Hip Pocket Builders' Forum

Outdoor Free Flight Forum => Power => Topic started by: RobinB on March 15, 2017, 01:46:03 PM

Title: K&B 3.25
Post by: RobinB on March 15, 2017, 01:46:03 PM
Sorting through some K&B 3.5 engines I’ve acquired over time, I found that one is actually a 3.25. It has the shorter stroke crank, and the face of the head has been machined to restore the correct clearance.
The 3.5 on the side has been removed and left blank. The side of one lug has been engraved with the word ‘small’. There’s also what looks like ‘SS’ or ’55’ in the case.(see pic.)
Also the conrod is different to those on my other engines, having a single oilway on the top of the big end.
It’s the later type case with the larger 0.765″ sleeve.
Any K&B experts out there know if this a factory engine, or could it be a conversion from a 3.5?

Title: Re: K&B 3.25
Post by: danberry on March 15, 2017, 01:52:13 PM
I cannot imagine why anyone would've converted one since they were available from the factory.
The 3.25 came with no markings as they ground away the 3.5 on the case. I would assume that yours is stock.
Marvin Mace told me that using the round exhaust thingy would add 2oo-500 rpm.

Title: Re: K&B 3.25
Post by: RobinB on March 15, 2017, 04:26:35 PM
Thanks, Dan.
I've read different opinions - that the size marking was ground off, as you say, but also that they were marked  '3.25'.

The 'round exhaust thingy' - is that the stub pipe, or the muffler adapter ?


Title: Re: K&B 3.25
Post by: danberry on March 15, 2017, 09:19:59 PM
I've never seen one with 3.25 on the case.

I don't know if the thingy is a muffler adapter or pipe adapter. It's the round exhaust thing that bolts to case at the exhaust leaving a round exhaust hole. I guess it's the stub pipe.
Common sense told me it was extra weight but Marvin said it was good for some RPM. Marvin knew his stuff.

Title: Re: K&B 3.25
Post by: RobinB on April 20, 2017, 11:45:22 AM
Just to finish this thread:

It's actually a 3.5. I misread one of the measurements - probably due to trying to read a digital
caliper upside down, or some similar stupidity.
My thanks to Mike Thompson, whose photos and advice helped put me right.

The ground-off number on the case remains a mystery.


Title: Re: K&B 3.25
Post by: Big G on April 21, 2017, 07:45:28 AM
I flew K&B 21's (3.5cc) in Goodyear and Open FF Power lord knows how many years ago. K&B did produce a 3.25cc version so it could be flown under AMA Class A rules, up to 0.20cu ins or 3.25cc as near as dammit. They were great engines, 'bullet-proof' as we used to say. I've still got some 21's and have loaned them to a friend who wants to try one in Weatherman. I would suggest that someone took a 3.25cc, converted it to 3.5cc presumably with stock parts, and ground off the i.d. on the case. Only a guess.

The exhaust stub was for a mini-pipe or the header of a tuned pipe. I used a 3.5cc for my first attempt at speed and fitted a mini-pipe, which made a worthwhile improvement in r.p.m.