TYPICAL BRITISH TWINS CRANKSHAFT ISSUES WHEN REBUILDING BOTTOM END
Now if you are considering moving down stairs and looking to increase your knowledge or angst then read on. Basically, most of my experience comes by way of the Triumph ,BSA and Norton twins and triples. My first being at age 19 with 2 x A10 BSA engines laid out on the floor and figuring the puzzle into one goer. My old man Jack, himself a jack of all trades was able to assist with a small booklet that had some info. Ignition timing etc. I had watched dad build his GMC bus motor 4 yrs earlier. My first effort wasn’t the best of engines for sure but it did carry me around for a couple of yrs. I was a 3rd yr apprentice electrician at the time. So the main thing to remember about any work around alloy cases is that heat is a good friend. Belting bearings in and out of gearbox or engine casings cold is best left to the less enlightened. In my time i can’t remember the number of damaged bearing recesses I have encountered. This sort of brutality will cause things like bearings coming loose due to less than effective interference fitment.
So the odd puller is handy, a basic 3 bolt steering wheel puller helps with the primary drive sprocket, some 4” 5/16 and 3’8 unf bolts are handy, a puller for the clutch hub on twins and the triple has a special sized one for it. A 3 finger timing pinion puller is nice but they can be fragile, I have lent 2 in the past , never to be seen again. So in this instance, heating the case and removing the crank from the timing side half can be done with pinion in place then with a small wedge/pincer type puller the pinion can be removed. They are not usually that tight. I use my air rattle drive to remove the crank nut. On bush motors BSA etc, this may work but is determined by the size of the journal as under size bushes can stop the gear coming through with the crank. On Triumphs the pinion nut is right hand thread but the cam nuts are left hand. BSA timing pinion nuts are left hand also as is the Norton pinion/pump gear nut. Most others are righty tighty - lefty loosey but the Norton gearbox sprocket nut is LH. I bought a cheapie ¾ drive socket set yrs ago and then with the sizes required I welded ½” drive sockets to them .This allows use in cases where a shaft extends, like gearbox sprockets. Much better than most of the feeble so called factory tools available. I have deep sockets too but sizes are limited. Another home made tool I have is 2 x large sturdy G clamps with a 1” piece of old fork tube welded onto the fixed end and cutaway at different angles . These I use for a valve spring compressor. I had an over the counter style early on but it is in landfill somewhere. You may also wish to buy yourself at least one micrometer with a 1-2” range to measure the crank journals. Typically these journals are anywhere from 1 3/8” to 1 7/8”. One more thing, a torque wrench is handy, mine is a 3/8 drive for the less bulky sockets.
With crank out in a vise with jaw protection and conrods removed , some whitworth sockets most likely required, the journal can be measured . Check in a few places around the journal with the mic. Most wear happens at the top /bottom of the journal. The specs here allow about ½ thou, 5 tenths of a thou but a few more tenths is usually acceptable. How did the rods feel before dismantling, side to side play necessary but rock ? Hold the rod to one side at the base, with one finger and feel for rock at the top, this should be minimum and if the rod bearings are still in good condition and journal wear minimum to nil, then rock will also be minimum. There may be some rock and the bearing shells may be worn and not the journal, only a micrometer can tell for sure. Triumphs have a nice wide shells and it takes more wear to show rock than both Norton and in particular BSA, which have narrower shells. Funny as BSA have to rely on oil passage to the rods via a bush which can wear and therefore lose oil required by the rods. Sludge traps should always be cleaned regardless of the amount of wear and I use a ½” drive screwdriver blade, a big one, a Snapon piece. If you see a Snapon truck wave him over. Until I found this ½” driver they make, removing sludge traps was always a bit of a nightmare. More often than not drilling multiple holes and punching out the residue . I bought a 7/8unef tap to cleanup the thread. I have never had any trouble since and with a small drill to relieve the punch mark, the rattle gun and a little heat never fails. To remove the trap tube itself I got a tap and a nut , a couple of threads cut first into the tube then remove the top crank bolt and use the nut on the tap as a puller. The traps are always part or completely full. One crank I did recently from a ’70 Bonnie with about 12,000mile was very full. Obviously little oil changing went on by the original owner. The oil is your only filter without a spin on. This is why a removeable oil filter is a great thing. Nortons have a much bigger gallery than BSA or Triumph and in ’72 they added a replaceable filter. I have never seen a full Norton crank. Triples have small oilway drillings but have an internal replaceable filter inline before the oil gets to the crank. The drillings still need cleaning though , similar to the square fours.. Once the crank is clean, high pressure air, a can of carb clean and a long wire brush in the drill. Much gouging and scraping later and if the journals are on spec or it has been to the machine shop for a grind, then one can reassemble the tube and nut. A dab of loctite on top bolt but not on the tube nut. You may be the one to visit this again. Make sure the oiling holes on the journals are clean. With a Norton crank 6 x 5/16 studs and nuts or 3/8 studs and nuts on the late 850 crank removed, a clean is simple. Best to fit new studs and nuts here as they have usually been punched and this affects the threads. I have seen an engine of a friend who did his Norton and forgot to loctite or punch the clamping studs on assembly and it was a mess. Removing and replacing main bearings may require pullers on a Triumph and definitely on a Norton. I heat the case and holding the case with crank using welding gloves I tap crank nose downwards onto a block of wood on something solid , like a concrete floor. Likewise when removing a bearing or bearing race only, the jarring on a wood block and heat should get the outer ring or full bearing to drop. I use the BBQ to get a full heat and a bit extra around the bearing with MAP gas. A quick clean and while hot drop in the new bearing or bearing race and or crank assembly. If removing a roller race and it is recalcitrant the run a bead of weld in 2-3 places on the actual race. This should be enough to shrink it but if not then the weld gives something for a drift to find purchase. Often though the roller bearing is low mileage and will show little to no wear and may be reusable. They are good for 100,000 miles. Ball bearings are cheap by comparison and available at the local bearing shop. No need at all to put bearings in the freezer? With Nortons I use a 6306 metric 10-11 ball high capacity on the timing side , same as I use in Triumph twin 750 timing side. Never an 8 ball. This allows end float capture and saves piddling about with setting end float on 2 roller bearings. I have seen many Nortons in original low mileage condition that have lots of crank end float and make a loud clap or whack when revved as the crank slides across. These would have had end float set at the factory and need it done again at 10-12,000 miles. So a high capacity ball it is for me and they have almost the same dynamic rating as a roller. On a BSA if the timing side main is worn and in need of a new bush I will have the crank ground to .008 instead of .010 then set up the case in a mill machine after the bush is fitted and bore bush for .0005 that’s ½ a thou clearance . This gives a better tighter tolerance than grind .010 and fit .010 bush. If you have a bush made you may wish to clean up to whatever size you wish .003-.004 etc and not waste main bearing crank material. Lots of heat cycles and a couple of oil changes in the first few miles and a filter added is best here. Helps make sure the oil gets to the rods and not out the end of the bush.
Check the conrod housings for evidence of fretting ( caused by the shells moving) and may require the rods to be resized at the machine shop. Resizing is a must in BSA’s as the rod housing can and will show ovality because the factory didn’t bother to go with the more practised route of using steel rod caps until the last year of production. I use new conrod nuts where possible but unless the rod bolts are damaged I will reuse them, except on triples . I have had two triple engines fail both at idle thankfully, when a rod bolt broke. One was still factory fit as far as I know and the other is a ’74 I still own and had done a freshen up and had started 2-3 times, then while setting the idle it went CLUNK and stopped. A tear down revealed a bolt snapped in the middle. Not a thread thing, so this time I fitted Carrillo rods after having 6lb removed from the crank, then balanced and boy does it react to the throttle. So always new bolts and nuts at a minimum with Triples , for me anyway. Norton rod bolts are a sturdy 3/8” and new nuts is all I see the need for here. If racing I go to better quality chrome moloy H pattern rods anyway. Most 5/16 bolt tensions call for 18-24 lb and I have a basic rule, ¼” -10lb, 5/16” 20lb and 3/8” 30lb or less. It seems to work. The modern steel rods use measured bolt stretch or tension when setting up and are usually high quality ARP bolts. Rebalancing is normally a good idea. Dynamically if possible as this method can detect end to end out of balance, ie. rocking couple, something a static balance on parellel bars can’t. Desirable on wider/longer cranks like triples. I tend to balance most twins at about 74% unless an Atlas Norton which seems best at about 68%. Commandos are recommended at 52-55% depending on who you listen to. If no changes are being made to rod or piston weight, ie. oversize piston, and the balance ,ie. vibration was satisfactory when last ridden then all should be fine. If you are rebuilding an engine due to a holed piston for whatever reason, timing or fuel incorrect or both then the conrod under that burn through, should be replaced. Failure to do this may see the conrod fail later due to chrystalisation, I believe is the term. I have seen failed rods in this area. Also make sure any small end bushes are replaced if the piston pin shows rock. Badly worn pin bushes will make a tick/tick sound and will cause a rod failure. This I have seen. Triples, 750 10 stud short rod, late Tri 500 and Norton run directly in the rod and I have never seen a worn rod in the small end.
I don’t normally remove the cams unless replacing the cams if worn , the bushes or grit blasting the cases. Cam bushes will wear and when checking it is best to put the cases back together and feel for wear up and down. It will be obvious if wear is more than minimal. New bushes can be a pain to fit on the D/Side and I use a tap into the drive side bush then with a bolt , a piece of tube and a nut as the puller. The timing side is an easy drift. Heat the cases in and out. The drive side I use a set size ream and an adjustable ream can be used on the T/Side. Be careful. I have had an extension made for the adjustable ream to fit neatly into the D/Side bush and keeps alignment. The factory tool for the T/Side bush is a big ball bearing knocked through the bush to size it. I like to fit the cam wheels to the cams before closing up the cases for the final time. Set the cam end on something solid but soft to prevent damage. Use a socket to drift the wheel on. Bit of a juggle for sure. Pull on tools are also available for when cases are together as are pull off tools. Be careful not to damage the thread for the puller on pre Oil in frame models. If an early model I prefer to use the late model ½” wide timing gears. The narrow 3/8” wide gears can and do wear and cause valve train rattle. 68 to 72 are wide but hollowed out , good for performance upgrades. 73 on are full thickness and reduce rattle. Great in pre-unit engines as well. The pinion gear late type nut may need to be trimmed a bit to clear the timing cover though. Beside loctite on the crank top bolt I use it on the std type conrod nuts.
Be careful on assembly with the drive side main bearing. Make sure you have the right one. 650s use the 71-2879 as do 750s but the 650 has a CN or normal clearance. The 750 uses a C2 as the 750 has a tighter fit on the crank. Best not to mix as too tight a fit may cause a whine and long term damage.
Hope this bit of info helps.