Home › Forums › Free Flight › The Engine Shop › KB 3.5 differences?
- This topic has 7 replies, 4 voices, and was last updated 6 years, 4 months ago by Anonymous.
07/29/2016 at 8:06 pm #77570
I am looking at two 3.5’s and in one of the exhaust ports there is a bar going across it. Is this an early vs late thing or something else? Which one is better?07/30/2016 at 10:00 pm #77571
There are 3 versions of the free flight K&B 3.5 built. The early version had a part number 8450. This was replaced by part number 8900. This version had the pressure fitting mounted thru the left side mounting lug looking from the rear. This version was replaced by part number 8918. The pressure fitting on this model was moved to the upper left rear crankcase bolt hole. The P/N 8918 had the following improvements made to it compared to the early P/N 8450. New brass sleeve with a heavier wall brass sleeve made from a harder and stronger brass alloy. New advanced technology ball bearings. New crankcase has been bored out larger to accept the heavier wall sleeve. The new sleeve and crankcase are not interchangeable with the older engine versions. New connecting rod with a fully machined con-rod made from high tensile strength aluminum stock with bushings at both ends.Crankshaft improvements thru a new machining process. I have both P/N 8900 and P/N 8918. I believe P/N 8900 has the same improvements as the later P/N 8918 with the exception of the pressure fitting placement. The engines I have are front intake/rear exhaust versions. They don’t have the bar you are talking about. Are yours side exhaust versions?09/04/2016 at 9:43 pm #77860
I noticed a few more differences not mentioned. One has strengtheners on the mounts. Also one is lacking a cooling fin at the top. Can you identify which ones these are from your list?
Attachments:You must be logged in to view attached files.09/04/2016 at 9:54 pm #77868
Neither of them have pressure fittings currently.
Attachments:You must be logged in to view attached files.09/26/2016 at 11:17 am #78002
The early 3.5 p/n 8900 has the bar in the exhaust port. The later 3.5 p/n 8918 does not have the bar. The later 3.5 p/n 8918 has all the latest modifications from the factory. Your photo marked 2016-09-04-22.55.41-1 jpg is the later 3.5. The other photo marked 2016-09-04-21.57.31.jpg. is the early 3.5 On my early 3.5 p/n 8900 there is a factory installed pressure fitting on the outside edge of the left side mounting lug. My later 3.5 p/n 8918 has the pressure fitting mounted in the top left hand corner of the rear back plate. It replaced the mounting screw on the back plate. The free flight versions of the 3.5 had the pressure fittings mounted. The standard 3.5 engines did not have the pressure fittings. Other then the pressure fittings, the engines are the same.
Mike10/17/2016 at 2:38 pm #78203Robin BeckfordParticipant
I have one with a crankcase like the photo on the right, but it has the liner bar / web.
The makes it an 8900, right?
I took off the pressure nipple so long ago that I can’t remember which hole it was in.
Robin10/17/2016 at 4:25 pm #78204
The 3.5 motor on the right is the 8900 model number. All the later 3.5 Aero engines number 8918 has the pressure fitting in the upper left rear crankcase cover hole.The 8918 motor also has internal upgrades compared to the early ones. In my earlier post, I stated that the 8900 motor has the same improvements as the later 8918. This is incorrect and only the 8918 motor has the improvements. My mistake when typing.
Mike10/29/2016 at 2:23 pm #78248AnonymousInactive
Oh jeeez……..is there any point to all the “i got to be right Mike?”
to put this to rest, the early 3.5’s had one exhaust window…………..the pistons would get buggered up quickly because of the rocking and lipping the top of the pistion, hence that later version(s) that have the vertical bar you now see.
both run the same and perform the same, but the latter will outlast the early single window by a good 50%.
Sorry to be blunt, just get tired of all the misinformation from those who really didn’t compete during the years that FF was at it’s prime when the 3.5’s hit the scene.
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