Upshur Farm Engine - Plugging a hole in the head

[mikehinz]

Hi, I have a question about plugging a hole in the head of an Upshur farm engine.  I've not started documenting the build here on the forum, but I've progressed to getting the head finished.  But I've run into a problem and need suggestions on how to get a particular hole plugged successfully.

The engine I'm building is a horizontal air cooled version.  I've built the head design that has the spark plug screwed into the head from the parameter.  I built it as per the drawing and drilled and reamed a 3/16" hole thru the head and then drilled and tapped 1/4-32 for the spark plug from the side.  Then to blank off the hole from the top side, I made a small plug from AL, .3/16" OD by .175 long and then applied some Loctite 620 (the high temp stuff) to the plug and pressed it into place.  I'd call it a light press fit.  That's all exactly as called out in the drawings.

After I assembled the head, I rigged a test fixture and submerged the head assembly under water.  The good news is, is that the valves didn't leak!  The bad news is that the small blanking plug leaked.  Not much but at 20 psi air there was a very noticeable stream of small bubbles from the blanking plug.  I did make a small, very precise punch and a fixture to hold the head and did manage to press the old plug out without a lot of trouble. So, at least I'm left with a good hole!

So the question is, what's the best way to get that blanking plug to seal?  It's short so that's a  possible issue.  I thought Loctite 620 would be good but it didn't seal.  Is there a different product that I should use?  Should I make a tighter press fit?  Should I use a bit of drill rod instead of AL?  Or maybe some JB Weld??

I'm pretty concerned as this plug does see full pressure from the engine whenever it fires, so for sure it's got to be good, tight seal.

Or should I give up on this head and make the alternative version with the spark plug entering from the front of the head vs the side??  That would entirely eliminate the issue. 

Thanks in advance for any and all advise!

Mike.

[Dave Otto]

You could get some #290 wicking Loctite and apply it to both sides of the plug, it will wick in and seal up the tiny leak.
One of it's advertised uses is to seal porosity in welds.

Dave

[RReid]

Dave's suggestion seems good, but since you've already removed the plug, I think I'd go with JB Weld or equivalent, something that will be gap filling.

[mikehinz]

Dave's suggestion seems good, but since you've already removed the plug, I think I'd go with JB Weld or equivalent, something that will be gap filling.

Do you think JB Weld will stand up to the engine compression when it fires?  Also, do you think the plug should be AL or should I instead use a bit of drill rod?

Thanks for your help!

Mike

[Dave Otto]

Oops, I missed the part where you had removed the plug. 

Dave

[RReid]

I'd stick with aluminum for the plug, to avoid a different rate of thermal expansion, if nothing else. Combined with the press fit, I think JBW will hold just fine, probably better than Loctite would. But it would be best to roughen up both the hole and the plug a bit, you actually want some gaps for the epoxy to fill, plus that gives some "tooth" to improve the holding power. Another option might be to tap the hole and put in a set screw, combined with sealant or epoxy.

[mikehinz]

I'd stick with aluminum for the plug, to avoid a different rate of thermal expansion, if nothing else. Combined with the press fit, I think JBW will hold just fine, probably better than Loctite would. But it would be best to roughen up both the hole and the plug a bit, you actually want some gaps for the epoxy to fill, plus that gives some "tooth" to improve the holding power. Another option might be to tap the hole and put in a set screw, combined with sealant or epoxy.

Ron, thanks for the very helpful suggestions!  I'll definitely try the JB Weld.  I do like the idea of threading and installed a set screw.  The challenge I have with that is the hole is currently .188"  I'd have to go up to something like 1/4-32 in order to make some threads.  I'll need to carefully look and see if there's enough clearance to do that.  If I can't manage that, I'll make another plug out of AL, as you suggest, score it, and try the JB Weld.

Again thanks!

Mike

[AOG]

one more quick suggestion. I would make the plug a little long and peen the outside edges. that will help with the sealing and add mechanical strength to the assembly by upsetting the material into any small voids and forming a head that will help keep the plug in mechanically.

my 2 pennies

Tony

[john mills]

if you are going to use loctite you need to have room for the loctite if you have a tight fit it will wipe the loctite off
so the loctite will do nothing and if the press fit won't hold it won't .it must either be a press fit or a loctite fit
with( room for the loctite )
John

[mikehinz]

Tony, John

Thanks for your most excellent suggestions.  I'm pretty sure that my fit was too tight and I squeezed out the Loctite as I pressed the plug in.  When looking at the removed plug, there was very little Loctite that I could see on the plug or in the hole.  And it pressed out quite easily which certainly isn't typical experience with Loctite, unless the part is heated.

My next attempt will be JB Weld and I'll give a bit more clearance and will try to make some small annular groove in the plug and will roughen the ID of the hole in some fashion.  Plus I think I'll make a small head on the plug to make sure some JB Weld gets under the head and hopefully improves the seal.

Again, thanks to all!  Hopefully I'll have some results to report tomorrow.

Mike

[internal_fire]

There are many types of Loctite beyond the common red and blue threadlockers. I use Loctite 648 for high strength assemblies such as crankshafts. The data sheet claims it is even usable for press fit and shrink fit assemblies.

The warnings in the data sheet include, "For Press Fitted Assemblies, apply adhesive thoroughly to both bond surfaces and assemble at high press on rates."

I have never used it for a press fit, but for a close slip fit the setup time is less than 10 seconds.

All in all I would use JB Weld. I recall there some ancient who said, "Give me enough JB Weld and I can move the world". 

Gene

[mikehinz]

First, thanks to all that offered many very useful suggestions.  I really appreciate it!

So, I did have success in getting the hole plugged!  I did also have a minor setback as the intake valve started leaking again, so I'll need to take it apart and inspect/clean/relap the intake valve.  It might have gotten a bit of crap under it when I had it all apart, hopefully I didn't somehow damage it.

Anyway, I thought I'd describe how I did the repair just for future reference in case anyone runs into the same issue.

First, I made a replacement plug, just a slightly longer to get the most surface area possible, and I made a small short head on it so it was spaced correctly when installed.  Then I roughed both parts by running a 10-32 tap in the hole and running a 10-32 die on the plug.  This in no way contributed any mechanical strength to the joint as the threads just barely touch as the hole size was entirely too large, but it was very effective in creating plenty of surface for the adhesive to adhere to. 

Second, I used JB Weld as was recommended.   I liberally applied it to both the ID and OD of the parts and pressed the plug in place and lightly secured it with a small clamp.  Then I let the assembly cure for 24 hours as recommended by the manufacturer. 

I reassembled to my test fixture and this time, it didn't leak!  So I was quite happy about that. 

I'm attaching a single pix of the plug and the head just to illustrate the what I did for the repair.  You can see the head on the plug and the very poor threads cut on both pieces.  The plug body is .200" long and the head is about .050" thick. 



If I had to do it all over, I'd try to avoid drilling thru the head.  In fact I redid the head in CAD so that if this failed, I'd remake it in such a way that the spark plug would enter through the face of the head vs the perimeter.  In my redesign, I angled the hole so that where it enters the cylinder, the hole is entirely within that diameter.  The Upshur design has the hole about 50% intersecting the cylinder wall and I just didn't like that approach, so I angled the hole to avoid that issue.

Anyway, again thanks to all!

Mike

[RReid]

Glad you got that sorted out, Mike.

I've found the Upshur plans to be generally fine, everything is there and seems to be accurate, but the presentation of information for the cylinder head, Sheet 12,  is confusing. I'd seen the detail on the next page, Sheet 12A, showing the plugged through hole, but assumed it related to the optional top mounted spark plug, which is the main subject of that page, and really just ignored it. On the main drawing, in the top view, that hole is shown as a dashed outline, which I read as a the hidden outline of a blind hole.  Apparently, Hamilton meant it to show the outline of the plug! I now realize that is really an assembly view, and the drawing to its left, which I took to be the bottom view (but wondered why it was not a mirror image). A section view A-A through the spark plug hole is indicated, but is not found unless you look to the next page.  A section view through the intake valve is shown, but not called out. As I say, confusing. At least the problem you had helped me finally sort it out in my own head, so Thank You for that, Mike!

I don't understand why he didn't just show a hole drilled up from the cylinder side to meet the spark plug hole, which is what I intend to do and you've realized you could have done.

[mikehinz]

Glad you got that sorted out, Mike.

I've found the Upshur plans to be generally fine, everything is there and seems to be accurate, but the presentation of information for the cylinder head, Sheet 12,  is confusing. I'd seen the detail on the next page, Sheet 12A, showing the plugged through hole, but assumed it related to the optional top mounted spark plug, which is the main subject of that page, and really just ignored it. On the main drawing, in the top view, that hole is shown as a dashed outline, which I read as a the hidden outline of a blind hole.  Apparently, Hamilton meant it to show the outline of the plug! I now realize that is really an assembly view, and the drawing to its left, which I took to be the bottom view (but wondered why it was not a mirror image). A section view A-A through the spark plug hole is indicated, but is not found unless you look to the next page.  A section view through the intake valve is shown, but not called out. As I say, confusing. At least the problem you had helped me finally sort it out in my own head, so Thank You for that, Mike!

I don't understand why he didn't just show a hole drilled up from the cylinder side to meet the spark plug hole, which is what I intend to do and you've realized you could have done.

Ron, I spent ages on pages 12 and 12a trying to figure out what he meant and ended up modeling the entire engine in CAD since I was going to change things anyway.  In retrospect, I should have just drilled up from the bottom to intersect the spark plug hole.  That's the best approach if you intend to have a side mounted spark plug.  I went ahead and made a model and drawing of mounting it from the top, but not like he shows.  I moved it a bit and angled it inward so as to fully intersect the cylinder bore, vs how it comes out if you do it straight from his location. 

I can give you another hint to avoid a possible problem.   The rocker arm pivot mounting hole shows to be drilled thru and the round .1875 section is 1.00" long.  I did it as per his drawings and most probably since there's more length there, it did seal.  However, if I do it again, I'd drill down to perhaps .40" and shorten the rocker post accordingly.  That way there's no penetration into the cylinder area to give problems. 

I think Mr. Upshur's intent was to make the build as easy as possible, in terms of machining operations, but the old mariner's warning should apply "Here Be Dragons" as issues can arise from the design.

I hope this is helpful.

MIke

[RReid]

I can give you another hint to avoid a possible problem.   The rocker arm pivot mounting hole shows to be drilled thru and the round .1875 section is 1.00" long.  I did it as per his drawings and most probably since there's more length there, it did seal.  However, if I do it again, I'd drill down to perhaps .40" and shorten the rocker post accordingly.  That way there's no penetration into the cylinder area to give problems. 

That's a good tip. Unfortunately I already drilled it all the way through. Then asked myself why. As you say, it shouldn't be a problem, but it is an unnecessary potential problem.