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Hello Everyone, i am pretty puzzled by the cylinder head bolt lengths of pre unit 650 triumphs with alloy head. i ordered head bolts as listed in a 1961 650 parts book. the head bolts i ordered were 4 outer domed head bolts 70-2873 they have a under head length of 3 inch 3/4 long. also 2 cylinder head inner bolts with threaded extension for head steadies 70-1596A under head length 5 inch 7/8 long and two inner head bolts with no extension also under head length 5 inch 7/8 long. when the parts arrived the long inner head bolts has written on the label cyl-head bolt p/unit TR5 T100. i think this is a parts book mistake. it lists the iron cyl head with outer domed head bolts as 70-0327 which is the same as unit 650 triumphs. also the long inner bolts for the iron head are 2 cylinder head inner bolts with threaded extension for head steadies as 70-1596 which is the same as unit 650 triumph. and two inner head bolts as 70-1484 with no extension, the book quotes 5 inch 3/8 under head length but they are 5 inch 5/8 long which is probably another parts book mistake. which ones are correct for the 650 pre unit with a single carb alloy head fitted. does anyone have a bike apart where they can measure the under head lengths of the bolts and please let me know. this is battering my head. just as a note, i have a 500 close pitch fin alloy cyl head plus a alloy 650 single carb 8 stud cyl head and a unit 650 9 stud cyl head to compare with and the depth of outer head bolt hole is the same on all 3 cyl heads. i am pretty good on triumph knowledge but today might be a learning day and i am always finding something new even though i am approaching 60 years old. as stated above, any info gratefully received. cheers. billy.

Last edited by billy banger; 02/03/17 4:48 am.
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Part # 70-1956 and 70-1956A are different lengths. 70-1956 has an under head length of 5.625" (5 5/8") and 70-1956A is 5.875" (5 7/8"). The 1956A is used for alloy 500 pre unit heads only. The head washers are .095" thick so keep this in mind when mocking it up.
Here are the under head lengths for the .375" head bolts I have in my notes:
70-1956 5.625" (w/standoff)
70-1956A 5.875" (w/standoff)
70-0327 3.400"
70-1484 5.625"
70-2873 5.750"
Hope this helps out with figuring out what you need- Dan


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