NOR­TON'S UN­BUILT TWIN RE­BUILT

From time to time, tales sur­face con­cern­ing the great What-Ifs of the old in­dus­try. One such is the story of the unit-con­struc­tion Nor­ton heavy twin. An­thony Cur­zon has a cou­ple of the en­gines, and Richard Ne­gus re­built one. This is his story…

Real Classic - - Front Page - Photos by An­thony Cur­zon, Richard Ne­gus

The first cylin­der supplied to me, when ex­am­ined closely, turned out to have been fit­ted with a thin steel liner and, more wor­ry­ing, ap­peared to have a fault in its cast­ing. Some of the cool­ing fins had sec­tions that were not ac­tu­ally joined to the cylin­der sec­tion and would not have been very ef­fec­tive in dis­si­pat­ing heat. I couldn’t imag­ine any log­i­cal rea­son for this ar­range­ment, ex­cept an er­ror ei­ther by the pat­tern-maker or by the foundry per­son­nel.

Af­ter a brief dis­cus­sion with An­thony, another cylin­der ar­rived, and was ob­vi­ously of an im­proved de­sign and car­ried a later cast­ing part num­ber. This cylin­der too had been fit­ted with a steel liner, lead­ing me to be­lieve that these had been sal­vaged when orig­i­nal ma­chin­ing re­vealed poros­ity in one or both main bores. The ‘good’ cylin­der was Aquablasted and pow­der coated gloss black.

The cylin­der em­ploys stan­dard Nor­ton twin cam fol­low­ers and guide plates. Slight ma­chin­ing er­rors in the guide plate lo­ca­tions of this pro­to­type cylin­der re­quired the plate pro­files to be mod­i­fied to suit, for­tu­nately with­out af­fect­ing their func­tion. The pis­tons were fit­ted to the con­nect­ing rods and sup­ported at TDC with two hard­wood strips while the rings were fit­ted.

With ring gaps equally spaced around each pis­ton, the rings were then com­pressed with ring clamps, a gen­er­ous squirt of Torco MPZ assem­bly lu­bri­cant on ev­ery­thing, and then the cylin­der placed over the pis­tons. With the ring clamps re­moved the cylin­der skirts were al­lowed to rest one of the wooden strips while the up­per face of the base gas­ket was coated with Wellseal sealant. Wash­ers and nuts were then fit­ted to the cylin­der base studs, tight­ened evenly, and ex­cess sealant wiped away.

Un­usu­ally, the pis­tons pro­trude from the cylin­der at TDC but a check against the ar­range­ment draw­ing con­firmed that this is cor­rect.

Drive to the camshaft is via a con­ven­tional 3/8” pitch chain with 15 and 30 teeth sprock­ets. The lat­ter were made from off-theshelf sprocket blanks with the key­way slots filed by hand. With­out any doc­u­men­ta­tion to sug­gest the in­tended cam tim­ing for this en­gine, I be­lieve the nd­cam tim­ing and pro­files of the newly man­u­fac­tured cam are the same as a stan­dard Com­mando; in­let and ex­haust pro­files are iden­ti­cal with a du­ra­tion of 304 de­grees. The en­gine was set up with a de­gree disc on the drive side main­shaft and an ex­tended dial gauge onto the cam fol­low­ers.

In the­ory, open­ing and clos­ing an­gles should be mea­sured at the cam fol­lower af­ter an ini­tial lift of 0.013”. As lift in this part of the cam pro­file is rel­a­tively gen­tle, I opted to mea­sure at the cam peak. Us­ing this method, in­let cam peak is cal­cu­lated at 102 de­grees af­ter TDC and ex­haust peak at 110 de­grees be­fore TDC. A key slot was la­bo­ri­ously filed in the cam sprocket and the fi­nal fig­ures of 94 de­grees and 107 de­grees were far closer than an­tic­i­pated.

In­stal­la­tion tim­ing, with the pis­tons at TDC, is align­ment of a stamped arrow ( " ) on the large sprocket with a stamped ( ! ) on the ad­ja­cent crank­case cast­ing. With­out an in­ter­me­di­ate gear and only one key slot in each sprocket, this drive is very sim­ple to in­stall cor­rectly.

Re­pair work on the cylin­der head, to re­place bro­ken fins, had al­ready been com­pleted but, in my opin­ion, the fins be­tween the ex­haust ports were a sig­nif­i­cant re­stric­tion to air flow. As an aid to in­creas­ing air flow to the cen­tre rear of the head, the up­per four front fins were ma­chined away and dressed with a file to look as though they were ‘as-cast’. The com­plete head was then Aquablasted to give a uni­form fin­ish and

thor­oughly washed in sol­vent.

With valves lightly ground to their seats, they were in­stalled with stan­dard valve springs and, learn­ing from later Com­mando de­vel­op­ments, oil seals fit­ted to the in­let guides. The head is se­cured to the cylin­der by a to­tal of ten fas­ten­ers, eight of which are 3/8” x 26 TPI bolts and the re­main­der 3/8” studs, as used on the At­las and Com­mando. With a very light smear of Three Bond sealant around the pushrod tun­nels, the cop­per head gas­ket was placed in po­si­tion and the cylin­der head fit­ted. The bolts and nuts were se­quen­tially tight­ened to 30 lbs.ft. With the rocker­box as a sep­a­rate assem­bly, there were none of the usual Nor­ton pushrod align­ment wor­ries.

At­tempt­ing to as­sem­ble rock­ers into the rocker­box re­vealed that ma­chin­ing of the cast­ing was in­com­plete. The rocker spin­dle bores were fin­ish ma­chined but the in­ter­nal faces were still rough-cast. Even worse, ac­cess to these ar­eas is very re­stricted and im­pos­si­ble to ma­chine us­ing con­ven­tional tools. As would have been done for ev­ery Dom­i­na­tor-type en­gine, tool­ing with de­mount­able cut­ters on an ar­bor through the rocker spin­dle bores was de­signed and made by my lo­cal ma­chin­ist.

In the belief that the rocker­box was now ready for assem­bly, it was placed on the cylin­der head – to re­veal another chal­lenge. There was in­suf­fi­cient nd clear­ance in­side the rocker­box to ac­com­mo­date the in­let valve spring col­lars. Work­ing with a ro­tary file, me­tal was cau­tiously re­moved to pro­vide suf­fi­cient clear­ance with­out break­ing through to the out­side of the cast­ing.

Hav­ing rec­ti­fied that de­fect, it was then noted that another ma­chin­ing fea­ture in­side the rocker­box had bro­ken through into one of its fix­ing bolt holes. As this could be a po­ten­tial source for an oil leak, the area was thor­oughly de­greased and filled with ‘Dev­con’ epoxy/alu­minium putty.

With that chal­lenge over­come, the next was not un­ex­pected. The rock­ers, in­di­vid­u­ally made by Mal­colm from EN24T bar and handfin­ished, needed mi­nor ‘tweak­ing’ to align each ad­juster with its valve stem. This was done by a lit­tle ‘per­cus­sive en­gi­neer­ing’ us­ing a cop­per-faced mal­let with each rocker firmly clamped in the ma­chine vice. Three rock­ers have near-per­fect align­ment with their valves, but the right-hand in­let rocker was well out of align­ment and needed re-man­u­fac­tur­ing as the dis­tance be­tween spin­dle and tap­pet ad­juster bore was 3.00mm too short. With this new ex­tended rocker, align­ment with the in­let valve was cor­rected. Both ex­haust rock­ers, af­ter ad­just­ment, were spot-on. Oil feed to the rock­ers is via a sin­gle cen­tral union and in­ter­nal drill-ways in the rocker­box to the in­ner end of each rocker spin­dle.

The ar­range­ment draw­ing for this en­gine shows the pri­mary drive hav­ing a Dom­i­na­torstyle clutch with a 3/8” pitch du­plex chain. Nei­ther en­gine sprocket nor clutch drum was avail­able, so the de­ci­sion was made to re­vert to stan­dard Dom­i­na­tor parts and ½” pitch sim­plex chain.

With a stan­dard 41 tooth clutch drum, 19 tooth en­gine sprocket, and a 64 pitch chain, ten­sion was per­fect, and any wear would be ac­com­mo­dated by the ten­sioner. Fur­ther, align­ment of the en­gine sprocket and clutch sprocket was also per­fect, lead­ing me to be­lieve that it was orig­i­nally de­signed with a sim­plex chain be­fore be­ing re­placed with the du­plex.

Un­for­tu­nately, fit­ting the alu­minium in­ner chain­case proved this not to be the case as the clutch back­plate fouled that cast­ing. Look­ing more closely at the ar­range­ment draw­ing re­vealed that the clutch back­plate had am­ple clear­ance from the in­ner chain­case be­cause the gear­box main­shaft was some 3/8” longer than stan­dard Dom­i­na­tor. Us­ing a Com­mando main­shaft and a lit­tle ‘en­gi­neer­ing’ in the clutch hub pro­vided the req­ui­site clear­ance but, ob­vi­ously, now the clutch sprocket did not align with the en­gine sprocket by 3.15mm. With the en­gine sprocket fit­ted on a ta­per, the only so­lu­tion was a new, non-stan­dard, en­gine sprocket.

Us­ing fine grind­ing paste, the new en­gine sprocket was lapped onto the crankshaft ta­per and the clutch hub shimmed on the gear­box main­shaft to align the sprock­ets. With the chain and gen­er­a­tor ro­tor now in­stalled, fi­nal assem­bly of the pri­mary drive was com­pleted.

To al­low for stro­bo­scopic tim­ing of an elec­tronic ig­ni­tion, tim­ing marks were scribed on the gen­er­a­tor sta­tor ev­ery 10 de­grees up to 40 de­grees BTDC. There is no pro­vi­sion in the pri­mary cover for check­ing the ig­ni­tion

tim­ing with a stro­bo­scope. Any such check will re­quire the cover to be re­moved.

With the oil pump now fit­ted, a goodly smear of Three Bond was ap­plied to the tim­ing cover joint face, the gas­ket placed in po­si­tion, another smear of sealant ap­plied to the gas­ket outer face, and the cover placed in po­si­tion. Af­ter check­ing that the nd camshaft oil seal had not in­verted, the fix­ing screws were fit­ted and then tight­ened evenly. Ex­cess sealant was wiped off the out­side of the joint face.

The gear­box se­lec­tor plate was also fit­ted, with a gen­er­ous amount of sealant, and op­er­a­tion of the gearchange ratchet mech­a­nism checked be­fore this had time to cure. Ini­tially stiff, the gearchange ac­tion im­proved af­ter a good squirt of oil over the mov­ing parts.

No in­for­ma­tion is avail­able re­gard­ing ac­tual ig­ni­tion tim­ing for this en­gine, so the Pa­zon was set to 31 de­grees BTDC as rec­om­mended for an 850 Com­mando. This seems rea­son­able in view of the sim­i­lar com­bus­tion cham­ber shapes and com­pres­sion ra­tios.

Miss­ing from the gear­box is a fea­ture which is stan­dard on al­most all Nor­tons – the gear po­si­tion in­di­ca­tor. The gear se­lec­tor mech­a­nism com­po­nents are mostly stan­dard Nor­ton and it would have been sim­ple enough to in­cor­po­rate the in­di­ca­tor. Progress per­haps?

An ob­vi­ous de­sign omis­sion in this en­gine is that, un­like the Com­mando, there is no pro­vi­sion for an in­te­gral tun­nel for the con­tact breaker ca­ble. For this en­gine, the ca­ble has to pass over the out­side of the tim­ing cover, some­thing which, I’m sure, would have been cor­rected had the en­gine reached pro­duc­tion.

One of the things that puz­zle about this en­gine is that nei­ther side of the crank­case makes me think ‘Nor­ton’. The pri­mary side quite clearly looks like a BSA, but the tim­ing side is an amor­phous lump quite lack­ing any recog­nis­able de­sign fea­tures. How­ever, add a bal­ancer shaft in the usual Nor­ton camshaft po­si­tion at the front of the en­gine, driven by an in­verted-tooth chain, and you have a unit con­struc­tion en­gine with strong de­sign clues to its pre­de­ces­sors. For con­fir­ma­tion, take the ex­am­ple of the mod­ern Nor­ton 961 en­gine

which, de­spite be­ing a ‘clean-sheet-of-pa­per de­sign’, is in­stantly recog­nis­able as a Nor­ton Com­mando by virtue of its for­ward-lean­ing cylin­der and the shape of its tim­ing cover. A bal­ancer shaft would re­duce the peren­nial prob­lem com­mon to all rigidly-mounted par­al­lel twins – that of vi­bra­tion ex­pe­ri­enced to a greater or lesser de­gree by the rider.

To place the trans­mis­sion close to the en­gine, it was nec­es­sary to ro­tate the gears and se­lec­tor set by 45 de­grees. One per­haps un­wanted re­sult of this is that the kick­start shaft is now higher than be­fore, higher than the crankshaft even, and may be dif­fi­cult for the av­er­age rider to op­er­ate. This will be­come clear once the en­gine is in­stalled in a chas­sis.

Fi­nally, it is said that the orig­i­nal pro­to­type en­gine seized when run­ning on the test bed and seized again when be­ing rid­den on the road by Fred Swift. Why that should hap­pen is not recorded, but over­heat­ing may have been the cause. Cer­tainly both cylin­der and cylin­der head of this en­gine have had con­sid­er­able work done to im­prove the flow of cool­ing air through the fins.

I see sev­eral pos­i­tive en­gi­neer­ing fea­tures in this en­gine and, had it pro­gressed suc­cess­fully through de­vel­op­ment and had some de­sign in­put, it could have been the ba­sis for a new fam­ily of Nor­ton mo­tor­cy­cles. But that wasn’t the de­ci­sion taken 55 years ago at a time when I was still at school, study­ing for GCE O-level ex­ams. Now back in the real world, and the en­gine as­sem­bled as far as pos­si­ble, thoughts turned to the cy­cle parts and, par­tic­u­larly, what might the com­plete mo­tor­cy­cle have looked like if the pro­ject had been car­ried through. None of the pro­ject’s se­nior de­sign staff now sur­vive so we will per­haps never know what they had in mind. Cue op­ti­mistic me again, with a crys­tal ball, soft pen­cil, and those empty fag pack­ets.

To be con­tin­ued…

An al­ter­na­tive bar­rel was supplied by Antony Cur­zon, and was re­fur­bished ap­pro­pri­ately

The first cylin­der bar­rels had se­ri­ous cast­ing faults The bar­rel uses cam fol­low­ers in the usual Nor­ton twin way, and re­quired only gen­tle re­pair work in this area

Pis­tons at­tached to their rods and ready for the bar­rels. Which fit, care­fully, in the con­ven­tional way

One tim­ing chain ten­sioner You can work out where Richard re­moved me­tal from the front top four fins to aid air­flow over the com­bus­tion cham­bers. The com­plete en­gine shot shows the orig­i­nal ar­range­ment

It’s un­usual for pis­tons to pro­trude above their cylin­ders, but this is how the en­gine was de­signed. The sec­ond shot shows a dial gauge be­ing used to work out the valve tim­ing by mea­sur­ing the cam lifts. The tim­ing marks are now the ar­rows shown in the third pic. Just in case the en­gine ever needs re­build­ing… The head, from the rear, with valves and springs The four rock­ers were hand made by Mal­colm Sag­gers, and are true works of art If you look closely at the in­let rock­ers, you can see that a lit­tle ju­di­cious ‘per­cus­sive en­gi­neer­ing’ was needed to align rock­ers and valve stems The right-hand in­let rocker needed to be re­man­u­fac­tured to fit ac­cu­rately

The pri­mary side show­ing the ar­range­ment. Un­usual for a Nor­ton twin, and noth­ing like a Dom­i­na­tor! Tim­ing marks were made onto the al­ter­na­tor sta­tor to match up with the ro­tor The tim­ing side. Oil pump fit­ted (skew drive from the end of the crank) and ready for the cover The pri­mary drive, par­tially as­sem­bled. The de­tail shows the slip­per ten­sioner The points hous­ing has no ob­vi­ous pro­vi­sion for a lead, so Richard pro­duced a tem­po­rary de­vel­op­ment for the Pa­zon unit used in­stead of con­tact break­ers. Neat enough, but not en­tirely el­e­gant. It does, how­ever, work to spark the en­gine

Job done!

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