Tech Torque

The last episode of our Bang for Buck ar­ti­cle saw the mo­tor fi­nalised for long term use and in par­tic­u­lar a change of pis­ton and a long stroke crank added, up from stan­dard 58/107mm rod to 60/110mm, with con­fir­ma­tion of re­sults. But it is im­por­tant to kno

Scootering - - Contents -

Bang for Buck tun­ing: Part 6 of the SIL200 tun­ing de­vel­op­ments – this month long-stroking the Lambretta.


To try and keep the en­gine re­li­able at this new found power level, a de­ci­sion was made early on to swap out items that on oc­ca­sion ex­pe­ri­ence has shown to fail. With the mo­tor re­moved and on the bench a full strip, in­spec­tion and re­build was done. The gear­box end of the mo­tor had proven fine with good gear se­lec­tion, the gear arm was shimmed well and the gear­box shim­ming was good also. The wheel had no ex­cess move­ment, in­di­cat­ing the rear hub end bear­ing was good. The gear clus­ter bush was re­moved though and the al­loy shav­ing of swarf, which oc­curs on fit­ting at the fac­tory, was also re­moved.

The case was dressed off at the bot­tom to en­sure its re­place­ment didn’t re­peat the process, a new bush was fit­ted with a new nee­dle roller, end-plate studs and dow­els were re­moved and Loc­tited in and set to the cor­rect height flush with the nuts to stop the chain catch­ing. The wish­bone was checked and de­burred and its bolt head checked for good clear­ance on the case be­fore re­fit­ting with a drop of Loc­tite.

The layshaft was re­moved to check the O-ring was fit­ted and okay and a de­burr and clean-up of the edges of both the layshaft and slid­ing gear se­lec­tor was car­ried out. The gears se­lected nicely with a pos­i­tive ac­tion which was pre­cise yet light, so this was left alone on the stan­dard se­lec­tor spring. An up­grade to a stiffer spring is done at this stage if it shows any risk of jump­ing out of gear. The end plate was re­fit­ted with new nuts and spring wash­ers, also with a drop of Loc­tite.

The clutch re­ceived a de­burr of its com­po­nents and was swapped out for a five-plate item which has proven to be good up to 25bhp when used with the right springs. The chain had al­ready stretched and be­come loose and would con­tinue to need ad­just­ment so was swapped for an IWIS 81 link item, and a 19 tooth sprocket, giv­ing the gear­ing in­crease which is ad­vis­able on the SIL GP 200 gear­box, pro­vid­ing a more re­laxed cruis­ing top speed. The SIL front sprocket bolt that has the sep­a­rate washer of­ten ‘dishes in’ so was swapped for a re­made one-piece item.

The orig­i­nal pressed steel chain guides were also re­placed with a pull-down ten­sioner. The crank­shaft at 60 stroke with 110 rod re­quires ei­ther the case ma­chin­ing in places or some­times all the way round to clear the back of the con rod, or the con rod ma­chin­ing down to sit near to flush with the crank webs. This has to be done with the crank split; ask about this when buy­ing a crank if you don’t want to get in­volved with do­ing too much work on the cases.

The crank bear­ings and seals were re­placed with high qual­ity items along with a new oil seal plate and fix­ings to re­place the SIL al­loy item which dis­torts when over­tight­ened. The orig­i­nal steel oil seal plates are best and have been re­made by many sup­pli­ers but a warn­ing here: they are not all to the same di­men­sions, so upon assem­bly great care is re­quired; check the cen­tre hole sits an equal

dis­tance left/right from the bear­ing and try one of each of the four po­si­tions it can be fit­ted to as one is of­ten bet­ter than the rest. The next is­sue is the oil seal plate gas­ket which, upon fit­ting the oil seal plate, must com­press the gas­ket.

This can be checked be­fore by mea­sur­ing the re­bate depth down to the bear­ing face then mea­sur­ing the oil seal plate spigot and do­ing a quick cal­cu­la­tion. You can find the di­men­sion to be for ex­am­ple 0.4mm depth, so a 0.5mm gas­ket will be com­pressed okay but some­times it’s around 0.8mm, mean­ing a 1mm gas­ket is re­quired. Al­ter­na­tively, do a dummy build of the plate with a small blob of plas­ticine or sim­i­lar to check the gas­ket is def­i­nitely be­ing nipped or check for the gas­ket leav­ing the bear­ing maker’s name em­bossed into it if you fit the bear­ing that way up be­fore­hand.

The plate was as­sem­bled and the gas­ket gave a guar­an­teed petrol re­sis­tant sealant around all sur­faces where there’s a gap. Af­ter see­ing many leak-down tests fail in this area, it’s worth the ex­tra ef­fort now rather than re­build it again later. The mag hous­ing was re­built with new bear­ings and seals and the mag gas­ket re­placed with one of an ap­pro­pri­ate thick­ness to give an even clear­ance on the crank webs to cas­ing.

One last item that’s re­ally im­por­tant – we found on our SIL test mo­tor that the short crank­case studs which se­cure the mag hous­ing were made of cheese so all three were re­placed. With the mo­tor built up, the top end was fit­ted again and more cal­cu­la­tions were needed as the crank and rod length change throws up lots of things to re­con­sider that are not al­ways ob­vi­ous.

The top end’s pre­vi­ous set-up on the 58/107 crank was as fol­lows: ex­haust du­ra­tion 188º, trans­fer du­ra­tion 126º, in­let du­ra­tion 149º and blow­down 31º.

If you built the mo­tor on the 60/110 and ad­justed the base gas­kets and pack­ers un­til the same squish clear­ance was achieved as be­fore, it would be as fol­lows: ex­haust du­ra­tion 193º, trans­fer du­ra­tion 134º, in­let du­ra­tion 145.5º and blow­down 29.24º. So a straight swap and build to this spec would not be a fair com­par­i­son of crank stroking.


On a 60 stroke, the pis­ton trav­els an ex­tra 1mm up and ex­tra 1mm down than the 58. If you ad­just the cylin­der height so that the pis­ton sits 1mm higher than it was pre­vi­ously, then it al­most cor­rects the ex-du­ra­tion to near what it was be­fore, as it’s near the top of the crank’s swing. But the trans­fer du­ra­tion doesn’t follow as close, be­cause they sit lower down the bore and fall onto the larger swing of the crank’s stroke, so have a greater change com­par­a­tively. The in­let be­ing nearer the bot­tom of the stroke again re­mains close to what it was on the 58 build.

So with the bar­rel height ad­justed and pis­ton sat 1mm higher than pre­vi­ous we get as close to pre­vi­ous fig­ures as pos­si­ble: ex­haust du­ra­tion 189º, trans­fer du­ra­tion 129.5º, in­let du­ra­tion 149.19º and blow­down 29.75º.

So what other changes oc­cur as a re­sult of stroking?

The pre­vi­ous 58 set-up was 198.43cc and the new 60 set up makes it 205.27cc so we now have a larger cu­bic ca­pac­ity. This, along with the mild ex­haust port du­ra­tion change, now al­ters the com­pres­sion ra­tio de­spite run­ning the same squish clear­ance. On a 58 stroke it was 10.5-1 un­cor­rected tdc. and 6.07-1 cor­rected on a 60 stroke it be­comes 10.87-1 un­cor­rected and 6.23-1 cor­rected.

With the 60 stroke and 110 rod we also add around 3mm of base packer so this in­creases the to­tal crank­case vol­ume by around 12cc, the length of the trans­fer duct is ex­tended by 3mm, and the en­try into the trans­fer feeds from the base of the crank­case will have a larger open­ing for the charge which re­sides be­low the pis­ton crown to travel through.

The longer con rod length sits the pis­ton crown higher in re­la­tion to the case so more vol­ume is cre­ated be­low it also. Com­bined with a dif­fer­ent an­gle it pushes the pis­ton up the bore, chang­ing the thrust ap­plied to the pis­ton faces. So with all these vari­ables it’s dif­fi­cult to at­tribute which one has pro­vided any dif­fer­ence in per­for­mance, be it for bet­ter or worse.

To com­plete the build a re­place­ment pis­ton was to be used (an­other change) made by Woss­ner for AF Rayspeed at 66mm. They pro­duced them for use in Rapido kits with the same crown pro­file but upon in­stalling I found a sur­prise that these ex­tra wide trans­fers on the SIL bar­rel meant that the bot­tom ring peg po­si­tion was not able to be used, so de­cided to run it on a sin­gle ring un­til a re­place­ment could be sourced (note: AF Rayspeed now has a new batch of 66 pis­tons with re­vised peg po­si­tions for use on kits that may be tuned with wide trans­fer ports).

So fully built and leak down tested, back in the chas­sis it was now time for its last dyno test. As you can see from the graphs, power and torque has gone up slightly and moved the power curve over to the left. This demon­strates an ear­lier low-down power de­liv­ery, but crop­ping a lit­tle at the end of the run again as was seen on the last com­pres­sion in­crease.

The re­sult is typ­i­cal of what I see on the many oc­ca­sions a change to longer stroke is ap­plied along with the other vari­ables that go hand in hand with this mod­i­fi­ca­tion. The net ef­fect is a small in­crease in power, de­liv­ered ear­lier. That’s all for this month.

Words & Images: Dar­rell Tay­lor

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