Hello and Happy New Year....

I was wondering if anyone has any ideas of why the piston locks on my OS 25 FX when I tighten the head bolts. 

*  The piston moves freely when the head is not tight.
*  The sleeve is seated in the block correctly. 
*  I tightened in a cross pattern and tried tightening different areas first like on a Fox 35, but no help.
*  I have tried different connecting rods, same result.
*  I tried a head off another engine, no help.
*  I checked the block and sleeve mating surface for flashing, etc.  None found.

Thanks

Russell Pieper

I have a 46FX that has a similar or perhaps the same problem.  Mine responds to the "Fox" method though and I can stop the "locking" with careful progressive tightening.  I have no way of figuring out the cause but, having been a machinist at one point in my life, I can venture a guess that something is not a perfect match at the cylinder/head joint.  I tried adding an extra head washer and it did not help.  Only tightening until it started locking then backing off till I found the bolt that caused it helped.  Engine runs fine once set.

One thing you might try is running all of the head bolts in as far as they will go and seeing if it is equal and more than would be required to seat the head.  Does it turn over smooth without the head?  There should be a slight "grab" at top dead center even without the head.  Mine will push the cylendar out of the case about 1/16" at TDC if it is dry.

Ken

... The distance from the bolt seat on the head down to the top of the block is 5.75mm.  The bolt hole in the block is 10mm.  That means the bolt has a distance of 15.75mm, so a 12mm bolt isn't going to bottom out...

Unless the holes were threaded with a bottoming tap, the threads aren't good all the way to the bottom.  I don't know how OS usually does this job, but it would certainly be less expensive in a production environment to make the holes a bit deep and tap them with a normal taper tap.  That would leave several threads worth of taper at the bottom of the hole; screwing a screw tight into that would distort the block.

Take one of your 12mm screws and screw it in with your fingers until you feel some resistance, then measure the distance from the deck to the bottom of the screw.  If it's clearly going to tighten up the head with room to spare I'm full of gas and can safely be ignored.  If it's not distinctly smaller than the head thickness, you've got a problem.  If it's more than a mm taller than the head thickness, you've really got a problem.

Progress, but not solved.

It is the cylinder.   I took the cylinder out of a known good engine and put it in the engine I am working on.  Worked perfectly. 

Also, I took the suspect cylinder and put it in the good engine and that engine locked up with I tightened the head bolts.

Now, what on earth can be wrong with the cylinder?

Did it get seriously overheated or get a load of wet stuff dumped on it when it was really hot?  Sounds like the cylinder is not round at the top.  A new cylinder might be in your future.

Ken

Hi,

I checked the bolts.  The bolts are 12mm long.  The distance from the bolt seat on the head down to the top of the block is 5.75mm.  The bolt hole in the block is 10mm.  That means the bolt has a distance of 15.75mm, so a 12mm bolt isn't going to bottom out.

Randy, you are correct.  I pushed the sleeve locator pin into the block so I could mark the land where the cylinder sits with a black marker and rotate the sleeve to see if it is seating evenly, which I found it was.   This did not cause the sleeve to stick. I have seen the locator pins pushed in before and it didn't cause this, but they were not FX engines either.    What can I do to check and correct? 

Thanks

HI
That will  not  tell  you that it is  OK , That may  tell you that  the top of the  case  is  OK, but not the  sleeve,  To see  for  sure  you would need  to check  the  run out, with a  dial indicator , of the  underneath part of the  sleeve  top
Sounds like  something  is  warped, so  I would just check everything  I could,  and  one thing  you could  also  check is  the  ROD , a warped  rod  will  also cause this

Randy

Air-ster,

Thanks for the engine history. That helps. Also good to know you don't have a boatload of bucks into this one already....

I didn't understand the way you assembled the parts inverted and squeezed in a vise. Did you simply rotate the sleeve in the case, squeeze, rotate, squeeze, etc.? I'd be real concerned that the sleeve might buckle around the ports, since there's a lot of meat removed there.

I've had a couple of discussions with guys about "compression."  Here are my basic thoughts:  Compression ratio is the fixed geometry of the engine. (Volumetric ratios.) An engine with a high CR may feel like it has more pop when you flip it--but only if the fits and seal was really good. Pinch would be the progressive tightening of the fit as you approach TDC. And, near the top the clearance may effectively go to zero. Binding could occur anywhere, and has little to do with "compression." In your case, the worry is that as the clearances get smaller at the top, a slight distortion is giving you binding, on top of any pinch, so not sure how you could accurately assess the seal at this point. (ie. "that the engine has a lot of compression.")

Something you might try is to put some kerosene or WD-40 in the exhaust port and turn it slowly thru TDC looking for bubbles. If you get a lot of leakage, and it turns pretty stiff at the top, then your current diagnosis of warped liner seem to be further validated.

Something you might try is to repeatedly heat cycle just the liner by itself in the oven. I might try a setting of 300F or 350F for 10 minutes, followed by an air cool, and then repeat 4-5 times. With aluminum, for example, it will relax partway back to the original shape. (We did this on certain critical aerospace parts to stabilize them prior to any final finishing.) I've never tried this on a liner--but for $10 you can risk learning something.

Since you're a parts swappin' kind of guy, did you try replacing only the head? If the original head had damage, that might tell you something.

Not sure anyone will ever have a junk barrel like Marvin. Made me think of McGee's closet--but bigger. Marvin's "barrel" must have been the size of one of those storage tanks at an oil refinery. Never heard of some part that somebody needed that wasn't in that barrel somewhere. It was legendary....

The Divot

OldRod,

Welcome to the 'hanger.  Interesting story. On your engine, if the head wasn't dinged up; if the muffler/mounts aren't damaged; if the front of the crank was not dinged or bent, then I  might guess that your engine was hydro-locked and someone went after it with an electric starter. I wouldn't have expected a fracture at the crank pin, but as you say, something had to cause it....

Go run it and tell us how it holds up after an hour or so. Perspiring minds want to know....

The Divot