Board index Flathead Power-Technical Questions, Answers, and Suggestions Knuckles 61" Cylinders??

61" Cylinders??

Post Thu Jan 29, 2015 5:47 pm

Posts: 13
Location: Northern Michigan USA
I'm trying to put together a parts list for a Knuckle project I've been putting off for 10 years or more. Flywheels I have are 61" and the cylinders that I have are bored out for 74" over sized .050. pistons. Also one of the cylinders is a 4 fin and the other is a 5 fin. Kind of a mutt. My question is what is the best source for replacement cylinders and pistons? It seems that 74" are easier to find than the 61". I really like the 61", got no need for speed, show or glow and was really hoping to use the parts I already have from the original engine to keep the costs down. But now I'm wondering if it wouldn't be more reasonable to invest in a new set of 74" flywheels and go from there? Thanks in advance. Steve

Post Thu Jan 29, 2015 9:51 pm

Posts: 2717
Location: Los Angeles, CA
eBay is a good spot to find original cylinders.
http://www.ebay.com/sch/i.html?_ftrt=90 ... SS:US:1150

Post Sat Jan 31, 2015 11:33 am

Posts: 13
Location: Northern Michigan USA
Chris Haynes wrote:
eBay is a good spot to find original cylinders.
http://www.ebay.com/sch/i.html?_ftrt=90 ... SS:US:1150


Thanks for the reply Chris. I'm a little leery of ebay listings. Can these used cylinders with cracked or repaired bases be useable? Do you know of any quality repops?

Steve

Post Sun Feb 01, 2015 1:39 am

Posts: 2717
Location: Los Angeles, CA
Welded cylinders are a no go in my world. I am using Flathead Power cylinders on the bobber I'm building. V-Twin repops cylinders with the MCC casting marks. I don't remember if they make 61" or not.
74" cylinders will work and give you 68". Custom pistons can be ordered.

Post Sun Feb 01, 2015 2:26 am

Posts: 1729
FWIW, my first Panhead was a 61" engine, an EL. I actually found it to be very similar to the 74" in general performance, a bit less acceleration and punch but much smoother.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sun Feb 01, 2015 9:45 am

Posts: 13
Location: Northern Michigan USA
Chris Haynes wrote:
Welded cylinders are a no go in my world. I am using Flathead Power cylinders on the bobber I'm building. V-Twin repops cylinders with the MCC casting marks. I don't remember if they make 61" or not.
74" cylinders will work and give you 68". Custom pistons can be ordered.


I thought the welded cylinders idea sounded a little scarey. As for the V-Twin repops, that's probably the direction I'll end up at. This engine/project is not intended to be a "restore" and will have a MVN title anyway. What are the advantages with using the 5 fin versus the 4 fin? Also if using the the 74" cylinders are you saying that there are custom pistons available that place the piston higher to take care of the low compression issue?

Steve

Post Sun Feb 01, 2015 9:53 am

Posts: 13
Location: Northern Michigan USA
45Brit wrote:
FWIW, my first Panhead was a 61" engine, an EL. I actually found it to be very similar to the 74" in general performance, a bit less acceleration and punch but much smoother.


I don't have a problem with the 61" either. My flywheels are EL's and in great shape so unless the 61" cylinders and pistons are to difficult to find, or to expensive, I'll be staying with the 61".

Steve

Post Sun Feb 01, 2015 5:26 pm

Posts: 2717
Location: Los Angeles, CA
4 fin cylinders are correct for 1936-1939. 5 fin for '40-'47.
Last edited by Chris Haynes on Fri Apr 10, 2015 10:31 pm, edited 1 time in total.

Post Mon Apr 06, 2015 6:47 pm

Posts: 607
Location: Menomonie, Wisconsin, USA
Last year for 4 fin cylinders was 1939. 1940 got the 5 fin ones. They can be shared on the same motor as they both have the same deck height.

Post Thu Apr 09, 2015 12:31 am

Posts: 1729
Surely compression ratio is a function of swept volume, piston crown profile and cylinder head capacity, not deck height?
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Thu Apr 09, 2015 9:23 am

Posts: 1189
Location: Ojo Caliente,NM,USA
45Brit wrote:
Surely compression ratio is a function of swept volume, piston crown profile and cylinder head capacity, not deck height?

Surely deck height raises or lowers the chamber in relation to the piston changing its effective volume.
Dusty

Post Sun Apr 12, 2015 2:33 pm

Posts: 396
Jerry's comment as to deck height just refers to the '39 and earlier crankcases being the same as the '40 and later, so therefore either type '61 cylinder interchanges. My personal favorite engine configuration is a 74 top end on a 61 bottom. The cylinders need shortening (approx .090) to work with 74 pistons at standard compression ratio (the piston pin location is different on 61), or use high compression pistons to achieve the correct working ratio. At .060 over on 74 the bore and stroke are the same, cresating a square engine ratio and just under 68 CI . H-D built a few like that themselves.

The aftermarket 74 cylinders are fine and cutting a set down doesn't make a person cringe...


Return to Knuckles

cron