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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Black Shadow Crankcase Threads
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<blockquote data-quote="highbury731" data-source="post: 176962" data-attributes="member: 1557"><p>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.</p><p>The standard cylinder stud thread is 9/16" + 0.006" BSF, 20 tpi. It is - shall we say - a little unusual.</p><p>I coresponded with an Australian member </p><p>(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.</p><p>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.</p><p>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.</p><p>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?</p><p>Paul</p></blockquote><p></p>
[QUOTE="highbury731, post: 176962, member: 1557"] 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 [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Black Shadow Crankcase Threads
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