppner

G, I was told by my machinist that these are upgraded pistons that allow for expansion and have the extra oil ring. I think most new pistons today are set up for 5 rings.


WMR Bill

George,
It just dawned on me that your crank may be just fine, are your bearings labled 0.50 or .050 ? Bearings that are marked in metric 0.50 are the same as English measurement .020. Did you measure the crank with a mike and is it really 50 thousands under? Or did you misread a 20 thousands crank by looking at 0.50 METRIC?
Many of the engine parts that fit a MGA will also fit some Datsun (Nissan) engines. As such the come marked in METRIC like the rod bearing in the picture.
This is a .020 under rod brg for a Mitsubishi.


R J Brown

RJ, I measured the crank with a micrometer, and with a digital caliper. Interesting thought though, I'll have a look at the bearings. I removed the crank from my spare engine and you can tell by looking that the mains journals are larger in diameter, and in fact do measure exactly to spec, so it has never been turned. Do you have an opinion on the 4-ring vs 5-ring MGA pistons?
Thanks!
G Goeppner

Fewer rings mean less friction, less wear and more power. A few years ago I had a set of pistons made for me by Venolia. They were forged not cast and only used 3 rings like a modern car. Even though they were of a much better quality they cost less than the AP pistons Moss sold at the time.
I have seen the 5 ring pistons you ask about but have never been told a reason for them. So without a better explanation I say pass.
R J Brown

We used to use those pistons but omitted the second oil ring
Bill Spohn

I purchased a Hillman Minx in 1950. When it reached 10000 miles I had to return it to the dealer. At that time they dropped the oil pan and installed the lower oil control ring. Apparently that's the way Hillman did it. this dealer was also an Austin dealer and apparently they did it also on the Devon and
Dorset models.
Sandy
conrad sanders

G,
Those five ring pistons are Hepolite pistons. When I had my MkII engine pulled apart in 1985. That is what I found. I thought I would never find a set of rings for them back then. Fortunately I found a parts man who knew what they were and ordered a set of rings. He said the Hepolite were the norm back then for replacement pistons.
Back in 1985 new pistons were hard to find or at least in this part of the world.

Ray
Ray Ammeter

These are "Split skirt" pistons. The split compensates for piston expansion, allowing designers to specify tighter piston-cylinder clearances. Long-stroke motors (almost all British motors are long stroke due to tax laws) generate a lot of piston side force and piston wear quickly. Air cooling compounds the problem. Worn pistons + long stroke = piston slap.

They are most often seen in motorbikes (air cooled) and luxury motorcars such as Jaguar XK's and sedans. Luxury versions often included an extra oil control ring at the bottom to further manage noise and wear. As Bill Spohn said, removing the bottom ring trades a bit of noise for less friction and heat.

I guess they are "premium" parts- they are more expensive and used in expensive cars- but they are not performance parts. They are heavy and relatively fragile so high rpm's are a no-no. Today we have better machinery, better aluminum, better rings, and better oil. They really don't add any value.

Hepolite was (is?) a British manufacturer of automotive and aero engine parts. Brit bike guys love them- probably a combo of Anglo-centric snobicity and fear of cheap, knockoffs. They are well-made but kind of heavy. They made "normal" three-ring pistons as well as split skirt, 4-ring, and 5-ring designs. They are now part of Federal-Mogul. While they no longer support old Brit cars, I've heard rumor that Buick Grand National Turbo motor pistons are stamped Hepolite inside.

Peter Hepworth also markets components under the Hepolite name.

http://www.peterhepworth.com/html/cars/history.htm

Have fun,
Bill
Bill Eastman

This thread was discussed between 26/08/2008 and 01/09/2008

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