Don God­den

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…

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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!

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 RIGHT: Cam fol­low­ers get the pol­ish­ing and re­liev­ing treat­ment, too. It’s all about stress re­lease.

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.

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 it­self to bits.

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:

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