Does anyone know if I need to get the crankshaft balanced before re-assembling? And if so, any suggestions for where I can get this done near Swansea please?
What changes have you made from the factory build? Especially with the isolastics, they would surely need to be extreme before re balance is needed.
The previous owner opened up the crankcase mouths & did a 920cc conversion but it failed catastrophically in the Isle of Man apparently. Bent conrods, not good. But no changes to the crankshaft as far as I know thanks.
Are you keeping it as a 920 or reverting to 850 ?
The reason I was wondering about crankshaft balancing is that I watched a U tube video of a Commando engine reassembly (English chap but now living in the US) and he said that he was going to send the crankshaft off to an engineering company for balancing before reassembly. Not sure what his name was now but very popular instructional videos and he seems very knowledgeable.
Keeping it as an 850 .. it came with the 850 parts but the crankcase mouths have been opened up slightly … not sure if this is going to be a problem, any advise will be gratefully received, thanks
If your new pistons are much the same weight as OEM standard then leave it be, if they are heavier then get it rebalanced statically to the factory spec and then dynamically balanced to get rid of any rocking couple eg side to side inbalance. Regardless of what way you go get hold of some accurate scales and match the piston etc weights LH and RH.
You should probably reassemble the crank with the old fasteners and check it runs true as a first step, big blow ups can bend things! If the crank was rebalanced to suit the 920 pistons and rods then you might have to consider getting it re balanced again if you are going back to 850. this will probably only be necessary if the old heavy Lotus type pistons were used for the 920 or steel rods (these also have bigger diameter gudgeon pins). I had a crank ground and balanced at SRM recently, £460 with postage. Don`t bother reassembling with new fasteners before a regrind like I did as they strip the crank to clean it afterwards, bin the fasteners and fit new ones at extra cost. A good secondhand crank could be an option if yours has issues.
Many thanks for your feedback. I don’t know much about the history of the bike because I bought it from someone who was just storing it for his friend so it’s anyone’s guess whether the crankshaft was rebalanced to suit the 920 conrods.. judging by the rough & ready way the crankcase mouths were opened up I’m guessing they were not. so I was hoping I wouldn’t have to balance the crankshaft but obviously want to make sure it’s all good for years to come. I’ll have to find a local engineering company near Swansea that can check it out for me. Thanks again Al,
When finding someone local to you, please make 100%sure that they are familiar with British bike cranks. The volume of oil inside the crank pins when it is running is quite large and will affect any balance readings.
A check in the services section will suggest good companies. Someone like Norman White, RGM, Norvil, SRM etc. will know what they are doing. SRM aren't a million miles from you either. They will akso know that Commando's have a different balance factor to an Atlas for example.
Yes you will pay more, but you will end up with something that is right, not worse.
Enjoy the rebuild,
Brian, you're not my cousin are you? I have a cousin with the same name who used to have a Bantam in his youth and it's been some time since we spoke.
Thank you George … good advice on the balancing considering the volume of oil in there, much appreciated. Regards, Brian
I don’t think so .. I did have a Bantam in my youth but I’m from Hendy