Since November 2008, reports have been seen in other clubs’ newsletters about problems members all across the country have been experiencing with the Bosch replacement distributor. Experiencing mis-firing on my Box myself and tracing it to a completely worn rotor arm on my Bosch distributor, I thought it is time to bring all the reports together.

The first report was seen in the Bristol A7C (BA7C) in November 2009:

“Whilst waiting for some of our group to get through the maze of traffic lights at one of these new road junctions, Stewart Creasey's car just stopped for no reason at all. After much fiddling about and changing of coils etc it was found not to be this problem at all - but the Bosch rotor arm. Apparently the resistors in these are prone to packing up without warning. Now, you wouldn't get that problem in an original one 'cos originals don't have resistors. Therefore, warning ........ If you use a Bosch rotor arm, always carry a good spare.”

“Bob Downey, has been running his 1934 Tourer’s engine with a Bosch distributor for over 3 years, but it recently, and without warning, destroyed itself. On inspection Bob reports that the base plate fixing to the main shaft failed, leading to extensive damage. This joint is apparently just crimped together so the advice to anyone else using this equipment is to have a quick check to avoid the same fate!! Following this, the pivot pin for the bob-weights subsequently fell out, causing further damage!”

“I’ve had a yet more disappointing experience with the Bosch unit. On the morning of departure from Edinburgh to the Yorkshire Experience 8 it suffered mechanical failure before we left home. When I came to start it to leave, as the starter motor turned there was no movement of the needle of the ammeter, and when I removed the top of the distributor I found why: the rotor arm stayed stationary as the engine rotated. When I grasped the rotor arm I found that I could turn it freely through 360 degrees – it being still attached to its shaft! I surmised that whatever secured the drive gear on the shaft had failed. After our return home I sought the cause of the failure and found that the shaft of the distributor is in two parts, joined end-to-end by a small setscrew (about 2BA) which had come out. Indeed, I could see nothing to hold it in: no signs of Loctite, and no securing washer, etc. The lower part of the shaft had then fallen under its own weight (there being no restraint provided by the Austin’s dynamo end-casting), with the gear taking up a position where it jammed with the worm gear (on the end of the dynamo shaft) which drives it. Cranking the engine had then resulted in both gears getting “chewed up” (see photo) to the point of needing to be scrapped. I rectified the fault by fitting a new worm drive-gear and substituting my original distributor (type DJ4). The absence of lubrication of the distributor shaft is potentially a cause of “pickup” on the shaft, which could contribute to its failure as here described. Ian Milford of Milford Vintage Engineering – himself a long-time user of a Seven – after reading the foregoing he has given me permission to quote him: “The VW distributor drive is of a slot form and so has no thrust applied in either direction (up or down), but our beloved Austin 7, owing to the skew gear, applies thrust to the distributor which is what is causing the destruction of the press-fit shaft. All the Bosch units are at risk unless the press-fit plate is welded or brazed into position. This, with the aforementioned lack of a lubrication, makes the upgrade dubious at best.”

“Accurate ignition timing requires your timing marks to be correctly positioned and always checked at the flywheel, as this gives the most accurate readings. We are not expecting 100% readings as there are too many variables, called gears, between the crankshaft and the distributor - all we can do is try to eliminate as many as possible. On all distributors maximum advance will vary by a few degrees due to manufacturing tolerances and, as the Bosch 009 distributor was produced in several countries, the advance range does vary. However, this is not a problem as we intend to set the timing at the maximum centrifugal advance. If you have checked ignition timing with a strobe timing light you will have noticed the timing mark jumps about to some degree. This is because we have five gears between the crankshaft and the distributor. The first two, the crank gear drives the cam gear, and should have a small amount of backlash. Anything excessive will also affect cam timing, so either find a better pair or replace with new gears. The dynamo is driven from the cam gear with a skew type gear. You can improve the mesh of these by surfacing off the base of the dynamo housing. At the distributor end of the dynamo a worm gear drives the distributor gear. If these are worn, new gears are obtainable. To set the timing you need a reference point. The TDC 1/4 mark on the flywheel will need to be checked and marked at actual TDC with either a pointer or a mark on the crankcase. Why use an auto advance distributor? With fixed advance your timing will only be correct at one point in the rev range and will be over advanced at the start-up and low revs meaning excess loading on bearings and starter. The Bosch 009 distributor is a modern alternative for the Lucas originally fitted and can replace any of the Lucas types. It will require some modification as the drive and fixing are different. You will also need to add lubrication as it was originally lubricated by a breather. You may be able to buy a ready-to-fit distributor from some Seven suppliers with body machined for fixing clamp and drive gear fitted but no provision for lubrication. If your distributor is as Bosch supplied, this is what you need to modify:- Remove cap, rotor arm, spark shield, condenser and wiring, then remove screws retaining cap clips. This releases the points base plate. Removing the base plate reveals the centrifugal advance mechanism. Remove wire circlip from the offset drive peg and punch out the 4mm pin. There will be shims fitted under the drive peg. Now remove the shaft assembly from the distributor body. There are shims under the advance base plate - retain these. To fit the Lucas distributor clamp you need to machine off 3/16” (5mm) from the shoulder of the distributor body stem. This is to ensure the new drive gear meshes properly. There is a groove on the body stem that normally takes a seal ring. This groove lines up with the screw peg on the dynamo end. Drill a 1/8” (3mm) in the centre of the groove and clean out any metal particles. This hole is between the drive shaft bushes and provides a point of lubrication when a grease nipple is fitted in the peg thread hole. While you are machining off the shoulder for the clamp, machine a new 2mm ‘O’ ring groove above the original groove to prevent the grease oozing out from under the clamp. A new drive gear with a 4mm fixing hole can be obtained from your friendly Seven supplier but make sure it is a good fit on the shaft. Refit the shaft with base shims, oil bushes and shaft before fitting. Temporarily fit gear and pin. Measure gap between gear and body and fit appropriate number of shims. You might find a 4mm roll pin is easier to fit than the original solid pin. Re-assemble the points base plate, clips and condenser. Check points gap - .016/.018 thou. A smaller than recommended gap retards timing and shortens coil LT resulting in a poor HT spark. Too large a gap advances ignition and could result in a misfire. Make sure the clamp fits square on the top of the dynamo and is not bent. Light grease distributor stem and ‘O’ ring, also grease drive gears before fitting to dynamo. Fit grease nipple to dynamo and give a couple of shots of grease - any excess will pass through original lower lubrication holes and on to the drive gears. How do we arrive at this optimum advance setting? By a lot of testing on the road and rolling road tests, watching power and torque dials while advancing and retarding the timing? No, you don’t have to do all this testing! The figure we arrived at was 27 degrees max advance. This is easy to measure on the flywheel ring gear as each tooth is 4.5 degrees apart so six teeth before TDC gives you 27 degrees of advance. This figure works with any state of engine tune from standard to full race spec. To set up timing, set distributor at points just breaking TDC on No 1 cylinder, connect up strobe timing light, rev engine to about 3000 rpm. This fully advances the centrifugal weights. Turn distributor anti-clockwise until your advance mark lines up with the TDC pointer. That’s it! Don’t forget to tighten up your distributor clamp bolt. The timing setting is not so critical at idle or low revs, correct carburation being more important to give smooth transition from idle. But retarded or over advanced ignition at high revs can cause piston or valve failure.

“Having done several Bosch distributor mods myself, I was very interested in Terry Griffin’s article in the October mag. He’s absolutely right that the best way to set the max advance is with a strobe off the flywheel. However, there’s a slight oversight which means that he might not be getting the best from his mod. It’s true that the Bosch distributors vary a bit in terms of maximum advance from manufacturing plant to plant, and for peak performance, setting them with a strobe at high revs is ideal. However, unless you make a small additional modification to reduce their “swing”, the 27 degrees of advance travel will mean that at very low revs, the car will be grossly over retarded. The later Lucas distributors had about 8 degrees of travel, (earlier ones were 5 degrees). This means that with the right advance “on song”, the spark falls back 8 degrees at tick-over. Now, if you don’t reduce the “swing” on the Bosch, for the same timing point at high revs, the idle timing will now be 27 degrees further back, or in effect, 19 degrees FURTHER retarded than with a Lucas. It will tick-over nicely, be very quiet, but from take-off won’t pull the skin from a rice pudding! A bit more won’t hurt, but it’s advantageous to constrain the change to about ten degrees so that a fully tuned advance at high-ish road speed (say 25mph in 3rd in my Box, to lug us up a hill) doesn’t result in weak performance at take-off.

Having compiled this summary of articles, I thought it only proper that I try the Geoff Hardman mod with the insulated female electrical bullet connector on my Box. The job was straightforward enough, taking about _ hour, but a standard blue bullet wouldn’t fit over the post without being opened up with a flat-bladed screwdriver. I didn’t expose the flywheel to set the ignition properly and only used the “by ear” method of getting the fastest tickover by rotating the distributor. There does seem to be a slight improvement in pulling power and smoother running but only marginal. I’ll set the ignition more accurately when there’s more time and see if that makes a difference.