Help with my 7.5 Redhead OB K&B please !

[CapeTown73]

Hi Guys,

I would appreciate your advice with the following problem.

Since I started racing 7.5cc OB tunnels in 2006, I first used Rossi powerheads and when the Nova OB powerhead arrived about 3 years ago I switched and have had nothing but great performance from the Nova. However, without going into the details, I managed to damage the engine and don’t currently have the required spare parts to fix it. Our final club race day for the current season is in a few weeks. We then have a two and a half month winter break during which time I plan to rebuild the Nova.

So for the first time in my life I’m preparing/rebuilding a used 7.5cc Redhead K&B that I bought last year but have never run. As the old piston/sleeve is way past it’s sell-by-date, I’ve decided to install a new piston/sleeve/conrod assembly that Joe Monohan kindly obtained for me but here’s the problem. After installing the new assembly and bolting down the cooling head (without the glow-plug), I tried to ratate the crank by turning the flywheel by hand and it is absolutely impossible to turn the crank further than the position as shown in the two attached photos.

Please tell me how to ‘lap’ the piston into the liner manually as I’m concerned that if I tried to start the engine with my electric starter, I may damage the conrod or it’s bushings.

Joe mentioned that he would be away from home for the next two weeks so I’m not sure if he will see this message which is the only reason why I’m asking the other K&B specialists here on our forum. Thanks up front for any help.

 

[HTV Boats]

Wennie,

I have used a simple brake cylinder hone to loosen up over tight sleeves. Use a cordless drill and not too many RPM's. Use the finest grit stones you can find. K&B's have a lot of sleeve taper, so concentrate on the upper end.

Mic

 

[normlee]

Just a suggestion, pre heat the engine with a heat gun to 160 - 200 f and idle the engine rich. An infrared temp gauge is the go to monitor the temp, you will be able to balance the temp with the HSN and the heat gun. Idle for 5 minutes then shut down with piston at the bottom of stroke to let cool. Repeat the heat cycle a couple of times running slightly longer each time.
With the engine pre heated the sleeve will expand and take the stress of the rod. Even once you hit the water initially pre heat the engine to give extra life.

I wouldn't risk a brake hone in the sleeve, too much chance to get out of round or take too much off.

 

[Eric Bourlet]

That head fits flat on top of the cylinder. What is your head clearance? I would not touch a hone to that unobtanium liner. Can you rotate the motor with the head off? Set the deck clearance at .008" . If its set and you can't rotate over tdc try Normlee method. I have used the piston itself and cutting fluid mixed with the gray pollishing compound to lap a piston to the liner. Takes a long time but hard do destroy a piston like is likely if you haven't used a brake hone before.

 

[Matt Minassian]

I agree with heating the motor up to loosen it up. Check your head clearance and if head clearance is ok, warm back up and start motor, run rich for several tanks of fuel, and then start to lean out as you break in the motor.

 

[CapeTown73]

Hi Guys,

Thanks for your contributions, I'll try the heat treatment this weekend.

Kind regards,

Wennie.

 

[TUNLBOTR]

Wennie,

What Matt said....

Hope all is well, long time since we last communicated.

Take care,

Randy Premo

 

[CapeTown73]

Thanks Randy, and yes time flies...... even if one is a pensioner. I've now been retired for 10 years.

I'll write you a PM after this weekend's testing.

Cheers,

Wennie.

 

[CapeTown73]

[SIZE=14pt]Hi Guys,[/SIZE]

[SIZE=14pt]Thanks to your guidance and advice, I had three successful runs with the K&B last weekend. Although still letting it run slightly rich, the engine performed well with really nice response to the throttle lever from fast idle to top-end. Just love the K&B carb which is so easy to set, although it's probably due to running the standard little muffler and not a tuned pipe. Have now made a slight adjustment to the angle and depth of the Lawless lower and will be testing again with some other props as well. I'm going to use this engine in our last club race day for this season on 16 May. I'm sure you all know what a 7.5 K&B looks like but here are a few photos of mine anyway.[/SIZE]

[SIZE=14pt]Cheers,[/SIZE]

 

[Brian Schymik]

mostly interested in the silencer devices. Is this a noise policy requirement where you run??

 

[dion smith]

What prop shafts are you running Wennie?

 

[CapeTown73]

Hi Brian,

Yes our club has a noise limit rule but it's not strictly monitored because our lake is in a wine farm area with no residences near to us. However, the standard K&B muffler is quite noisy and I had these two mini-mufflers (from Joe Monohan) and they fit perfectly on the little drum outlet-pipes.

Hey Dion,

You have sharp eyes! The propshaft assembly on my Lawless lower is made up from a Hyperformance propshaft (which has a longer 1/4" section than the K&B propshaft), running inside a locally machined and threaded phosphor bronze bush which is also longer than a standard K&B unit. This combination provides much longer life than the K&B PSA.

Cheers,

 

[dion smith]

That's a nice set up mate!! If there is a supply of them over there, I would love to try one

 

[CapeTown73]

Hi Dion,

My friend the precision engineer did me a very big favor to machine a few of these complete new PSA's (extended Phosphor bronze bush and new prop-shaft with extended 1/4 inch thick running surface) and he did not charge me his normal hourly labor rate for his work. I will ask him what he would charge to make up a few of these PSA assemblies for export. I'll send you an e-mail when I get his reply.

Cheers till later.

 

[dion smith]

Thanks Wennie, I appreciate the effort. Cheers mate!!