5/16" cylinder barrel threads

[APRRSV]

Has anyone changed the 5/16" threaded holes in pre-Commando barrels to 3/8" to update and accomodate for later cylinder heads?
Are there any special considerations?

Thanks,
Ed

 

[dave M]

Has anyone changed the 5/16" threaded holes in pre-Commando barrels to 3/8" to update and accomodate for later cylinder heads?
Are there any special considerations?

Thanks,
Ed

I turned a G15 cylinder into a Commando cylinder by simply enlarging the holes and rethreading to take the larger head bolts and experienced no problems at all. The cylinder I used was the later version and did NOT have the spiggot on top.

 

[APRRSV]

Thanks Dave. I assume then that the 5/16 and 3/8 bolts have the same tpi? You didn't need to use helicoils?

Ed

 

[YukoNorton]

Thanks Dave. I assume then that the 5/16 and 3/8 bolts have the same tpi? You didn't need to use helicoils?

Ed

Hello Ed, assuming your pre-commando barrels are 750cc, here is some info that may be helpful: http://atlanticgreen.com/nhth.htm
"The head bolts have been down sized for atlas only to 5/16-26cei" and for Commando 750,"The head bolts have wisely been upsized back to 3/8-26cei". Other considerations: "The C-C head stud spacing, for the holes forward from the spark plugs, continues at 7-1/8" for all 750 heads".
If not 750 pre-commando barrels, "The Commando head also has slightly different centres for the four bolts adjacent to the spark plugs, so that it is necessary to 'pull' the holes to fit on a 500cc or 650cc barrel. I have seen this done once, but it can only be described as a bodge". https://www.nortonownersclub.org/support/technical-support-commando/cylinder-head-identification

 

[APRRSV]

Hello Ed, assuming your pre-commando barrels are 750cc, here is some info that may be helpful: http://atlanticgreen.com/nhth.htm
"The head bolts have been down sized for atlas only to 5/16-26cei" and for Commando 750,"The head bolts have wisely been upsized back to 3/8-26cei". Other considerations: "The C-C head stud spacing, for the holes forward from the spark plugs, continues at 7-1/8" for all 750 heads".
If not 750 pre-commando barrels, "The Commando head also has slightly different centres for the four bolts adjacent to the spark plugs, so that it is necessary to 'pull' the holes to fit on a 500cc or 650cc barrel. I have seen this done once, but it can only be described as a bodge". https://www.nortonownersclub.org/support/technical-support-commando/cylinder-head-identification

Thank you Yuko,
That info is very helpful.

Ed