Sal­vaging a knack­ered cylin­der head

Classic Motorcycle Mechanics - - WORKSHOP CMM -

Mak­ing do and mending is our stock-in-trade at CMM – es­pe­cially with rare or scarce parts. And it’s here that Stan has a plan…

As parts for clas­sic bikes get ever scarcer (be they new-old-stock parts or sec­ond-hand) we have to look at things slightly dif­fer­ently. A cylin­der head, which a few years ago would be con­sid­ered as scrap be­cause sec­ond-hand ones were plen­ti­ful, is now worth re­pair­ing. A head that’s just mildly dam­aged can be re-ma­chined fairly eas­ily. A head, which is badly dam­aged from say a big-end break­ing-up and the pis­ton ham­mer­ing the head, takes a lit­tle more work but it is eas­ily within the scope of your lo­cal welder and ma­chine shop, or your­self if you have ac­cess to a lathe. I will cover here how I re­claim a head – although I will say here and now that other en­gi­neers may well have their own meth­ods. The head I am us­ing in the pho­tos is an LC 350 head but the method will be the same on any twin cylin­der two-stroke head. The re­ally bad dam­age on a head is nearly al­ways around the squish band area where ei­ther the pis­ton has hit or de­bris has tried to be com­pressed. Any slight dam­age in the com­bus­tion cham­ber dome can usu­ally be cleaned up with emery cloth while the head is on the lathe. On a twin, one thing to re­mem­ber is that what­ever you do to one cylin­der you must do to the other. It isn’t pos­si­ble to ma­chine the head right down and then ma­chine the dam­aged area un­less the dam­age is min­i­mal. I would say if you have to take more than 0.5mm off the head face the com­pres­sion will be too high. The dam­age we are go­ing to tackle here would re­quire ap­prox­i­mately 2mm to clear. First of all the tops of the spark-plug holes have to be ma­chined to the ex­act same depth, this is eas­i­est done on a mill but if you are care­ful and ex­act it can be done on a pil­lar drill. Next, have the dam­age around the squish area al­loy welded-up by your lo­cal friendly welder. Now you need to ma­chine the weld back to the orig­i­nal di­men­sions. I have what is called a face plate on the lathe, it has many dif­fer­ent drillings and tapped threads on it for the dif­fer­ent heads I have ma­chined over the years. I have two spac­ers 50mm long and two bolts with a shaped domed spacer on each. I put the domed spac­ers on the bolts with the domed spacer into the com­bus­tion cham­ber dome, and screw and tighten the bolts into the threaded holes marked LC on my face plate. The rea­son for ma­chin­ing the plug-holes to the same depth is so that when bolted to the face plate the head runs true. I then ma­chine the min­i­mum off of the face of the head to clean the weld from the head face to give a flat sur­face to work to. If you want to keep the squish clear­ance the same as it was, mea­sure the depth of the squish on the un­dam­aged side.

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