Article kindly reproduced from the Herefordshire A7 Club.

Part 1 – The crankcase

Part 2 to follow

There is plenty of available information on dismantling engines that I won’t repeat here (Woodrow is particularly good, also the 750 Club ‘Companion’ pages 107 et seq.) and will therefore assume we are starting with a completely stripped and cleaned set of engine components.

Two-bearing A7 coil ignition engines enjoyed three different crankcase configurations. The first two were for the high frame chassis and had solid engine mountings. The earliest having its starter motor facing forwards i.e. alongside the gearbox in the passenger compartment (sometimes confusingly referred-to as facing backwards) with the starter ring gear on the clutch cover plate. This was followed by the starter motor facing backwards and located in the engine compartment. Finally, a flexibly mounted crankcase with bigger feet was fitted to the low frame chassis cars that retained the backwards facing starter. The following notes apply to all three versions.

In order to build a reliable, oil-tight engine, it is first necessary to remove all studs – i.e. those locating the bellhousing, block and fuel pump. These can usually be removed by very firmly tightening together, two full steel nuts (not the ghastly modern countersunk ones) on the protruding thread and lean hard on the lower nut with a well-fitting ring or combination spanner repeatedly until it ‘gives’. If this fails, then try applying heat (a hot air gun
works well) to the surrounding aluminium. On the odd occasion this doesn’t work either, it will necessary to use a stud extractor but this will almost certainly damage the stud, thus precluding its future use.
It is unlikely that studs will break when being removed from aluminium but not uncommon when being removed from cast iron. I will discuss the removal of broken studs in a later Part – ‘The cylinder block’ and head.
Studs should then be cleaned (a rotary wire brush is ideal) and inspected because only those with perfect threads should be re-used. Damaged studs should be discarded and replaced.
All threaded stud and bolt holes in mating surfaces should be lightly countersunk to prevent the aluminium ‘pulling-up’ when tightened, thus preventing a good seal. Also, all crankcase threads should be gently cleaned with the appropriate BSW or BSF tap, taking care to remove only dirt/sealant and not any aluminium. Then, nosepiece, sump, fuel pump and front bearing housing mating surfaces should be carefully dressed with a file to remove any damage. Also the bell housing face but this is less critical.
Any damaged threads will need to be repaired. I am not aware of any data that compare the pull-out strength and costs of the following different methods but I like to think that my favoured approach might compare well.
The quickest way to repair a damaged thread is undoubtedly the coil insert process (‘Helicoil’, ‘V-coil’ or similar) – but this requires a specific kit for each thread type and diameter. Each kit includes an appropriate drill and tap, together with an insert tool, a number of coil inserts and a tang breaker. The process is quick and straightforward – the damaged thread is drilled-out, the new hole tapped and then the coil insert is wound-in maintaining slight downward pressure and a clockwise motion with the tool provided until it is just below the mating surface. Finally, the ‘lead tag’ of the coil is broken-off with the punch and the wire fragment removed. Interestingly, coil repairs are claimed to be stronger than the original configuration but it is useful to remember that they can sometimes be less oil-tight than their conventional counterparts.
An alternative approach for repairing threads that take a stud, is to buy (or make) a stepped stud with a larger thread at one end. The crankcase being drilled and tapped oversize to suit. A possible extension of this approach would simply be the deployment of a larger diameter stud. However, this might entail drilling a larger clearance hole in the mating component which obviously has the potential to weaken it.
Another approach and the one I usually adopt, is to drill-out and tap the damaged thread to a size larger than would be used for a stepped stud, then turn-up a bush that externally matches this newly threaded larger hole and is threaded internally as per the original. This bush can be made in aluminium but I prefer to use steel or brass to make it more robust (steel is cheaper but brass is easier to machine – so, ‘you pays your money and you takes etc etc’). The bush is secured in position using a high-strength industrial adhesive such as Loctite 648, then carefully filed exactly flush with the surrounding surface. This approach takes a little longer but it’s cheap and has never given me any problems.
Next, the top surface of the crankcase needs to be made perfectly flat if you want to keep your oil inside the engine. I do this by lapping; starting with coarse (then finishing with fine) grinding paste mixed with a little diesel or paraffin. Interestingly, the well-known Chris Gould writes on this very subject in the October 2018 A7OC magazine and he advocates a totally different approach.
Anyway, whilst lapping, the crankcase should be held firmly in contact with the plate and moved in a figure of eight motion. This is very important, because circular or fore/aft movements will tend to leave a high area in the middle. I use an offcut of granite kitchen work-top about two feet square for lapping but a sheet of plate glass would also suffice. Some people remove the tappet guides and lap the crankcase directly to the block but I have always found the underside of A7 blocks to be reasonably flat. In addition, it can sometimes be difficult to accurately replace the guides so-that the tappet blocks sit exactly square to the camshaft. Incidentally, nearly all the A7 crankcases I’ve worked-on, have had noticeably distorted top surfaces, no-surprise then that so many engines leak oil between block and crankcase. Some engine builders say that the top surface of the crankcase is sufficiently flat when a two thou’ feeler gauge is rejected under a straight-edge. However, I simply continue lapping until there is an even matt finish over the entire block mating area.

Oil galleries
If modern multi-grade oil is known to have been used in the engine, you might skip this step. Otherwise, it is essential to thoroughly clean-out all the galleries before using modern and arguably superior oils. This means removing the hex’ plugs at each end of the main oil gallery and the one covering the cross-drilling to the front camshaft bearing. The oil pressure relief valve - ball, spring and cover plug should be removed together with the two threaded plugs located at the bottom of the two vertical oil galleries at the back of the engine. To remove these, it is necessary to clear the screwdriver slots of the peened aluminium that prevents them from unscrewing in service. This can be achieved by driving a properly ground screwdriver along the slot in each direction to clear the peened material, then holding a well-fitting screwdriver firmly into the slot these plugs will usually unscrew without much trouble.
All the oil passages must be thoroughly cleaned and I use a number of model traction engine brass wire flue brushes for this purpose (every home should have these!) together with liberal doses of a petrol/diesel mix. It is amazing how much crud can usually be extracted.
Front main bearing lip
Sadly, many crankcases have damaged front main bearing lips. Often the result of the bearings being previously removed by ham-fisted ‘mechanics’ or occasionally perhaps by poor brakes leading to a frontal impact on the starting handle?
Happily, there is no great load on this front lip in normal service, therefore some slight damage is acceptable so long as the remainder of the lip is not cracked. Nevertheless, it is not difficult or expensive to replace a damaged lip – just a few hours of rather rewarding work – as follows …

Some years ago I made a very simple device (see photos) to machine-away a damaged aluminium lip. It is hand operated and typically takes about forty minutes of healthy exercise to provide a suitable surface upon which to screw a steel lip replacement ring.  Several of our well-known suppliers stock reasonably priced ready cut replacement steel rings but the ones I have seen appear much too thick (at about 5/32”) which in my view are likely to give insufficient clearance to the front of the crankshaft. 

I simply mark-out and cut a 3.5” diameter. disk from 0.10” thick mild steel to the dimensions shown in the diagram and initially drill six 2BA tapping holes equally spaced on a 3.10” PCD. The outer edge (which is not critical, unless you plan to show photos to your friends!) is roughly sawn then filed almost to size and the inner hole created by drilling a row of holes in increasing sizes – inside the marked line to remove the inner portion, which is then filed close to the finished circle dimension. I then trim the inner and outer edges in the lathe, exactly to size and create a slight chamfer on the inside that exactly replicates the geometry of the original Austin flange. If you get stuck-in, this will take less than an hour and is quite satisfying.
To secure this new plate, some people advocate drilling through from the front main bearing retaining plate holes and using ¼” diameter countersunk through bolts. However, once the original flange has been removed, there seems to be precious little radial width in the remaining aluminium boss and even less surrounding the ¼” Dia clearance holes in the new plate which might weaken it. I therefore use six 2BA by 5/8” long steel countersunk set-screws put-in with high strength Loctite. I have done this on several engines and never had any problems.

The photos show the turned hardwood jig that I use to hold the ring concentric with the bearing housing whilst drilling for the screws. I rotate the ring on the jig until none of the six holes correspond with the four front bearing retaining plate drillings, then drill one hole 2BA tapping (5/32” Dia is OK but a No 24 drill will give much higher engagement ) to a depth of about 1” into the crankcase. It is also useful to mark the top of the ring, just-in-case the six holes do not lie on a precise regular hexagon.
This first hole in the plate is then opened-up to 2 BA clearance (3/16” Dia), the crankcase hole then tapped 2 BA and a temporary screw tightened gently in position to hold the ring in place whilst still supported by the jig. The same operation is then repeated for the opposite hole which then holds the ring exactly in-place before dispensing with the jig and dealing with the four remaining holes. Finally, the holes in the ring are countersunk to ensure the screw heads lie perfectly flush when tightened and the ring secured with high-strength Loctite (e.g. Type 648) on the threads. Job done!