K&B 7.5 outboard piston replacement

[R1rider]

K&B 7.5 outboard piston replacement

I'm new to nitro boating and need help.I just bought a prather lapcat with a 7.5 outboard.Before running I took the head off to inspect piston/cylinder.The sleeve has 2 scars on intake side.I think I should replace piston/sleeve.I tried to remove the starter cone to disassemble but it seem as though you need a special puller?(removed nut and stud).How do I get this off?How about tips on the rest of removal? ???

 

[Ron Olson]

Re:K&B 7.5 outboard piston replacement

If you are just going to replace the piston and sleeve, you don't have to remove the front plate, just the back plate (power take-off). After removing the head, pull the sleeve first then the piston will come out easily.

Now, if you really need to pull the flywheel, you can use a small flywheel or gear puller if it is on tight enough or loosen the acorn nut or Allen bolt depending on which motor you have, then tap on the nut or bolt until it loosens.

 

[duc]

Re:K&B 7.5 outboard piston replacement

Maybe some other tips:

To remove the liner: remove the head, put the piston in it's lowest position and stick a soft peace of tissue in the liner with a lot of it in the ports. Then you can turn the flywheel and the liner wil come out.

Why removing the flyweel? Ok but be very carefull to tighten and secure it again. I lost two of them. They just flew away during running.

Are you sure you will change the liner and piston? Did you run the motor allready. Maybe it will run as fast as you would like to go or even faster (for now )

And Remember: K&B parts are very hard to get!!

 

[Propjockey]

Re:K&B 7.5 outboard piston replacement

And still more tips..............

Loosen the flywheel retaining nut a few turns so that the top edge is just above the end of the stud (you don't want to damage the stud threads). If it has an acorn nut, just back it off a couple of turns. Set a wooden block--about 1" X 1"--on top of the nut. While holding JUST the flywheel tightly in your hand, smack the block of wood with a hammer. Make sure the engine isn't sitting on anything, just hold it out in the air and the flywheel should pop right off.

Another way to pull the sleeve is to remove the head, carb, rear housing (PTO), and front housing/crank assembly. Heat your oven to 400 degrees, and set the engine in there for 5-7 minutes. Take it out with oven mitts, flip it up-side down and tap it. The sleeve should fall out.

 

[R1rider]

Re:K&B 7.5 outboard piston replacement

Thanks for the help friends.Now where could I find replacement cylinder/piston since removed?It appears that the piston pin caused the scoring in the cylinder then the piston was damaged.There is what looks like a piece of teflon in end of pin.Is that what prevents pin from contacting cylinder and if so is it supposed to have a button head on end?It just looks like the inside part of an ink pen end sticking out.Also looks like 2 head shims outer diameter is too big because the head screws damaged the outer edges through contact any thoughts?

 

[Propjockey]

Re:K&B 7.5 outboard piston replacement

Yup, the teflon insert should have a button head, small--but there.

As for parts, I don't know. I get mine off eBay.

Here is a pic of a new 7.5 wrist pin and the teflon insert.

 

[John Dorinzi]

While we’re here what way does the nut on top of cone?
It’s right or left hand thread
Thanks

 

[Mike Hughes]

Lefty tighty.......

 

[Alan Elzer]

Re:K&B 7.5 outboard piston replacement

Maybe some other tips:

To remove the liner: remove the head, put the piston in it's lowest position and stick a soft peace of tissue in the liner with a lot of it in the ports. Then you can turn the flywheel and the liner wil come out.

Why removing the flyweel? Ok but be very carefull to tighten and secure it again. I lost two of them. They just flew away during running.

Are you sure you will change the liner and piston? Did you run the motor allready. Maybe it will run as fast as you would like to go or even faster (for now )

And Remember: K&B parts are very hard to get!!

Just replace the stud & nut with a 1/4"-28 headed Allen bolt, if it's a recessed flywheel use one an 1 inch long, if not recessed use 1 1/4 inch long you should never need any loc-tite to hold these on./