Kind of an odd duck engine here, at least 'round these parts. One of the last engines before K&B closed up shop. Some serious leakage going on.

I think this engine was designed so that a head gasket was not needed at the head & cylinder interface (in the past, it ran that way). Both surfaces look good: Cylinder top is smooth. A very narrow, ever so slightly raised mating ring on the head, about 1mm wide between the bolt holes and the squish band. With carefully tightened head bolts, it still leaks. Looks like aluminum mating surfaces. This engine took a serious dive into the dirt at its last run a couple of years ago.

What is to be done?  Could the head and cylinder be warped? I'm thinking of fashioning a head gasket out of aluminum can. Or, hyper fine sand paper on the surfaces. Taking it to a machine shop; well maybe, but there's so little meat on the head to machine off.

Any suggestions here?

Thanks...

Dave Mo

Dave Mo,

Possible troubleshooting and repair sequence:

00. Determine which engine you are dealing with. I am guessing it is a K&B .20 Sportster. Know that the early engines had an o-ring for a head seal. Your comment that yours was one of the last produced implies that it does not.

0. Take off head and clean parts. Eliminate any FOD from the crash. Check screws, especially to see if they are bent from the crash. The head/cylinder bolts in a Sportster are long, and more critical than in the more usual engine designs. Roll them on the edge of a  ground steel or cast iron surface, or something you know is very flat. The underside of the heads should not wobble either. Check that the screws run free in the threaded holes. (The screw threads may have stretched in the crash. This would likely be on the case side of things. If the threaded hole in the case is "pulled up" that would be an indication.) If the screws hit the sides of the clearance holes in the head that is another sign of problems. Reinstall head and torque in a star pattern. Check in soapy water for compression leak at the head to confirm initial diagnosis. If you think it is leaking, be sure to leave in the plug and gasket you had in it so you are checking it at the same time. If the head is all chewed up by the plug, you may have a problem there.

1. inspect both the case and the head for flatness. Figure out if one or both are part of the problem. The top of the cylinder should be the sealing surface, so you should be able to wring it on a very flat surface (granite surface plate, ground cast iron tabletop, etc.) and look at the polish marks to see how flat it is. Same for the head.

1a. If it hit hard enough for the cylinder to be bent out of line, then it probably ovalled the mating interface which would pucker the normally flat surface. If the top surface of the cylinder has high spots on the right and the left of the engine, that is likely what happened. If the piston fit still seems reasonable, then you have nothing to lose by lapping the head interface and trying it out again.

1b. If the head bent, the cylinder probably did too.

2. I would be best to hand lap both surfaces back to a good fit. I don't have a Sportster to look at, but on other "gasketless" K und Beasts, they typically would cut a ribbed surface in the liner where it mated to the head. The head mating surface was machined flat. If the cylinder and head are both as I suspect, with the mating surfaces protruding, lay a piece of 400 grit silicon carbide paper on a very flat surface and using some light oil lap them both. Take off only enough to remove previous machine marks.

3. If you cannot get a seal by hand lapping, then the soda can aluminum gasket is a good idea. Recall that soda cans are made by back extrusion methods, so they have to be nearly pure aluminum, which is the softest grade you are going to get. Exactly what you want. The cans I have used were around .0043 thick. Don't make the gasket any wider than you have to. Reassemble and test again.

4. If that does not solve your head leak, then you might consider replacement parts, if the rest of the engine is still in good enough condition to merit this. MECOA still has heads and piston/cylinder sets. The head isn't that pricey, but the P/C cost may make the project impractical. But since the parts are available, you have a choice.

Good luck with your repairs in the land of trees and green forests,

Divot McSlow


PS--One last thought:  if the screws are not factory parts, and have been replaced, it is possible that one or more of them are slightly too long and bottoming out in the case. So you apply torque, but are not getting the preload that you would expect. I have seen things like this on other engines, so it is worth checking.

Hmmm,

Sounds like it hit pretty hard. Where was the cylinder chunk from? Is a picture possible?

Will the lower end seal up with the chunk missing?

Divot

Another bit of info. Three of the four screws pass the roll test. The other does a little shimmy shimmy on the flat terra cotta surface - possibly the one adjacent to the break on the low end. All four screws thread in and out and set into the head with no trouble.

Dave Mo...

Presumably, the engine did not sustain (serious) internal damage from the broken piece of the case floating around inside? Be sure to deburr the edge of the break to avoid damage to the piston skirt and having the sharp edge strip off the film of lubrication the piston needs.

The fretted area on the head is the sealing band. It is nice and narrow, and is inboard of the screws. You should be able to lap both parts and restore the seal. I agree that a scratch across the seal is a concern. Try lapping it first. If the scratch won't come out without removing a couple of thousandths, then what I would do is install the head with something like Loctite 5189. This is a CyA gasket material developed for use on car engines. It does not appear to be available thru retail outlets. So I'd just use medium Zap. Put a dab (way less than a drop) on the end of a pin and drag it along the scratch. Then immediately bolt the head on. And leave it to cure for a good while.

Before anyone gets too worried about CyA'ing your engine together, by doing it this way with the pin and only in the groove, you are just restoring the surface mate-up. Also note that the high temperature strength is practically nil. It will be trapped in the scratch and will not likely "blow out" which is the very definition of a gasket or sealant. And, remember that machinists glue down small and complex shaped parts to hold them in the mill all the time. A bit of heat and they pop right off. Less chance of distorting or damaging an odd shaped part using this method.

If you don't like that option, then go to a coarser grit silicon carbide paper and take off enough to clean up the scratch. Follow with the 400. Then install your soda can gasket. Shoot for getting back the original head clearance or greater. The soda can shimstock will be a bit over .004".  You can also get thinner material from an industrial supply store and possibly a local auto parts store or farm store. Smaller head clearance is not likely better for sport flying.

Good luck and let us know how you make out,

Divot McSlow

I agree that a scratch across the seal is a concern. Try lapping it first. If the scratch won't come out without removing a couple of thousandths, then what I would do is install the head with something like Loctite 5189. This is a CyA gasket material developed for use on car engines. It does not appear to be available thru retail outlets. So I'd just use medium Zap. Put a dab (way less than a drop) on the end of a pin and drag it along the scratch. Then immediately bolt the head on. And leave it to cure for a good while.

    That's even more hacky than what I was thinking, but, it should work!  I don't think there is any way to remove the maybe .001-.002 required to remove the scratch across the entire surface (presumably with sandpaper on a surface plate, or a precision stone). My idea was to use JB-Weld like body filler, just in the scratch, let it cure,  then sand that flat on a surface plate. All you need is to fill the scratch, so you end up with a microscopic thread of glue in the scratch, and it looks the same otherwise. Then just bolt it back together. There's no way you are pushing .001" thick bit of glue out of the scratch with the head on it, even if it breaks down from heat. The same theory works with header sealant, you can apply it, screw down the header, and then go fly it immediately, and for the first few seconds, it's completely uncured. But it won't push out of the micro-scratches from the surface prep.

    BTW, not to be negative, but the pictures of the head show it to be very, very dirty with plenty of particles that could cause the seal to fail in this (very poor) seal design.

     Brett

       

Breaking news:

I tried the flat surface sanding trick on both parts (didn't trust myself with lapping). Cleaned with acetone first and when finished. Used 1,000 grit silicone paper with WD-40, a light touch and figure eight. The results: Cylinder top - Nice even cut, the scratch is less distinct. Nothing unusual here.

Banded area on the head -  About half the band disappeared! Must not have been very much there at all.

Re-installed the head, and it continues to leak like a sieve. Rotated the head 180 and re-installed. Nice and "poppy" with just a tiny leak around one of the head screws. Much better!

Might call this good enough and just go with it. Thoughts amongst the brethren?

Dave Mo...

HI
I have fixed a leak like your having with duct tape, since you have sanded the area flat, and its still leaking, clean it with acetone, and get a piece of duct sealing tape, this is metal tape with a one side sticky back covered with paper, You can cut out a piece, install it onto the top of the engine, and carefully use a xacto knife and trim the tape around the inside of the bore, trim out the bolt holes, and the outer surface, This tape is very soft and will seal most any small imperfections you have.  Make sure you get the proper metal duct tape  and  not the  cloth type

Randy

Cooking wood?  Are we talking KD hardwood or....?

If you are going to temp cycle, do it before the adhesive. If you already have the JB on it, skip the oven cycles.

The aluminum tape is a great idea if that works. I would have never attempted it though because it comes with an acrylic adhesive (PSA) on it. I would have thought that the heat and the pressure would cause problems on a head gasket. It is very malleable which would be great for gaskets. It will cut you badly if you handle the roll carelessly. I have my fair share of scars from using it for instrumentation placement during temperature/altitude tests. If guys are successfully using it for head gaskets, that is a great tip.

Divot

Hi Dave

There is  no problem with the  tape, it is the exact tape  we use as a header gasket for the EXHAUST , and it stands up to exhaust with ZERO problems,  and  as I said I have used it on heads before, so  yes it works.
The glue  is  extreme thin coating  so it does not cause  any problems

   It's thin adhesive to start with, and whatever is left is quickly extruded out except in the scratches. That's why the header gasket sealant works, even if it is not cured.

   I like it less well in this case, because the metal duct tape is about .008-.010, or at least the kind I have is. That's a pretty big change in the compression. Using the JB-weld like a filler should result in nearly no change to the clearance (aside from what has already been done).
 
   Brett

Mo,

Glad to hear you are making progress on your K&B repairs. Hope you have it flying soon!


Some follow-up on the aluminum tape techniques--

   The stuff I used a lot for temporary installation of instrumentation during environmental test was the 3M series, such as the 425. This is .0046" thickness.

     https://www.uline.com/Product/Detail/S-10311/3M-Single-Coated-Tapes/3M-425-Aluminum-Foil-Tape-2-x-60-yds?pricode=WB3547&utm_source=Bing&utm_medium=pla&utm_term=S-10311&utm_campaign=3M%2BTapes&utm_source=Bing&utm_medium=pla&utm_term=S-10311&utm_campaign=3M%2BTapes&msclkid=b67548be957a12291509141289e2cf1a&gclid=CLfTzcCTwuECFUOTxQIdLYkOSg&gclsrc=ds

They do list the upper temp limit of 300F, which is higher than I would have guessed. Is the stuff being used on header gaskets the same or some special high temp material?

I'm going to have to try this out, as soon as a decrepit engine opportunity avails....

Dave H

That aluminum tape can be expensive. If you'd like some without having to buy a roll, I can send you a little.

Great info, Randy and Brett. I will add it to my methods.

Dave

  For a quickie fix that I have used on other engines, is good old  #2 Permatex. Wipe on a very thin layer and let sit a while, then bolt the head on and run it. It never dries hard, but still seals some how. I keep a tube in my flight box. It gets to be a sticky mess sometimes, but we're talking about a K&B .20 here, correct??
   Type at you later,
    Dan McEntee

BTW, the probably best, machine shop way of fixing this would have been to fly-cut it flat on a milling machine, face it off in a lathe, or grind it with a surface grinder as deep as necessary (probably wouldn't be able to reasonably remove less than maybe .005" without the surface grinder. Then use a brass or aluminum head gasket to space it back out. Of course, by the time you got done paying someone to do that (probably requires a specialty machine shop), it would have cost $150 or so, which is considerably in excess of this:


https://www.mecoa.biz/shopdisplayproducts.asp?id=156

Part number 33-5601  $39.50

   Which, no offense intended, is probably far more than I would be willing to pay for a complete working engine, even a brand new one!

   Brett