I have a new K&B 28 Sportster set up for CL. While breaking it in I noticed that the head was leaking. Alot. Checked the headbolts, all were tight. Ordered new gaskets from Mecoa and it still leaked. I was told to lightly sand/polish the the head and cylinder with some fine grit. I did so and replaced the gasket again. Still leaking. I even tried a head from a different .28 on it. What am I missing? Thx.

Mike,
You need to lap both surfaces, not simply sand them or polish them up. (You may know this, but it was not clear from your post.) To get them truly flat to an acceptable number you will need a very flat plate. A granite surface plate is ideal. A float glass plate is probably ok. Then clean it carefully and tape down a new sheet of silicon carbide wet or dry paper. You'd like to get down to a 16 microinch finish if you can.* It's not hard if you have the right equipment. For technique, don't lap back and forth, you will round off the parts (think rocking chair runners). Lap in oscillating circles. Some machinists will tell you to lap in figure eights, but depending on the part that can lead to more issues. (Like a crankcase. Help yourself by holding it as close to the surface being lapped as you can to help eliminate rocking the part.) Use a light oil or kerosene as you lap and keep the paper clean. You may have to start with something as coarse as 400 grit, but you will need to get down much finer to be sure it won't leak. I'd guessitimate at least 1200 grit. It won't take long even if you are starting with a very non-flat part if you go thru progressive grit sizes.

If you have the granite surface plate, you can wring the part on the surface after you have lapped it and it will polish the metal everywhere it touches. That will show you the low spots--they are not polished. Depending on how out-of-flat each piece is you may have to keep backing up in grit size until you are in contact everywhere.

Don't assume that the gasket is constant thickness, either. Not sure, but would expect this one is metal, and should be dead soft aluminum (1100 alloy). If it has a rough edge from die stamping, that can cause the problem. I have made head gaskets using soda cans. This is the right kind of aluminum. It has to be unalloyed and dead soft for them to back-extrude the can from a slug of it. (And I'll neglect talking about work-hardening because this is no longer a "technical forum."  Sorry, that just slipped out....)

Remember that each screw only supplies so much compressive force to the joint. While it seems like a lot, it won't be enough if you have to deform the two metal pieces to seat and eliminate the leaks. And compressing a solid metal gasket will take a lot of force. So everything has to fit right. Trying to stop a leak with parts that aren't really flat or a gasket with issues can lead to too much torque and stripped bolts. If you have a small calibrated torque driver that can be useful to know when to stop.

K&B had a successful history of producing parts that sealed just fine after precision machining. They did have trouble with the Sportsters for some reason. The early Sportsters had an o-ring seal (with different machined parts.)  That's just not a great idea due to the temperature. About the only seal that I would think would work there would be Viton due to its high temperature capability, but they probably didn't specify that material.

Hope this helps,

Dave

*--I'm assuming that neither of these parts have a "ribbed" interface. That was often done to reduce the compressive load required to deform the metal gasket. So, for example, an older K&B head groove would be flat and very fine finish. The gasket would be aluminum, and machined not stamped (or at least they did not have stamping burrs.) But the top of the liner (the lip) was machined such that it had circular ribs that went all the way around. If you parts have this, do not lap them off!

Dave thx for all the info. I understand and was thinking along those lines. I used 400, 600 then 800 on a piece of glass. A buddy of mine should have the items you mentioned to do it properly. Thx

It is Brett. My search showed no results. I’ll try again. Thx

   Try this one:

https://stunthanger.com/smf/engine-set-up-tips/kb-20-leaking-head/msg550879/#msg550879

  That was a 20, but it sounds like the same thing.   Note the rather hacky suggestions about filling scratches, include my own super-hack idea, but ultimately they got it working.

       All of the K&B Sportsters, particularly the 61, ran very well and were very competitive in their day. I would have (and did...) chosen them over the ST60, and was working on an airplane for it, when Paul showed us the future with his 40VF.

     Brett

Make sure your head bolts aren't bottoming out before the head is tight.

Mike,
You need to lap both surfaces, not simply sand them or polish them up. (You may know this, but it was not clear from your post.) To get them truly flat to an acceptable number you will need a very flat plate. A granite surface plate is ideal. A float glass plate is probably ok. Then clean it carefully and tape down a new sheet of silicon carbide wet or dry paper. You'd like to get down to a 16 microinch finish if you can.* It's not hard if you have the right equipment. For technique, don't lap back and forth, you will round off the parts (think rocking chair runners). Lap in oscillating circles. Some machinists will tell you to lap in figure eights, but depending on the part that can lead to more issues. (Like a crankcase. Help yourself by holding it as close to the surface being lapped as you can to help eliminate rocking the part.) Use a light oil or kerosene as you lap and keep the paper clean. You may have to start with something as coarse as 400 grit, but you will need to get down much finer to be sure it won't leak. I'd guessitimate at least 1200 grit. It won't take long even if you are starting with a very non-flat part if you go thru progressive grit sizes.

If you have the granite surface plate, you can wring the part on the surface after you have lapped it and it will polish the metal everywhere it touches. That will show you the low spots--they are not polished. Depending on how out-of-flat each piece is you may have to keep backing up in grit size until you are in contact everywhere.

Don't assume that the gasket is constant thickness, either. Not sure, but would expect this one is metal, and should be dead soft aluminum (1100 alloy). If it has a rough edge from die stamping, that can cause the problem. I have made head gaskets using soda cans. This is the right kind of aluminum. It has to be unalloyed and dead soft for them to back-extrude the can from a slug of it. (And I'll neglect talking about work-hardening because this is no longer a "technical forum."  Sorry, that just slipped out....)

Remember that each screw only supplies so much compressive force to the joint. While it seems like a lot, it won't be enough if you have to deform the two metal pieces to seat and eliminate the leaks. And compressing a solid metal gasket will take a lot of force. So everything has to fit right. Trying to stop a leak with parts that aren't really flat or a gasket with issues can lead to too much torque and stripped bolts. If you have a small calibrated torque driver that can be useful to know when to stop.

K&B had a successful history of producing parts that sealed just fine after precision machining. They did have trouble with the Sportsters for some reason. The early Sportsters had an o-ring seal (with different machined parts.)  That's just not a great idea due to the temperature. About the only seal that I would think would work there would be Viton due to its high temperature capability, but they probably didn't specify that material.

Hope this helps,

Dave

*--I'm assuming that neither of these parts have a "ribbed" interface. That was often done to reduce the compressive load required to deform the metal gasket. So, for example, an older K&B head groove would be flat and very fine finish. The gasket would be aluminum, and machined not stamped (or at least they did not have stamping burrs.) But the top of the liner (the lip) was machined such that it had circular ribs that went all the way around. If you parts have this, do not lap them off!

Took your advice, it's fixed and running fine. It still had a small leak after the first try but another round of lapping took care of it. Thanks!

Hello
Nice to hear you solved the leaking head problem Mike and I should copy and paste Dave Hull's advice somewhere ready for my next leaker that turns up.
Incidentally how does the K&B 28 Sporster compare with the LA25 on Slow combat and also stunt ?
These two engines are among very few new engines now available that suit combat/stunt, many are looking at them now...
Regards Gerald

I haven't nor would I use them for combat. The OS 25's and 40's are all we use. We have a bunch of OS's because they are plentiful and cheap so I haven't been inclined to try a K&B on a bladder. As far as stunt, Sam has been flying them on RST's for a couple years now and they are great for that. I think it's a good in between motor when a .25 isn't enough and a .35 is too much. They are also extremely quiet with the muffler installed. Several have asked while Sam was flying "Is that electric?" I think many overlook them but with the new OS LA25II being $120 and only coming in an R/C version the K&B .28 may gain some support among sport flyers.