MASCOT November/December 1999

Roadster Notes - After the conversion:- head tightening and oil leaks

Bill Haverly

Though I've not done a great mileage since the unleaded conversion to the 4AB's engine earlier this year, there's enough on the clock now to make me feel confident that the operation has been a success. The fitting of hardened steel exhaust seats seems to have made little difference to the engine's behaviour at all, except that running on unleaded it gives the impression of being quieter, and I've little doubt that due to the new fuel's volatility it will be running hotter; whether this will be hot enough to cause any problems will only become apparent in the next spell of sizzling weather, or being stuck in a traffic jam with vapour lock somewhere east of Hammersmith. But so far, so good, and there proved no need to make the half expected changes to either carburetor settings or retard the ignition, as it runs as sweetly as before in operation.

Since going through this head operation again after many years I have come to realise even more the importance of regular head tightening on the 1074cc Roadster and 1193cc post-war '10' engine. There are few owners who have not experienced blown head gaskets between the pots, warped heads or internal water leakage, and the head stud configuration, it appears, is a design that needs far more attention than the traditional tighten down after 500 or so miles of 'running in'. `The copper/asbestos gasket used is around 0.75" thick and, with similar bedding in characteristics to the common fibre washer, needs tightening frequently until it will take no more at the given pressure (from 30-40lb psi depending on who is offering the advice). It can take three or four sessions with the spanner before the nuts are bedded down. I'm of the school that avoids the torque wrench and socket approach to this job, preferring a 5/16" WAIT combination spanner. Hard pressure all round using the correct sequence makes sure that the two rogue head nuts beneath the manifold get even treatment and these can also be reached with the combi ring end on further tightenings without removing the manifold; undertightening, uneven tightening and overtightening are all major contributory causes of head problems on this engine.

The other success story on this top end overhaul has been the effective seal of that notorious leak point on both '9' and 1500 Roadster engines - where the cylinder head mates with the front end plate via 3 small cork (9) or paper (most 1500's) gaskets. The leak occurs because there is no means (other than. the rocker box) of applying enough downward pressure on the plate to seal its bottom joint with the head. Oil leaks can also be aggravated by the plate being at a different level to the head, usually caused by oversight in not having the end plate machined to match the head after it's been skimmed. An additional leak will then be created at the rocker box/end plate/cylinder head joint. But with head and end plate a matching pair, helped by some thorough cleaning, it is possible to get an effective seal. If the longer Payen 106 gasket has been used, cut off the protruding end and stick the three gaskets in place with plenty of silicon sealant. When the end plate is offered up the two set bolts will enter at a slightly downward angle due to the oversize bottom gasket, needed to get a compressed seal. The bolts will level up as the end plate is pulled in to meet the head. So far, touch wood, there are no leaks but given time and a lot more miles we shall see what happens.