Poorish compression and piston leak by

[nkt267]

Unfortunately the borescope did not lie.
The barrell is scored above the ring line for the whole of the stroke. The top ring was stuck in the groove between the 2 studs on the timing side but the piston was nice and free.
The grooves can be felt with or without running your finger nail round them and I was hoping to find a broken ring, but no such luck.
The skirt clearance is still 6 thou but at the top of the barrell it is 12 thou.
On inspecting the rings the top and second ring both have burrs turned up on their outer edges which at the moment I cannot explain, and maybe never will.
The piston is only lightly scored and I reckon that if I were to fit a new liner it would be ok, but that's not going to happen.
Could the new liner I fitted be soft? --Unlikely.
Still I'll order a new piston and maybe a liner and get the nice reborer to work his magic and see what happens...John

 

[Comet Rider]

John,
Thinking outside the box a bit here,

1/ Is the crank case to muff joint square?
2/ Could the bore/liner be out of true to the piston/rod
3/ Did you remove the oil restrictor disk when running in? I had to do this to prevent pick up.

When I went to one of Vinparts 90 bore kits I had my local friendly machinist with a 5 axis mill, check out the joint face on the c/case and mine was 1 deg out of true.

HTH
Neil

 

[chankly bore]

To redbloke: I think it is called colclatteral damage; at least it was on my machine. I agree with Neil about having a round square liner. Also, old muffs cause no end of drama through indifferent contact and poor heat transfer.

 

[Albervin]

Hi John
I think you need to try to get your thumb nail down the plugole and have a feel of the bore, the endoscope may be not telling the whole story.
As to crankcase pressure , I have recently experimented with the breather valve timing on my Comet, the procedure is extremely technical and involves blowing up the breather pipe while feeling the piston crown with a rod and slowly pushing the kickstarter, the results were very surprising as for most of the downstroke the valve was shut and for most of the upstroke the valve was open!
I have now timed the valve to my own preference and ignored the B timing mark.
The engine now revs more freely and I may have found at least a half of a horse in the process

Half a horse?!? You could make a lot of beef burgers with it ;-)

 

[greg brillus]

Just as a side....what ignition timing are you running (at full advance) the top of your piston looks to me like it's been detonating.......Greg.

 

[nkt267]

That is a posibility as I had coil ignition on the bike when I last rebuilt it and I was starting to get starting problems, the timing was at 38 BTDC.I timed it at 4 to 6 degrees BTDC fully retarded and that cured the starting and all seemed ok.I wanted to go back to a magneto which I did earlier this year.When I had the recent mag failure I borrowed a mag but also investigated the coil ignition assy.
I set it up in my lathe with a degree plate mounted to the shaft and found that I was getting near to 19 degrees of advance( that equals 38 deg at the crankshaft).So maybe the damage was started early on after the rebuild,Although the bike has run well with no signs of pinking.
On closer inspection the top of the piston is a little rough on the sparkplug side so you are most likely correct especially taking into consideration that's the side the ring was jammed. John

 

[greg brillus]

Hi John, yes i think you will find that 38 deg advance is way too much on the modern fuels now thrust uppon us. It is quite likely that if the engine was pinking, you may not have heard it....I know just very roughly and someone will correct me on this, but normal combustion pressures are around 600 to 900 psi In the cylinder, whereas under the conditions of detonation they can exceed 2200 Psi, this is why the damage happens quite quickly. Any metal especially aluminium, will start to melt around the plug hole, the top and also down the side of the piston. I think this is what you may be seeing in your engine. At least it is better to find the fault and fix it, than to simply repair the condition and have the same thing happen again.....Cheers and good luck.....Greg.

 

[aldeburgh]

Hi John
Sorry to hear that you are "piston broke" do you have a good engine machinist in mind? if not I can highly recommend one who resides in our fine county PM me if you need details
regards Paul

 

[Big Sid]

Might I ask what torque figure did you use on the main head nuts ? No more than 30/ 32 ft lbs is needed cold . More can distort the muff and thus the liner , so that the rings can't follow the cylinder wall causing blowby and rapid wear . Sid .

 

[Big Sid]

Rings can be seated in minutes if the assembly is done towards that end . Modern pistons need 4 thou skirt clearance , no less and this measured half way or more down the bore , not near the top . The finish on the bore needs the correct tooth to do what's needed . The ring stack is kept dry of oil , a few drops rubbed onto the skirt . A little on the lower fourth of the barrel surface , NONE on the upper half of the bore , this clean but dry . Thus the dry rings ride over a dry liner over the upper distance . This seats the rings in the initial few strokes as they burnish against the dry surface . A few quick but short wrist blips within a few seconds after the initial firing does the trick . RIGHT NOW .
No fear because the oil flushes up over everything within moments .
A good car oil , Texaco Havoline is one , just regular , not super duper stuff hastens the entire break in proceidure , then drain it and replace with the better stuff , gets the iron swarf out of there came off the rings and cylinder walls .
The initial break in miles use short hard bursts over a brief hard pull and then back off to suck up oil into the bore , cool it , then repeat that a few times as you raise the road speed . This burst has cylinder pressure get behind the upper two rings forcing them out against the walls . Then backing off flushes the lot , cooling it .
Done properly this method has all ring seating and pistons bedded in under 50 miles . Even less . Then you can venture in stages up among higher speeds always backing off for a mo to cool er down .
We still use Racing Valvoline in a quick motor . Sid .