Crankshaft pulley change

Technical MGB discussion
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Not_Anumber
Posts: 99
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Crankshaft pulley change

Post by Not_Anumber »

Whilst setting the timing recently I spotted a noticeable wobble on the crankshaft pulley. I dismissed this at first assuming it was a solid pulley but then read up and realised it is a harmonic balancer consisting of inner and outer metal sections joined by a ring of rubber. The outer part of the pulley was about 2mm further forward on one side than the other and it was also marginally off centre. Its seemed stable enough and didnt move when levered but I didnt want any risk of this getting worse or of it causing any damage to the drivetrain. £ 30 bought me a good second hand replacement from the Wrexham MG centre.

Changing this was very straightforward, especially on a late B as the forward mounted radiator provides lots of clearance.

[*]Remove the undertray, if fitted.
[*]Using a flat screwdriver push the locking tab away from the pulley's central bolt. (This is just a large soft metal washer)
[*]Loosen the alternator and remove the belt
[*]Put a 1 & 5/16th" socket on the crank pulley bolt head. Use a decent length solid bar and rest this against the inner wing to your right. Its best to use something to protect the inner wing such as a length of wood or as i did a small old cushion. Make sure the socket is tight on the nut.
Note that if you dont have access to a 1 5/16th socket then a 33mm metric socket will do the job just as well- and if you happen to have some old Whitworth sockets in a drawer then a Whitworth 3/4 will do fine.
[*]Disconnect the ignition coil (unplugging it's HT lead is the easiest)
[*]Put the car in gear and turn the key to the start position for no more than a couple of seconds. (just turn and release straight away)
[*]The bolt is now sufficiently loosened and will be easy to unscrew. Recover the lock washer when removing the bolt. Replacements are available on Ebay if this looks too fragile or mangled to be reused.
[*]If you need to use a lever to ease the pulley off the crankshaft nose try to do this right at the bottom; be careful not to lever against the front timing cover as this is only made of bodywork thickness steel. In very rare instances you may need to use a puller to help ease the pulley off.
[*]Get the new pulley ready , ensure it is clean in the centre contact area by rubbing away any surface rust or paint in there with a strip of medium grade sandpaper, then apply a tiny coating of oil.
[*]Note the position of the woodruff key and slide the pulley onto the nose of the crankshaft.
[*]Fit the lockwasher and the bolt.
[*]Put the car in 1st gear and apply handbrake.
[*]Tighten the bolt to 70Lb ft (95Nm)
[*]Using a screwdriver bend up one edge of the locking washer so it touches one of the faces of the bolt. I used some grips to make sure it was tight against the side of the bolt. It only needs to be bent in one place.
[*]Replace and tension the belt, tighten alternator bolts.
[*]Refit the undertray if appropriate.
[*]Reconnect the ignition coil
[*]Put kettle on.

Pulleys from early and later cars differ in respect of the timing mark 'v' position. If changing from one to the other you would need to transfer the mark to the replacement pulley.


Chris
Ian Fozzard
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Re: Crankshaft pulley change

Post by Ian Fozzard »

Thanks for that Chris, useful information. Worth pointing out that as well as the possibility of failure/vibration, if the outer has moved relative to the inner, the timing marks will no longer be reliable!!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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Paul Hollingworth
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Location: Worcestershire

Re: Crankshaft pulley change

Post by Paul Hollingworth »

We cant have Americanisms here. Harmonic balancer indeed. The correct name for that part is torsional vibration damper which perfectly describes what it does. Its an absorber tuned to the crankshafts torsional natural frequency. This frequency is excited by harmonics of the firing torques of the engine principally 2,4,6,8 orders. It doesn't balance these, they are still present what it does is to reduce the crankshafts response to these harmonics. The most common type is the rubber in compression type where the rubber is trapped between the hub and the ring (which incidentally drives the alternator and water pump). If the original posters damper appears buckled then the ring has moved probably due to the rubber perishing, so it scrap and yes the timing marks will be out. Pleased to hear that Welsh MG were able to help out.
1971 MGB roadster & 2006 MGTF
Not_Anumber
Posts: 99
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Re: Crankshaft pulley change

Post by Not_Anumber »

The engine runs noticeably smoother with the pulley replaced. I will recheck the timing as has been suggested.
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