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XPAG Clatter

WORRY and STRESS.

 

“I have recently brought a ‘T’ series, why is its XPAG engine so noisy? It performs well and uses a little oil and has the usual oil leaks.”

 

That question is a compound of one of the most common I get, usually by email. It often comes from someone returning to the ‘older car scene’ after years of driving modern cars. The answer is that the modern car has spoilt the questioner with its good door seals and steeply raked windscreen, getting rid of nearly all wind noises. The sealing of the hull and its design along with good soundproofing also masks any mechanical noises. To help along with this, many modern engines use over-head camshafts running with hydraulic tappets, all driven by a rubber covered steel belt. So straight away there is no tappet clatter or chain noise; because it is the valve gear of the XPAG that causes most noise. It is a very proud over-head valve train and it lets everyone know it is there. Your modern car’s hydraulic tappets are expanded by the engine’s oil pressure to take up virtually all the clearances, so it runs without any gaps for sloppy push-rods and cam followers to bang about in.

 

YBenginenearsidebSo the XPAG is noisy, like all the other ohv engines of its day. The side-valve engines had fewer gaps between its camshaft and valve, and it was all buried down the side of the engine, so little 918cc Morris Eights sounded like sewing machines. The XPAG, coming from the Morris family of engines, had a thin pressed steel rocker cover that can almost act as a sounding board, and it did not prevent noise from getting out. The simple fact that an ohv engine changes the direction of its actions through 180 degrees from the cam lobe to the valve stem does not help.

 

The noise is far worse in the MG saloon cars, any YA or YB has its noise trapped in its cabin, where as the T series and the YT with their open cock-pits are much quieter until wind noise becomes a problem. A T series with its hood up and side screens fitted is quite a bit noisier than when open.

 

There is only a wooden floor between you and the road, and a single steel sheet between the cab and the engine bay. So, as that valve gear wears, the noise level rises. Adjusting the tappet clearances helps, but do not be tempted to close them up below the .019” or .012” depending on your camshaft type. The gap is part of the design, it permits oil to get between the various ends of push rods and rocker ends, as well as under the most highly stressed bit of the engine, the thin line of contact between the camshaft lobe and the base of the cam-follower, (called a ‘tappet’). The later ’12-thou’ camshaft has ‘silencing-ramps’ built into the lobe. (See earlier article).

 

Wear will cause the tip of the case-hardened camshaft lobe to wear, and this eats away the underside of the tappet, eventually causing it to concave and break up. The tappet also gets a lot of thrust in the direction of rotation of the cam, so it wears its hole oval, (you can get oversized +.010” tappets, many Gold Seal engine have them already). The oval hole with its sloppy tappet is a major cause of clatter, and no amount of adjusting the gap will cure it. It is very like piston-slap, but at half-engine speed. The ends of the push rods work loose, and cause more noise. But the item that is easy to change and will remove lots of excess noise, is the rocker shaft. It gets very badly worn on its under-side with the action of the valves and camshaft shoving upwards twice each cycle. It is also the last place to get any oil on starting up a cold engine.

 

Worn pads on the rockers, worn shaft bushes giving lots of side play, and the tiny camshaft bearing surface all contribute to the noises. The rocker pad has to be stoned to the correct ‘arc’ so it runs on the tip of the valve stem accurately. Get this wrong and you will cause the pad to shove the stem about sideways, and then wear out the valve guides. The guides can also cause some noise is they are worn, but heavy oil consumption on the over-run and high crankcase pressures on accelerating blowing out oil will indicate this problem. Worn pistons and bores lead to very high oil consumption at speed, (see earlier article on ring-pumping).

 

XPAGvalvegearbThere are lots of other culprits under the bonnet that cause noise, a very powerful one is any carburetters that have no air-filters fitted. Pancake air-filters do little silencing of the intake-air, and that is why the Y saloons have a large intake ‘silencer’ above their single carburettor. The fan will roar away at speed, its very inefficient flat-spade like paddles bashing the air stupid as it pulls it through the radiator. Worn and cold engines will display piston-slap if run at too low an rpm in gear. This will mostly disappear once everything warms up to its running temperature. A badly worn camshaft lobe will make a sound like a diesel engine idling, and has fooled some into thinking a big end is on the way out. A worn timing-chain will clatter about at idle. The vibrations of the propeller shaft spinning away inches to your left (or right on LHD cars) cause drumming. This can get quite bad if a universal joint is worn causing the shaft to spin out of true. Follow a few T types about, and you will soon see those with out of true wheels, or poorly fitted tyres that can cause vibrations.

 

Though whilst gearboxes and axles can cause noises, it is by far the faults of the valve gear that produces the most common racket your ears perceive. Having enjoyed the silence and reliability of a modern mechanical clone, going back fifty or more years to a very individual old M.G. can cause one to re-examine life. Suddenly there is something to worry about, is that oil pressure OK, what was that noise, is that vibration a fault, how much will it cost to fix?…and so on and on. When you go out in the M.G. you are probably driving your partner to distraction with your worry. There is no need to, as all old M.Gs have an infallible system fitted that helps you source faults. It is the rear view mirror, look into it after each odd noise, if nothing fell off, keep going.

 

Oh yes, and an XPAG that has no oil leaks has an empty sump. Like many animals, old M.Gs mark their territories with scent, they use old black engine oil.

 

Neil Cairns.