K&B 7.5 IB marine mods

[Scott Miller]

A few pictures of a JAE 45 I set up for my son (who is now 10 years old). It still is very respectable when it comes to speed and is very reliable (always comes back to the dock). A great trainer for my son.

 

[Andy Brown]

For what its worth, I ran K&B 7.5's for years. Disk and drum. In hydro's and mono's and sport 40's.Disk engines of course made more power but were prone to breakage, drum engines were timed a bit tamer and required some mods to the closing timing. With that being said, when you got them screaming, the bearings would not hold up because of the very small balls in the big bearing and little front bearing. I found that I could get 3 great heat races and then they would start to go away. These are older technology from the 70's and early 80's. When I switched to macs in the sport 40's, it was no contest. The mac 45 on a bad day would outpull the K&B 7.5 on its best day.

I ran them untill I ran out of bearings and front ends. Great engines in their day.

Rob wrote: "When I switched to macs in the sport 40's, it was no contest. The mac 45 on a bad day would outpull the K&B 7.5 on its best day."

 

[Rod Fonda]

Here's my JAE with a Kand B 7.5 I ran the other day lol,,190ex,T port,,30 blowdown,10deg earlier opening on drum housing,,45 deg epoxy ramp in the drum rotor to direct flow,30%nitro,standard head button at .012",OS 10 glow plug, .015" stroked crankpin, ceramic main bearing,,detongued and pitched up LE and cupped tips x452(some abc props arrived today),,Just for a bit of fun and lots of spare parts to use up lol,,boat seems to hold the turns ok,,,my brain can't keep up,,its been 28 years since I last raced haha,,,

 

[LohringMiller]

K&B's engine designer was the great Bill Wisniewski. The older K&B design had a detachable front and rear on the crankcase. That design could use similar parts for a large variety of applications. It was not ridgid by today's standards. It used die cast parts. In 1998 that design was updated to a one piece investment cast crankcase. This was a much better engine in both inboard and outboard configurations. It is a lot harder to find.

Lohring Miller

 

[Rod Fonda]

K&B's engine designer was the great Bill Wisniewski. The older K&B design had a detachable front and rear on the crankcase. That design could use similar parts for a large variety of applications. It was not ridgid by today's standards. It used die cast parts. In 1998 that design was updated to a one piece investment cast crankcase. This was a much better engine in both inboard and outboard configurations. It is a lot harder to find.

Lohring Miller

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Thanks Lohring,,yes,,I read about the great Bill Wisniewski designing the .40 size engine for pylon racing back in the day,,looks like the old 7.5 case is basically a bored out .40/6.5cc case,hence the smaller/shallow transfer passages and the same .750" stroke,,so the .40 is the better engine for what is was designed for,,,,Now that you mention it,,I havn't noticed many of the new 1 piece case 7.5s laying around,,probably has better transfer passages and I think .780" stroke and a smaller bore,,cheers

 

[Norm Doerr]

.015" stroked crankpin,

Interested in knowing what that is

 

[Rod Fonda]

Interested in knowing what that is

Norm
I made some crankshaft blanks, drilled the crankpin hole .015" offset to increase stroke by .030",,I silver brazed a drillbit pin in to it,,havn't broken one yet,,pulls a bit more prop so I can keep up with the big boys lol,,cheers