Here is what the designer , Bill Appleby, of the 'B' series engine said
about piston rings, ( in the MGA in this case.) Might prove interesting
to some.
neil.cairns@virgin.net wrote:
>
> Part Four.
>
> Gudgeon pin, Clamped type.(Wrist-pin.)
>
> Gudgeon pins are made from 55 ton case-hardend steel.
>
> Pin diameter .6875"
> Max pressure in piston boss 4570 lb/in.sq.
> Max bending stress at centre 23.2 ton/in.sq.
> Factor of safety 1.8
> Max deflection at inner edge of piston boss .00134"
>
> Big-End Bearings.
>
> These are of a steel-backed copper-lead alloy with a very fine coating
> of tin or lead-indium. It is claimed these will take loads under the
> best conditions of up to 9,000 lb/in.sq., but for average engine
> conditions 4,000 lb/in.sq. should be considered the maximum. On our
> current MGA engine we work to approximately 3,000 lb/in.sq. so we have
> plenty in hand.
>
> Connecting rod big-end bores are ground to recieve the bearings as a rod
> and cap, ( a pair,) and the small-end bores are reamed.
>
> Pistons and rings.
>
> For the touring car engines a split-skirt piston made of Lo-Ex aluminium
> is used. A 'T' slot is not recommended as it is prone to tighten up on
> the skirt. A piston clearance of .0007" to .0013" is used in a 2.875"
> bore, with about .009" clearance at the gudgeon-pin bosses.
>
> For the MG versions of this engine of higher output, with compression
> ratios above 8:1 a solid shirt piston should be used as the split skirt
> piston has a tendancy to crack at the corners of the horizontal slot. An
> extra .0005" clearance is required for solid skirt pitons. This makes
> these engines a little more noisy until the piston has expanded.
>
> The use of three compression rings and one oil scraper ring above the
> gudgeon pin is recommended. This enables the piston skirt to be
> copiously lubricated abd reduces blowby to a minimum. Whilst D/26 rings
> in B.S. 5004/4 material will give good enough oil-consumption figures in
> Great Britain with its winding roads; fast autobahn work will reduce a
> figure of 6,000 mpg at 40mph to 500mpg at 80mph.( ** The UK had NO
> motorways until 1959, then it was only 40 miles long and called the M1.)
>
> By increasing the radial depth of the rings to D/24 and reducing lands
> of the oil scraper ring to .030", both rings being in DTD.485, the
> consumption at 80mph can be reduced to 2,000 mpg.
>
> >From our tests it would appear that the top compression ring controls
> the oil consumption at low speeds and the scraper ring at high speeds.
> This is probably because the centre of thrust moves down the bore and on
> to the other side as the inertia loads increase. A chrome-plated
> parallel-sided ring should be used in the top groove to increase bore
> life. The thickness of the chromium is from .0025" to .003". The
> periphery should have a lapped finish. The diametral load on the
> chromium -plated piston ring should be kept as high as the material will
> permit.
>
> Two compression rings with one to one and a half degreetaper on the
> sides should be fitted in No. 2 and 3 grooves. For engines with speeds
> up to 5,500rpm, rings of 5/64" thickness can be used; but for 6,000rpm
> and above, we use 1/16" thickness in our MG engines. Thicker rings will
> cause ring flutter which sets in between 5,500rpm and 6,000rpm. Rings
> should be lathe turned on the periphery and ground on both top and
> bottom faces.
>
> Ring specs:-
>
> Top compression made from DTD.484 .119" deep, .0771 wide, 34-45 lb.
> Second ring B.S.5004 ,, ,, 24-31
> Third ring ,, ,, ,, ,,
> Scraper ring DTD.485 .119" .1552 56-80
>
> The last figure is the LB/in.sq on the bore.
>
> To be continued.....
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