Sunday, April 4, 2010

Gearbox out, flywheel off

Bugly arrived in Darwin with the advice that if the starter wouldn't engage but just sat there spinning, then a partial rotation with the crank handle would fix it ... no problem! Does this sound like a worn ring gear to you? About 12 months ago I bought a ring gear off eBay suitable for an MGA 1500 engine, so decided to fit it over Easter.

The workshop manual said "Support the power unit (engine), which can be done with a small portable crane through the cab door ..." The reason for this is that the gearbox mounts have to be lifted off the rear cross member before the transmission can be withdrawn rearwards from the engine. So ... coming up, one dodgy overhead crane ...

I had already taken the front quarter windows out, so a wooden packer under the 50x50 RHS worked wonders! The chain ran through the back door and hitched around the left rear chassis member. Not much strain on the chassis, as most of the direct weight was on the window sills. I have great faith in Mr Morris's welding!

Once the gearbox was removed, the wear on the ring gear where the starter motor sat was obvious. I have heard that an engine will usually come to rest on the cylinder with the highest compression, which is why one position of the ring gear gets most of the wear.

Once the flywheel was removed, the extent of the wear was evident, and little wonder that poor Bugly had starting problems! Even the pinion on the starter motor was worn, although not too badly.

The worn ring gear was removed by supporting the flywheel clear of the work bench by some wooden packers, and driving the gear off the flywheel with a punch, working gradually around the perimeter. Actually, I used a cold chisel as it had a wider contact area.

There are two ways to expand the ring gear to allow a "shrink-fit" to the prepared flywheel. Firstly, the ring gear can be placed over the flywheel, ensuring that it is right way up, and the flywheel is adequately supported. Heat via an oxy-acetylene torch is then applied evenly around the ring gear, until it expands sufficiently to drop into place on the flywheel. Once in position, it can be checked and adjusted to ensure that it is seated correctly before it cools and shrinks. But BEWARE it is HOT and will BURN IF TOUCHED!

The second method will ensure that a more even heat is applied to the ring gear ... simply cook it in the oven at around 250 degrees centigrade for twenty minutes or so. Take it out using a good set of welding gloves initially, then by means of multi-grips or vice-grips or similar tool, carefully place it right-way-up in position on the flywheel, and check to ensure that it seats correctly. The same warning regarding "HOT" also applies here.

I chose method 2. Silly me ... I used our hooded BBQ, which usually cooks a roast at 250 degrees no problems. But today, it struggled to make 220 degrees (it has a temperature gauge on the hood). I though I'd still try it, but as it was not sufficiently hot it cooled quicker, and didn't quite seat properly on the flywheel before it cooled and shrank seated higher than it should be up the flywheel.

Therefore tomorrow, I need to find an oxy welding set and finish fitting the ring gear using the first method!

While the gearbox was out, I checked and cleaned all components. The rear oil seal in the gearbox extension at the output shaft is well worn, and requires replacement. This was the reason for a substantial pool of oil under Bugly when parked. The seal presses into the cast aluminium housing, and seals against the tailshaft, or propellor shaft. The tailshaft bears in a long bush forward of the oil seal, which may also be worn ... I will check that tomorrow.

On the other end, the clutch drive plate is still serviceable but worn, and it would be a little silly not to replace it now while the gearbox is out. The clutch pressure plate has been replaced at some stage (it still has the "Coventry's, Perth" sticker on it) and appears to be in good condition. The carbon thrust bearing also appears to have been replaced at some stage, as it is a "Repco" part. It too is in good condition.

So the bits I am chasing are:

New starter pinion assembly (9-tooth)
New clutch drive plate
Rear gearbox extension oil seal
Rear gearbox extension tailshaft bush (possibly)
Clutch withdrawal lever rubber boot
Gearbox casing upper dust cover


  1. Hi Bugly
    Don't know if you have found the rear gearbox extension oil seal,if not try Obsolete Oil Seals on 01480 462 611,ask for Jonathan Welch,he seems to know the j type oil seals pretty well,good luck.

  2. Hi Alex -

    I can't find any listings for this as a Morris JB seal, but it is the same engine and drive-train as the MGA 1500. There are a few UK and US suppliers of MGA spares, and they list this seal as the second of three types that were fitted to the MGA. I am waiting for one from the "Little British Car Company" in the US ( Moss Motors (, who have a UK division, also stock MGA parts but are at present out of stock of this seal. MG Owners Club Spares in the UK ( were also helpful.