Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties 13 fin cylinders

13 fin cylinders

Moderators: Curt!, Pa

Post Wed Mar 04, 2009 8:00 pm

Posts: 536
Location: Wa, USA
Does anyone have any experience with running 13 fin cylinders at .080" over stock 80CI bore? I know it is thin but will they run or would sleeving be the better option?

Post Thu Mar 05, 2009 8:22 am

Posts: 1642
Location: Interlaken, NY USA

Hi Woody, don't know about .80" over, but I did have a set of 13fin cylinders sleeved 35 years ago. One lasted 1/2 a season and then the base flange separated from the rest of the cylinder. Didn't break anything else, just kinda stayed in place due to the top mount, I guess. I'm still running the other cylinder, and it's now .030" over. I'd guess it's a crap shoot. Maybe someone else has better info,

Post Fri Mar 06, 2009 6:41 am

Posts: 3010
Location: Central Illinois, USA
I did one at .070" over and it did fine in FL heat pulling a hack, powdercoated even. Probably still running for all I know....

But at that point, I determined that it only had .070" left at the thinnest spot, measured with the simple caliper shown.
Even at .080" over it would have to be stronger than a sleeve, as the boring for the sleeve risks reaching 'daylight'.

The manditory technique for huge overbores is to apply a torqueplate to the base (and head surface on OHVs) for honing the last .0035" of clearance: The spigot will bend inward .003" between the base fasteners, and outward .003" near each fastener, so hold on to your Sunnen carefully!

PS: I need a 13 fin rear cylinder to make a pair...
Measuring the thinnest spot over each base fastener
THINBORE.jpg (81.53 KiB) Viewed 2645 times

Post Sun Mar 08, 2009 5:37 am

Posts: 90
Location: Norway
Look at this tread:

One possibility might be to sleeve down to 3 5/16" bore.

Post Mon Mar 09, 2009 11:52 am

Posts: 17
Location: Bowdoin Maine
I recently sleeved a pair of 13 fin cylinders using a readily available LA sleeve that was 3.5 inch OD and 3.225 ID. After sleeves were installed I bored and fit standard size NOS 74 inch pistons(3.312). It looks like a good repair but the engine has not yet been run. 13 fin cylinders have substantially more meat and wall thickness at the base of the cylinder than the 11 fin cylinder. This past year I saw a sleeved 11 fin cylinder break off at the base after just 3 or 4 minutes running time. When you compare 11 fin to 13 fin side by side there is a big difference in the amount of material at the base of the cylinder. If my engine is OK with the sleeves in it, then your engine should be fine at eighty over eighty, but your finished bore size(approxamately 3.502) will exceed the fit requirement for the LA sleeve which is a standard catalog item. So my preference would be to sleeve with a readily available sleeve while I still have the chance. In my opinion the 11 fin cylinder is not sleeveable if I have to guarantee that it will stay together....Bill

Return to Big Twin Flatties