ET: Engine (Twin) Black Shadow Crankcase Threads

R

RayF

Forum User
Non-VOC Member
Hi all from West Australia. I have been asked by a friend to do some machining repairs on a Black Shadow crank case. I have previously repaired a crank case that had suffered a conrod coming out so I'm not a total newcomer to these.
There are a number of issues with this one. The main bearing bores have previously been sleeved and the bearings are now loose. That looks straight forward enough. The issue I want to start with is one of the cylinder bolts has been stripped out of the alloy and it has been over sized a a plug screwed in and tapped to the bolt size. I'm having a problem with that size. Initially I thought it was 9/16 BSW which has a 16 tpi thread as standard, The stud itself, and it looks original is close to 35/64 and it has a 20 tpi thread. There is nothing in my thread charts that show it. Is there a special size tap available for doing these threads? If I can tap that size I have the option of making a new insert or TIG welding the hole and tapping it to bring back to standard.
 
greg brillus

greg brillus

Well Known and Active Forum User
VOC Member
Maughan's in the Uk supply a new stud that has an oversize lower thread that is 5/8" UNF an easy fix, if the thread ends up bigger, we tend to machine the hole larger and make up a brass insert with the inner thread back to the standard 9/16" x 20 TPI thread. Either of these work well.....I've seen heli coils used here but not a fan of them.......they tend to grab on to the stud and if removed it winds out the heli coil with it.......Cheers.
 
R

RayF

Forum User
Non-VOC Member
Thankyou for the good replies. I had never come across BSC threads here. Can we buy a 9/16 BSC tap? I can get one made as a one off if need be. The exististing thread in the crank case looks around 7/8 UNC. I'm thinking opening it as little as possible and tapping it to a UNF size,machining a plug that is tapped to 9/16 BSC.
There are other issues to be addressed on this crankcase that I will ask about as we get to them. Thanks again
EDIT, I just had a look at the link Marcus put up. Thanks
 
Last edited:
Generic Beardy

Generic Beardy

Well Known and Active Forum User
VOC Member
RayF said:
Thankyou for the good replies. I had never come across BSC threads here. Can we buy a 9/16 BSC tap? I can get one made as a one off if need be. The exististing thread in the crank case looks around 7/8 UNC. I'm thinking opening it as little as possible and tapping it to a UNF size,machining a plug that is tapped to 9/16 BSC.
There are other issues to be addressed on this crankcase that I will ask about as we get to them. Thanks again
EDIT, I just had a look at the link Marcus put up. Thanks
Click to expand...
N.b it's coarse cycle thread. 20 rather than the more common 26 tpi cycle.
 
H

highbury731

Well Known and Active Forum User
VOC Member
I had to look deep into this area, because my engine had a stripped cylinder stud thread. It pulled at 28 lb/ft, due to an old bodge done in the 1950s.
The standard cylinder stud thread is 9/16" + 0.006" BSF, 20 tpi. It is - shall we say - a little unusual.
I coresponded with an Australian member
(I think it was Albervin) who had got some helicoils made to 9/16", only to discover that it is +0.006". He got a special helicoil set made to suit. I understand that Bob Culver has the helicoils also, tho' he does it in two inserts one above the other.
A threaded insert is another option, usually done in brass. It is surely less strong than a good aluminium alloy, and therefore weaker than the crankcase, so I think a high tensile alloy might be a better choice.
I decided to go with an oversize cylinder stud. Maughans make one, with a lower end thread 5/8" UNF. They chose this size because it was the smallest oversize they could go to after cleaning out a stripped thread. They had the studs around the end of June when I bought mine, but was told that I would have to wait a couple of years for them to do the job. 'Try Conways'..... I took my nearly complete bottom end, they set it up in a mill and did the machining for me to take it away in the afternoon.
If your engine has already had a repair and failed again, a 'standard' oversize stud from Maughans is not going to work. The crankcase is going to be rather thin around the thread. The hole is VERY deep to be trying to fill with weld. I do not like the options. Decidedly not an expert, but I suspect you might have to drill out the oversize threads, cut open the side of the hole into the cylinder mouth and weld up. Being very careful to avoid distortion... Then machine the cylinder base area and drill and tap the stud hole to standard. That's a lot of work - but what else is going to work and be reliable?
Paul
 
Last edited:
R

rapide049

Well Known and Active Forum User
Non-VOC Member
RayF said:
Thankyou for the good replies. I had never come across BSC threads here. Can we buy a 9/16 BSC tap? I can get one made as a one off if need be. The exististing thread in the crank case looks around 7/8 UNC. I'm thinking opening it as little as possible and tapping it to a UNF size,machining a plug that is tapped to 9/16 BSC.
There are other issues to be addressed on this crankcase that I will ask about as we get to them. Thanks again
EDIT, I just had a look at the link Marcus put up. Thanks
Click to expand...
9/16 Admiralty fine
 
Last edited:
Top