Totop grass and long track racer Ddon Godden was interviewed for TheTh Motor Cycle in 1968 to see just how he managed to make a 35-yearold engine design work so well…
For an engine designed in the 1930s to still be the choice of a racer determined to win in the then ‘modern’ era of 1968, the basic concept of that engine must have been pretty good. At that time, The Motor Cycle’s man Vicwilloughby visited multi-time British grass track champion and talented engineer Don Godden to find out what the lad did to make those engines work way beyond the maker’s wildest expectations.
Immediately he found there were remarkably few changes from its inception to the latest 1960s model. The crankcases were cast in the magnesium alloy known by its trade name of Elektron, the con rod was either in Duralumin – another trade name – or steel. Willoughby went as far as to say the drawing used in the feature was an updated one from the 1930s and all the artist had to do was to erase the track carburettor and replace it with a Concentric carburettor, shorten the cylinder and con rod a little and erase a pushrod return spring. The feature went on to say the speedway JAP had the right sort of power to propel a speedway bike and rider off the line in the best time possible. That this power was flagging at a little more than 5000rpm, and all but gone at 6000rpm, was of little matter to a race on a track of 440 yards. The JAP motor was, while not the only one in use, dominating the speedway scene.
When the race track was 1000 metres long, however, it was a different kettle of fish and with the standard JAP motor Godden barely had enough power to qualify when he made his early forays into the scene in Germany. Clearly something had to be done and he set about it with a good measure of common sense, engineering talent and a deal of determination. He did it to such good effect that three years later he was challenging for the top spot in the longer format. This, thought Willoughby, was a story worth hearing and detailing in the paper. With a grass track career going back as far as 1953 Godden had earned himself nine British championships using equipment he’d built himself, and latterly by co-operating with Alf Hagon to provide a commercial JAP tuning service. This was the basis of the motor Willoughby went along to hear about. In 1968 the basic JAP speedway motor was available for £140, however for another £30 Godden/hagon would provide a ‘Gold Top’ engine, likely a reference to the gold top milk which was extra creamy and had foil in gold rather than silver; the ‘Gold Top’ JAP was reckoned to be the cream of the crop. To distinguish their highperformance engine from the rest, Godden and Hagon had the rocker covers anodised yellow – it’s just a shame the effect can’t be seen in black and white photos.
Starting with the standard JAP motor, Godden first addressed the cam timing which went from the inlet opening at 45 degrees before TDC, and closing at 62 degrees after BDC – the exhaust opening at 65 degrees before BDC, and closing
at 35 degrees after TDC. Using his own cam grinding jig Godden altered these timings to 60, 85, 90 and 55, vastly increasing the overlap, but this on its own wasn’t enough and modifying one part of an engine has an effect on other areas. What was likely to happen with such modification to the cam timing would be the majority of the increased fresh charge coming into the cylinder would be pushed straight out the exhaust valve unless the port shape was changed. Willoughby reported that Godden’s handiwork in the inlet port prevented this from happening. Other work, aimed at stopping valve float at high revs, included progressive rate valve springs and alloy pushrods rather than steel. With a motor now safe to 7000rpm rather than 6000rpm it was a useful increase in performance for the expert rider who it was reckoned could quite easily cope with the slight loss of flexibility from the modified unit.
Even this performance-enhancing work wasn’t enough for Godden’s 1000-metre engines, and the work needed to make the venerable JAP cut it with the opposition in Europe amounted to major surgery. Gone was any semblance of power below 4000rpm, but in its place was reliable power way beyond the 7000rpm of the ‘Gold Top’ motor. To achieve this, Godden has a seriously wild cam profile that increased the valve lift by 50% and needed flat-grinding on the valves’ edges or they would tangle at high speed. Allied to this extra lift, the inlet port was opened out from 11⁄ to 13⁄
8 8 which needs an Amal GP carb with twin-float chambers to feed in the fuel. Naturally, Godden felt the standard Dural con rod was not quite right for this wild engine and used a steel one. At this level of competition the search for exxtra performance is intense; any littl le increase in power is welcome and an nything which prevents power loss is worth investigating. For instance, th hough important for providing sparks to o the plug a magneto can absorb power. O Okay, only a little power, but far better it be available for race winning than not t. Apparently running the magneto at quarter engine speed rather than half engine speed releases that absorbed power. JAPWRINKLES When interviewing Godden for the earlier feature, Willoughby realised there was a vast amount of untapped information on making a JAP engine work to the best of its ability. He also, in conversation with Don, realised that while not everyone wanted to super-tune an engine to howl it round the European long tracks there were an awful lot of enthusiasts who wanted to go grass tracking, second-half speedway riding, hill climbing and sprinting with the engine and they would benefit from knowing how to screw the unit together. So, the pair collaborated and produced a ‘how to’ feature for the 500cc JAP motor.
A lot of the work was under the heading ‘common sense’, but even so it doesn’t hurt to go back to basics with such things. For the basis of this, and indeed all such features, it has to be assumed the reader attempting to follow the advice has a work space, tools and some knowledge. Taking each part of the engine in turn, Godden related his experiences of the robust unit and how it holds up in service. Taking the crankcases first the main bearings
run in steel rings, Elecktron not being an ideal material for the housing. These rings should be a good fit in the case, but if there’s any suggestion of main shaft misalignment on the crank then this can loosen the rings. Godden suggested Araldite adhesive to tighten them up again. On the main bearings Godden advised it was best to renew them each racing season or problems would occur. Also, the crank pin had a limited racing life and the advice here was to junk it after 25 meetings. The pin is where the big end runs and has a hardened surface for the rollers. Under racing conditions this surface breaks up and all sorts of nasty things can happen. If the big end has failed then the cause must be investigated or it will happen again – it could be the oil feed, or a blockage somewhere.
To replace the crank pin – and this is an indication of how enthusiasts seemed to be much more practical 50 years ago because these days the crank would be shipped off to a specialist – the article advised to hold the flywheel by clamping a large spanner in a vice, sit the crank nut in it and heave on the other but with a socket ground to remove the lead in radius. Then use three bolts to put pressure on the inside of the flywheels and then fix an old crank pin to the original and give the end a sharp tap to release the taper. As time goes on the flywheel tapers can enlarge, so JAP provided oversize crank pins to cope with this and allow the proper side float of the rod to be achieved. Assembling was regarded as a simple task merely requiring a new crank pin to be bolted in place, the rod with new bearing rollers fitted then the other flywheel fitted and the whole lot bolted up with new nuts. Any misalignment found in the truing jig was cured by bumping the assembly on a stout surface with a sheet of soft metal in place. Willoughby suggested the balance factor should really be checked, though only as a courtesy as it was unlikely to be far out – but you can bet Godden always checked his.
Godden stepped away from standard JAP practice and used a Mahle piston with a thicker crown which was useful for coping with high lift cams which would need the valve pockets deepening so the valves didn’t clout the piston on full lift. Also a departure from JAP practice was the closing-up of the ring gaps. Most people will know the piston rings form part of the sealing of the combustion process, and the ring gap is an important part of that. Too big a gap and the engine burns oil, too small a gap and the ring ends will touch when the engine is hot and seize. Using alcohol as a fuel means the engine runs a lot cooler, and Don found a smaller gap worked perfectly.
Naturally with a man dedicated to performance, there were also a few tips of use to those working on a budget and if replacing standard cams with high performance ones isn’t viable then perhaps Godden’s ingenious method of grinding back the rocker pad on the arm may be of interest. Also, Godden’s method of timing the ignition is sure to create some discussion; he did it by ear!
These poor things have a tough life in a race engine. ABOVE:
With spreader bolts opened out to create some tension, a sharp tap on the crank pin will split the flywheels. Lining up the flywheels before tightening the crank pin nuts. BELOW:
LEFT: Thhere is often a limit on howmuch lift a big valve can have before something touches. The flat on the rim means the valvesv miss.
ABOVE: Godden made his own special cogs for various applications.
ABOVE: In an ultra-high compression engine there is the danger of valves hitting the piston crown. This is bad, so those that know what they’re doing relieve the valve pockets so if it does tangle it’s square on and doesn’t bend the valve.
BELOW: When fettled to his exacting standards, Don’s JAPS were a match for the foreign competition on the long tracks of Europe.
ABOVE: A small of observation here – spot the crank pin… test worn TOP LEFT: Magneto timing and operation is modified in a Godden ‘Gold Ttop’’ engine.i TOP MIDDLE: Even valve guides can be usefully modified to increase reliability and power. TOP...
All case faces are prepared so they mate fully and prevent leaks. ABOVE: Don aimed to have the crank running as true as possible. If they’re not then the ride can be uncomfortable and it’s not good for race-winning power, plus the engine can shake...
If you think logically about howmany times the rockers open and close a valve and how heavy they are as standard, then lightening them makes sense as it gives all other components an easy time. BELOW:
ABOVE: Clean and tidy, not overly glitzy, and blisteringly fast… a bit like the rider really.
Each cam is for a different purpose, with grass, sand and long tracks requiring slightly different power. ABOVE: