Board index Flathead Power-Technical Questions, Answers, and Suggestions Knuckles 68 inch motor

68 inch motor

Moderators: Curt!, Pa

Post Fri Nov 20, 2015 6:00 pm

Posts: 57
Is a 68" motor an EL bore and an FL stroke, or is it the other way around?

Post Sat Nov 21, 2015 7:15 am

Posts: 1634
The strokes appear to differ by 2/16", about 3mm, bores differ by 15/32" . Quick fag-packet arithmetic suggests that 68" would be FL bore, EL stroke.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sat Nov 21, 2015 8:27 am

Posts: 1660
Location: Interlaken, NY USA

Right, 61" stroke, 74" bore, and about 5:1 compression with regular 74 pistons! my 47EL came that way, took it up and down the block just to see if it would run, which it did. Can't remember what the performance was like, I was just so tickled that it actually ran and moved itself :-)
DL

Post Sat Nov 21, 2015 9:10 am

Posts: 57
Thank you both. I'll now be looking for 74" barrels.

Post Sun Nov 22, 2015 3:06 am

Posts: 1634
Good point about the compression. 3mm extra height on the combustion chamber would do the compression no favours at all. I'd want at least 7:1 compression on any ohv engine.

Personally I very much liked my 61" pan head; if I had one bike back that was lost and gone, that would probably be it. Very smooth, easy starting and not really any slower than a 74", although two up or fully loaded with baggage might change that
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Mon Nov 23, 2015 10:43 pm

Posts: 2684
Location: Los Angeles, CA
H-D actually built 68 inchers. They were used in the rare Military TA model.

Post Tue Nov 24, 2015 1:55 am

Posts: 1634
The military would probably have been happy enough with 5:1 compression.

To put that in perspective, compression ratios between 5.4 and 6 to 1 were fairly common on older "utility" ohv engines, 7:1 was a "sporting" compression ratio by 1930s standards
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Tue Nov 24, 2015 9:05 am

Posts: 1660
Location: Interlaken, NY USA

Or, you could buy yourself some T&O 4 1/4 wheels and make yourself a nice little stroker, keeping the jugs you have! I did that with my EL, (used UL wheels), just used some S&S stroker pistons to get the deck height right. If I recall rightly, I used pistons that should have given 10:1 compression to get a more reasonable 8:1. Been awhile, (30 yrs), so I'd make some measurements and talk to S&S for confirmation on that! Point being you don't need to replace your jugs.
DL

Post Tue Nov 24, 2015 2:05 pm

Posts: 57
Thank you. This bike will be pulling a sidecar. A stock 74 is probably as good a choice as any. I considered 68 because I thought I might already have the pieces, but I don't.


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