Round fin vs square fin

[Anomaly]

For equivalent displacement motors (say an 850 or a 1000), what are the differences- anything that would/should influence the choice when shopping for a classic from the past?

[Scout63]

More knowledgeable minds will weigh in Anomaly, but in looking at the parts fiches it seems like big block Tonti drivetrains are all pretty similar. I’m partial to round fins aesthetically. That being said, there are so many beautiful square fins in your area.

[Aaron D.]

Round fins are chrome bores except for LeMans, which are iron.

Square fin are Nikasil.

[Anomaly]

Round fins are chrome bores except for LeMans, which are iron.

Square fin are Nikasil.

Not sure about that as a blanket statement-- T4s are round fin, nikasil as far as I know. In any case, aside from the bore issue, other considerations? Is it just aesthetics after that?

[Antietam Classic Cycle]

Round fins are chrome bores except for LeMans, which are iron.

Square fin are Nikasil.

Round fins came in three varieties in the US: chrome (V700, Ambo, Eldo, V7 Sport, 850-T, 850-T3), iron liners (Convert, G5, 1000SP, CX100, 850 Le Mans) and Nigusil (late G5, 1000SP and CX100).

[fotoguzzi]

Aesthetically square fins look slightly beefier to me.

[Alfetta]

I can't speak directly about MG as i am new to the brand. However ive been around air cooled ICE engine for a very long time. It has been my experience that when a desing changes from round to square it is primarily an aesthetic choice, but oddly the overall size is changed very little (or not at all) and this results in the square shape yielding more surface area.

As an example my BSA441 came with a round fin design (up to 1968 i think ) and then it went square. the width across the square fins was very close to the diameter of the round fins. I never had heating issues that i was aware of, but i also never took temperature readings either.

Personally i would choose what ever design pleases your eye, unless i lived in Phoenix, then i might go square just for that miniscule amount of maybe more cooling.

PS >  Bigger fins tend to make more noise, many Jap singles even add rubber strips between fins to help reduce the "ringing"

[MattP]

   The stud pattern on sq fin is larger top end will exchange between sq fins but not between ,with round fins, From what i uderstand the cranks ect. be changed around. put a ambo crank in a 1100.                                                      matt                                                                           

[Frenchfrog]

Round fins are prettier by far for me ...but then you are more limited with bore increase size. 90 mm is likely to be the max you can go if you can find suitable pistons. For the squares the bellagio model goes straight up to 95 mm bore ( but with a very short stroke crank ) .

[Lee Bruns]

the crank case breathing handles high speeds better on the square fin engines. If you want to run at 80 mph and up for extended miles, you'll find that the square fin engines do a better job of handling the case pressures.

[moto-uno]

 ^ Strange you should mention that , I still own my Le Mans 2 (they were available in Canada) and for the longest time
that bike spent pretty much every day for 3 months going flat out without any breather problems ( I lived in
northern Alberta ) straight highways for miles and never a police officer . 1/4 million kilometers later and it still has
the original breather . Peter

[lucky phil]

Square fins are better, it's a WKF.

Ciao

[chuck peterson]

I think there was a difference in crankshaft counterbalance ratios…not sure if that’s the right terminology…rotating mass balanced against counterbalance weight per a ratio..

Roundfins are a touch quieter and smoother to me

But that could just be a difference in engine sizes,? Squarefin went up a size over time?

If you goose (ahem) a roundfin to match a square your likely running it harder

Differences in valve sizes I’m not aware of..

[moto-uno]

  Also not to upset others , BUT , the Le Mans 2 model ( in Canada) came with the nigusil bores , I'm
 still running them in my Ubilt here in B.C . One might call them durable :) . Peter