Is This Piston and Liner FUBAR?

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Tim Strange

Tim Strange

Well-Known Member
Joined
Apr 13, 2009
Messages
402
Here, for your viewing enjoyment is a CMB 67 with a bit of an issue. As you can see from the pictures a piece of debris was caught between the piston and liner and caused a bit of damage. The pics look a bit worse than in person...(just trying to make myself feel better about it)

Anyway, is there any way to save the Piston and liner?

P.S. The rust is proof positive as to why you should flush out the water jacket with WD-40!

img_3666.jpg

img_3667.jpg

img_3668.jpg
 
Definitely toast but smart money says it wasn't "debris" that killed it, you stuck the piston from either being too lean, too hot or both. The places damaged are where the piston and liner "swapped metal" and is typically the hottest area around the exhaust. I bring my water cooling in at the "7 o'clock" position (looking from overhead) for that reason even if it means drilling and retapping the water inlet/outlet. Check your cooling lines, was it over propped, what fuel mixture and oil content??
 
Last edited by a moderator:
Tim,

If you are carefull. You can take a Qtip with meratic acid on it and wipe the aluminum of of the liner. Then you will only need a new piston. I have had to do this many times on my fullsize OB hydroplane race engine. Which uses a nikisil as a cylinder coating.

Mark
 
I am not sure exactly how this happened...the motor was sold to me as a "not broken in yet" and "only one tank of fuel" kind of deal...Obviously that "one tank" was disasterous!

Other than the rust stains in the water jacket and the piston/liner the motor does look to be pretty new. I haven't pulled the bearings out yet, but with the liner pulled out it turns over smoothly with no play. Does it really need new bearings?
 
Yes, this looks like you stuck the piston. Could be caused by too much heat, too lean, or breaking in too fast. I'd get a new piston and sleeve.

I agree with Don, bring the cooling water in at or near the exhaust port.

AHobbs
 
lights said:
Tim,

If you are carefull. You can take a Qtip with meratic acid on it and wipe the aluminum of of the liner. Then you will only need a new piston. I have had to do this many times on my fullsize OB hydroplane race engine. Which uses a nikisil as a cylinder coating.

Mark
Click to expand...
Muratic acid is OK on nikasil but it is used to strip the hard chrome from ABC cylinders, not so good. :)
 
Don't know if this is the best thing to do or not but I have begun doing it to engines when I strip them down. I take a fine diamond bit in my Dremel and knock off all the sharp edges on all the ports on the sleeve. I hold it at ~45 deg and just take ever so slight a 'lick' off.

Got a pair of CMB 80 engines from Fleabay and upon examination I found the ports to have a near knife edge. I figured that was just asking for trouble and couldn't see any harm in knocking off that sharp edge.

Anyone else doing this?

Wally
 
I should probably be chastised for this...but I could not resist trying to save the Piston and Liner. I used some tiny pieces of 1500 grit sand paper to gently massage the affected areas to remove any burrs. Everything was flushed generously with WD and wiped down a few times and the motor was reassembled.

I successfully heat cycled the motor (using a very rich needle, low RPM just until the motor got hot to the touch) using 3-4 ounces of fuel. The motor was flushed out thoroughly with no visible metal bits. The piston skirt still looks bad but the liner looks a lot better...the marks visible in the first picture above have disappeared.

I doubt that the piston will last long, but I am hoping to run a couple of tanks through it over the weekend to get it broke in. It will then get thoroughly cleaned/oiled and put in a plastic bag until I get a new P/L. I really just want it as a back-up for the rest of the season...

How much is a new Piston and Liner anyway?
 
Save your money. And dont waste your fuel,all you will do is take the chance of destroying the rest of it by running it
 
Try to restore the oil-groove ,

and make new one above and a new one under the existing one ,

thus creating an oil-labyrinth , and engine wiil run OK much longer
 
Maybe I can say it in spanish, here I go. "Echalo al zafacon". Go to Google and translate what I said.
 
Last edited by a moderator:
"Echalo al zafacon" ... ha ha ... Not everybody has the funds to ditch suspicious parts.
 
I've seen worse P/L 's , but 'refurbished' they ran several gallons again.

Maybe not a good solution for a 'racer' but good enough for a 'play-boater'
 
I am sure you didn't mean that as an insult...I am here for the RACING!

This motor only needs to serve as a back-up with what I have until I get the funds for the new parts.

The worst case I was thinking would be the loss of compression. However, most of you must believe it will come apart...so it goes back in the box until I get the parts.
 
Tim in the mean time if you need a solid back up my .67 CMB has not seen any fuel for awhile.lol Its already broke in also.
 
walthor said:
Don't know if this is the best thing to do or not but I have begun doing it to engines when I strip them down. I take a fine diamond bit in my Dremel and knock off all the sharp edges on all the ports on the sleeve. I hold it at ~45 deg and just take ever so slight a 'lick' off.

Got a pair of CMB 80 engines from Fleabay and upon examination I found the ports to have a near knife edge. I figured that was just asking for trouble and couldn't see any harm in knocking off that sharp edge.

Anyone else doing this?

Wally
Click to expand...
instant timing change. i would never 45° the ports
 

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