Tight Piston, any suggestions??

[chunk t]

Hey all,

I working on a used K&B 3.5 o/b w/c engine. I've found the piston to fit the sleeve perfectly until the last 1/32 of the stroke where it binds badly :angry: (@ TDC). I've checked and it's not a head clearance issue it is a sleeve/piston fit issue. The fit is off by about .001-.002". I'm thinking about wet sanding the top of the sleeve (inside surface) to free up the fit with the piston and go through the break-in process again and see if that does the trick.

Anyone have a different suggestion???

chunk t

 

[Joe Warren]

Chuck Assemble the engine & spray WD40 thru the Ex Port. keep rolling the engine thru the TDC in rotation untill it frees up..... this is your FIT. The liner is tapered from the fac. On a new engine I like to roll it about 100 times thru the Fit While spraying it with WD40. Install & set the needle Rich for the first 5-6 tanks.

 

[Don Ferrette]

Hey all,
I working on a used K&B 3.5 o/b w/c engine.  I've found the piston to fit the sleeve perfectly until the last 1/32 of the stroke where it binds badly  :angry:   (@ TDC).  I've checked and it's not a head clearance issue it is a sleeve/piston fit issue.  The fit is off by about .001-.002".  I'm thinking about wet sanding the top of the sleeve (inside surface) to free up the fit with the piston and go through the break-in process again and see if that does the trick. 

Anyone have a different suggestion???

chunk t

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Don't wet sand it!! I remember way back when sitting in front of the TV with a can of WD40 & hand lapping pistons to fit the sleeves of K&B motors. :blink: I bet Rod G's got a good answer for this one too.

 

[dave roach]

Run the engine in on a drill press. put the engine in a vise on the drill press run it as fast as it will go for about 20 min. I hook up a fuel tank with w-d 40 to the carb set the thottle wide open and this will lube the engine when it's running.

 

[Don Ferrette]

Run the engine in on a drill press. put the engine in a vise on the drill press run it as fast as it will go for about 20 min. I hook up a fuel tank with w-d 40 to the carb set the thottle wide open and this will lube the engine when it's running.

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A drill press??? Putting that little 3.5 motor on a drill press being as tight as it is will probably destroy the fit almost instantly. That's if it doesn't bend the rod or rip out the wrist pin as the piston sticks when you first hit the power on the press. If anything a lower speed spinning with a cordless hand drill while keeping it drowning in WD40 would be the max I'd go. Joe W.'s way is good too but with a K&B you'd probably be looking at 200 or 300 rolls past TDC ..........

 

[mark voorhies]

Hey all,
I working on a used K&B 3.5 o/b w/c engine.  I've found the piston to fit the sleeve perfectly until the last 1/32 of the stroke where it binds badly  :angry:   (@ TDC).  I've checked and it's not a head clearance issue it is a sleeve/piston fit issue.  The fit is off by about .001-.002".  I'm thinking about wet sanding the top of the sleeve (inside surface) to free up the fit with the piston and go through the break-in process again and see if that does the trick. 

Anyone have a different suggestion???

chunk t

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The best way to break in a tight motor is to heat cycle it under load, if you can

start the motor, run it rich a bit on bench till it's hot, shut it down when you can't

leave your finger on head. AS SOON as you can launch it, run 1/2 lap on throttle

and !/2lap at 3/4 throttle. Continue for 2 tanks of fuel. Should have a nice fit

on P&L after. If weather is cold and water also, I restrict cooling, or use none.

Don't try and fit a piston to sleeve with lube, you'll get a scored piston, and, or,

a bad fit.

 

[Don Ferrette]

The best way to break in a tight motor is to heat cycle it under load, if you canstart the motor, run it rich a bit on bench till it's hot, shut it down when you can't

leave your finger on head. AS SOON as you can launch it, run 1/2 lap on throttle

and !/2lap  at  3/4 throttle. Continue for 2 tanks of fuel. Should have a nice fit

on P&L after. If weather is cold and water also, I restrict cooling, or use none.

Don't try and fit a piston to sleeve with lube, you'll get a scored piston, and, or,

a bad fit.

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Hey Mark-

I agree on the heat cycling once you can get it started & keep it running but alot of the K&B motor's piston to liner fitting was so bad that you'd break them trying to start it without some type of basic hand lapping. I saw this happen to people more times than I care to remember....... :blink:

 

[Rickracer]

I'm not a nitro or small motor guru by any means, but it seems that there must be some heat involved to get a good final fit. After all, there will be heat when it's running, which will change both piston and bore dimensions. Just my 2 cents worth(maybe only 1.5 due to inflation, ) B)

 

[dave roach]

You guys can think all you what to but I have been breaking 3.5 engines on a drill press for 20 years and have'nt had a problem. I have seen taper on a 3.5 k-b form .002 to .006.

 

[izitbrokeyet?]

I think those "plug fits" are some of the best fits. That motor will keep a good seal when you lean on it a little B)

If you keep it too cold/fat it will ruin your rods

let it drink nitro bro....no need to run it in again if it's got some time on it already. If you decrease the crown diameter you'll get blow-by

anyway...I dont know squat about K&B...maybe Rod G will post....I'd definitely listen to him.

 

[gene gager]

everybody has their own way of loosening up "too tight" a piston fit, I chuck the piston skirt in a lath (very delicately) and wrap 2000 grit wet around a popsicle stick and touch it to it. then take it out and check it for fit. again and again and again until it fits tight just before where it will at top dead center. ( not at the top of the cylinder housing) put the piston in backwards and make sure the skirt isn't what is too tight. if you take too much off , well then you have just learned what several other racers have learned. been there , done that. take your time, and lots of it when doing this process. I hand polish the piston when I think I'm getting close to fit. it should still be tight at tdc when you are done fitting, and then you will have to break it in. most k&b engines come with a fair fit , but "too tight is too tight" . what seems tight in your fingers pushing the piston up and down isn't the same as grabing the flywheel and turning it. :blink: geno

 

[J Solinger]

I don't know for sure if this will apply here but years ago I had a K&B 7.5 and it had what I considered to be a VERY tight fit. But, it ran great, never had a lock up. I did read somewhere, I can't remember where, that K&B made the interference at the top of the stroke heavy but they made the sleaves thinner than the other engine makers and with alot of clearance between the sleeve and block. The idea was to jam the piston up to TDC with a fresh layer of oil then the combustion presure would expand the sleeve and release the piston. Another benefit was that the sleeve bore didn't need to be as round, it would conform to the piston at the top of the stroke. I don't know if this concept was exclusive to the larger engines or not, it may not apply at all here.

 

[chunk t]

...I did read somewhere, I can't remember where, that K&B made the interference at the top of the stroke heavy but they made the sleaves thinner than the other engine makers and with alot of clearance between the sleeve and block. The idea was to jam the piston up to TDC with a fresh layer of oil then the combustion presure would expand the sleeve and release the piston. Another benefit was that the sleeve bore didn't need to be as round, it would conform to the piston at the top of the stroke. I don't know if this concept was exclusive to the larger engines or not, it may not apply at all here.

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Solinger,

Your comment makes sence with what I'm seeing. :huh: ...It's just that I've had other K&B and other brand engines and had never had one this tight. (remember this is a used engine) This fit wouldn't receive a second thought if it was new. At TDC the piston is unable to complete it's stroke by .015-.020". I can MAKE the piston travel through it's stroke holding on to the starter cone but it takes about 15 ft.lbs acording to my torque wrench. That's a lot of strain to put on the pin, rod & bearings.

Any other thoughts before I choose a course of action?

chunk t

 

[J Solinger]

You could check with the previous owner for the history of this engine, but it kinda sounds like it has been "pinched", or resized with a resizing tool.

15 ft/lbs just seems realy high to me.

 

[chunk t]

I'm not going to get any history on this engine. I picked it up through a friend who went to an estate sale for me. It was cheep enough to let him buy it sight unseen. I'll take the sleeve to work and mic. the ID for roundness. The compression is great and there is no signs of scaring on the sleeve or piston . :huh:

I'll keep plugging away intil I have an answer.

chunk t

 

[Mike Larson]

Hey Chunk! one thing..... mayyyybe... check that the bearings are seated in the case ( crank aligned) ive ripped into a few 3.5s there may be a speck of aluminum, or gunk under the main bearing not allowing the piston to align well in the bore..... thought, stupidity, who knows, good luck mike

 

[Geraghty]

I'm not going to get any history on this engine.  I picked it up through a friend who went to an estate sale for me.  It was cheep enough to let him buy it sight unseen.  I'll take the sleeve to work and mic. the ID for roundness.  The compression is great and there is no signs of scaring on the sleeve or piston .  :huh:
I'll keep plugging away intil I have an answer.

chunk t

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Tidbits for the K&B 3.5 outboard or inboard motor...............

1.First of all stop worrying about wristpin or rod failure because of a tight P&S fit,it is not a concern.

2.I have never broken an engine in on the drill press[not to say that is a bad way to do it].I have thought about it but I decided against it because I want the motor exposed to the fuel and oil that the motor is going to be run on.The oils used in our fuel have exceptional load carrying abilities and thermal dissipation qualities and I want the motor exposed to that while breaking in.

I have built a ton of K&B 3.5's that took a 7/16" wrench to turn them through TDC when they stuck while trying to start them with a 12VDC starter.Loosen the plug and bump it to start it.When you get the motor to start I guarantee it will run like hell and will eventually will get a perfect fit that won't stick at TDC.

When you get a motor like this,the kiss of death is to lift the head after it has been run in.If you lift the head and re-assemble to just look see and after you put it back together you will be asking the motor to break in all over again and the fit will go south immediately.

Put your motor back together trying different head bolt torques.You will find the fit will noticibly change when you turn the motor by hand with varying head bolt torques.K&B 3.5's are famous for this.

For customers who just can't stand a tight P&S fit,I have a helical lap I use to loosen up the fit but all I am doing is removing life from that P&S.I have also spun the piston in the lathe using 400 wet/dry to remove a little piston material to loosen up the fit but you have to be real careful doing it this way.

You can run the motor tight and it will not hurt a thing and it will run like a turpentined cat.

Any questions....call 1-715-926-6096 CST and I will be glad to talk to you about it.

P.S.All my personal K&B 3.5 motors that I have built for my own use "all" start out needing the 7/16" wrench.

 

[Don Ferrette]

There is no better information source on K&B o/b motors than Rod. He does a wicked job modding those little 3.5's too!

 

[Norm Doerr]

I'm not going to get any history on this engine.  I picked it up through a friend who went to an estate sale for me.  It was cheep enough to let him buy it sight unseen.  I'll take the sleeve to work and mic. the ID for roundness.  The compression is great and there is no signs of scaring on the sleeve or piston .  :huh:
I'll keep plugging away intil I have an answer.

chunk t

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Tidbits for the K&B 3.5 outboard or inboard motor...............

1.First of all stop worrying about wristpin or rod failure because of a tight P&S fit,it is not a concern.

2.I have never broken an engine in on the drill press[not to say that is a bad way to do it].I have thought about it but I decided against it because I want the motor exposed to the fuel and oil that the motor is going to be run on.The oils used in our fuel have exceptional load carrying abilities and thermal dissipation qualities and I want the motor exposed to that while breaking in.

I have built a ton of K&B 3.5's that took a 7/16" wrench to turn them through TDC when they stuck while trying to start them with a 12VDC starter.Loosen the plug and bump it to start it.When you get the motor to start I guarantee it will run like hell and will eventually will get a perfect fit that won't stick at TDC.

When you get a motor like this,the kiss of death is to lift the head after it has been run in.If you lift the head and re-assemble to just look see and after you put it back together you will be asking the motor to break in all over again and the fit will go south immediately.

Put your motor back together trying different head bolt torques.You will find the fit will noticibly change when you turn the motor by hand with varying head bolt torques.K&B 3.5's are famous for this.

For customers who just can't stand a tight P&S fit,I have a helical lap I use to loosen up the fit but all I am doing is removing life from that P&S.I have also spun the piston in the lathe using 400 wet/dry to remove a little piston material to loosen up the fit but you have to be real careful doing it this way.

You can run the motor tight and it will not hurt a thing and it will run like a turpentined cat.

Any questions....call 1-715-926-6096 CST and I will be glad to talk to you about it.

P.S.All my personal K&B 3.5 motors that I have built for my own use "all" start out needing the 7/16" wrench.

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There you have it. The best answer I have heard on this thread. I Have two Kabooms that Rod built for us a while back. Believe it or not they have NEVER been apart and yes when we started running them the top was tigher than a .......Ah well ....you get the picture. These engines still run today and scream. The only thing I did was run them really rich on the water with the water line pulled off to heat cycle them.

 

[chunk t]

Rod,

Thanks. I have a couple other questions for you if you're up to answering them. e-mail me <[email protected]> I think that would be easier than a phone call.

chuck "chunk" t