HONDA SS125

Part two sees the en­gine stripped.

Classic Motorcycle Mechanics - - CONTENTS -

The Honda SS125’S mo­tor goes un­der the sur­geon’s knife. Steve Cooper passed the for­ceps and took the pic­tures.

in our first in­stal­ment of Project SS125 Honda the bike had been bro­ken down ready for a to­tal over­haul. Owner, Peter Spence, has al­ready told re­storer, Andy Jones, that he’s determined to use the bike the way Honda in­tended and that means it’ll be worked hard. So, it’s likely to be spin­ning its crank at ten grand plus speeds at some point its life. Now we know th­ese bikes are well en­gi­neered but who in their right mind would be beast­ing an un­known en­gine this hard? The bot­tom end is a ro­bust roller bear­ing as­sem­bly and should be very re­li­able but if some­thing lets go how easy is it go­ing to be to find new crank parts? Ex­actly and with that in mind the en­gine is off to see Dr Dave Jupp in his Home Coun­ties surgery: gloves on, tools out and crank up that cam­era Scoop!

2 5 5/ Cover off and clutch ac­tu­at­ing mech­a­nism re­vealed on the in­side of the case. Sur­round­ing the cam on the end of the crank are the two arms of the ad­vance/re­tard mech­a­nism. 6/ The ro­tor has been pulled off the crank with cor­rect tools and the sta­tor un­bolted. The in­su­la­tion and wiring of the lat­ter will be in­spected for dam­age later. 7/ In­ner cover re­moved the cam chain ten­sioner to the right of the chain and its ad­just­ing rod are vis­i­ble with the jockey wheel to the left. There’s also some ev­i­dence of blue gas­ket ma­te­rial at the crank­case joint.

8 8/ The cam­chain leads a rel­a­tively easy life and so it’s fit­ted with a sim­ple split link which makes the next job that much eas­ier. 9/ With the cam sprocket re­moved from the camshaft it’s now pos­si­ble to re­move the cam chain and pre­pare to lift off the cylin­der head. 10/ Just a few gen­tle taps and the cylin­der head is off. Apart from some car­bon build up all looks good. Note the hol­low metal dow­els on the head and bar­rels; we need to make sure they’re all ac­counted for. 3 2/ The top en­gine mount now needs to be un­done as it is fixed to the studs that run right from the top en­gine case, through the cylin­ders and up through the cylin­der head. 3/ The mount also acts as a baf­fle and breather sys­tem for the en­gine. Our first look in­side the mo­tor here shows no build up of oil/wa­ter con­den­sate, which is en­cour­ag­ing. 4/ With the cover off on the left-hand side we have ac­cess to the points and con­denser. The pe­riph­eral crosshead screws need to be re­moved to gain ac­cess the gen­er­a­tor sys­tem. Note the tim­ing plate which looks like many two-stroke sys­tems. 4

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11 11/ The cam fol­low­ers or tap­pets and their re­spec­tive spin­dles are zip tied to­gether; red for ex­haust, green for in­let. The camshaft it­self is sim­ply a snug fit on its mounts. 12/ The cam­chain’s rub­ber wheels are re­moved and checked for dam­age. High run­ning tem­per­a­tures and old oil rarely do syn­thetic ma­te­ri­als any favours. 13/ With the base nuts re­moved the bar­rel is lifted clear to re­veal two re­mark­ably clean stan­dard-sized pis­tons. This con­firms our sus­pi­cions that the en­gine has seen lit­tle use since a rebuild some time in its past.

14 14/ And here’s fur­ther cor­rob­o­ra­tive ev­i­dence. The orig­i­nal cross hatch­ing of the fac­tory hone can still be clearly seen so what­ever hap­pened to the mo­tor it wasn’t a seizure. 15/ Who­ever went in­side also cleaned out the cen­trifu­gal oil fil­ter as the few pieces of crud in here cer­tainly don’t line up with the bike’s doc­u­mented mileage! 16/ Over on the other side of the en­gine the side case is off. More ev­i­dence of blue sil­i­cone gas­ket sealer and some vile brown sludge in the bot­tom of the side case begin to raise con­cerns. 17/ With clutch plates re­moved the bas­ket is lifted off along with the oil pump which is driven via a bell crank off the back of it. There’s more brown sludge in here as well. 13

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The plan now is get ev­ery­thing cleaned up, mea­sured and checked ready for the rebuild. We’ve al­ready picked up on the fact that the valve stem oil seals have gone hard and th­ese were prob­a­bly let­ting oil seep by into the com­bus­tion cham­ber. The good news is that the crank is in top shape. It’s been said be­fore but it still holds true; pro­vid­ing th­ese early Hon­das are kept fed with fresh, clean oil they are likely to run for years. The pis­tons look like they have seen min­i­mal use and there’s no ev­i­dence of blow-by so tol­er­ances must be good. Also the rings are still amazingly springy, again sug­gest­ing lit­tle use. Ad­mit­tedly the pis­ton crowns are heav­ily car­bonised but per­haps the dead and dis­eased air fil­ter we saw in Part I may well be the rea­son for the ap­par­ently rich mix­ture. What­ever, the carb will need a look­ing at as well just to make sure. So, why was the en­gine taken apart and re­built with blue sil­i­cone sealer? Well, with a blend of cir­cum­stan­tial ev­i­dence and in­formed sup­po­si­tion there’s a the­ory. Given the sooty pis­ton crowns per­haps a pre­vi­ous owner sur­mised the bike was burning oil and this, al­lied to a pos­si­ble oilseal leak, trigged the so-called over­haul. The cases were taken apart to sort out the seals (the ones in place all look quite fresh by the way) and a set of new stan­dard pis­tons and rings were dropped in for good mea­sure. The grungy gloop in the bot­tom crank­case needs to be cleared, but once done we’re ready to rebuild the mo­tor. Stay tuned!

1 1/ Start­ing on the top end the long bolt that passes through the cam and holds on the cam sprocket cover is re­moved al­low­ing us sight of the cam chain.

18 20 18/ This says it all; blue sil­i­cone gas­ket sealer ooz­ing out be­tween the up­per and lower cases. How­ever, we’ve yet to de­ter­mine why the en­gine was pre­vi­ously taken down. 19/ With all the bolts re­moved the cases are care­fully tapped apart lib­er­at­ing the crank and trans­mis­sion. There’s loads of sil­i­cone ev­i­dent along with some high-qual­ity crud in the bot­tom of the gear­box cas­ing. 20/ Noth­ing ob­vi­ously wrong here upon first in­spec­tion: the crank is a pressed-up item and typ­i­cally over en­gi­neered. Honda was aware th­ese bikes would get worked hard; none of the bear­ings are ex­actly small.

21 21/ The gear se­lec­tor drum car­ries some ev­i­dence of mild cor­ro­sion, which is pos­si­bly con­sis­tent with damp stor­age, but it’ll be eas­ily cleaned up. 22/ Dis­gust­ing: there’s no other word for it. This stodgy oily por­ridge is in­cred­i­bly thick and te­na­cious. It’s been there for years; why wasn’t it re­moved when the en­gine was apart? 23/ Us­ing a twin-forked spring com­press­ing tool the valve caps are pushed down to fa­cil­i­tate re­moval of the col­lets. The cylin­der head is amazingly pe­tite and com­pact. 19

24 24/ And there are all the com­po­nents stripped out of the head. Again noth­ing looks like it has seen par­tic­u­larly hard work and ev­ery­thing should clean up fine.

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