was having an issue with my #4 cylinder having oil on the piston top so i took the engine apart today. i didnt find anything horrific, but did find some wear on my intake skirts that i wasnt expecting. almost looks like a lil bit of the the ol' black death trying to start up.

these are cp's @.0045 piston to wall in my 84.5mm benson block. power level is between 400 and 500whp. air fuel never got leaner than 12:1 on pump gas and 12.5:1 on c12. are my tolerances just too tight?

#1 cyl intake skirt. this cyl wasnt burning any oil, and had 225 psi and about 7% leakdown


here are all the intake sides, #1 on the left and #4 on the right


none of the marks catch your fingernail so ill be cleaning up and reusing the pistons, but it looks like it could have gotten bad if i ran more power out of it

 

I would prob look at the valve seals 1st
look at the valve from through the port to see if the back is oily
as the pistons look fine

 

That looks like quite a bit of wear on the skirts. What kind of timing are you running? I'd check to be sure the bores are round

 

Thats piston slap Mr loosey goosey.

 

^^^ I agree. It doesn't look like the piston has grabbed the cylinder wall.

 

In the first pic it looks like there is signs of getting some material dragged from high friction. I never set cp's any looser than that so I wouldn't say you are too tight from my experience.

However, a/f doesnt tell the whole story on what kind of temps the piston is seeing.

 

timing is pretty low, im a wuss when it comes to pushing the envelope. around 14* on pump gas and about 18* on the leaded fuel above 7000 rpm at 18 psi.

i do have an egt gauge and ive never seen it about 800c give or take 20 degrees. normally at the end of a pass ill be in the mid to upper 700 range.

for the record none of the valves are oily through the ports. the exhaust valves are black showing extra carbon.

piston slap doesnt bother me as long as its not too tight and make contact with load on the engine. id rather be a bit loose than too tight. im going to the machine shop in a lil while we will put the bore gauge in and check everything, then hopefully do a little deglaze hone and i can re assemble as soon as possible

 

a pic of the cylinders would help but by looking at the pistons they dont look bad...check out this link for some pics when they have severe contact

http://www.laskeyracing.com/shop/breakin.htm

 

ohhh trust me i know what they look like when it makes serious contact, this engine had #3 hit piston to wall on the dyno last year using .0035 ptw.
yeah, not so good. learning from that i went bigger ptw this go around.

current situation:
the walls have vertical wear but they are still smooth and you can see the crosshatch through most of it.

 

7% leakdown would lead me to think about a valve job. The guide may be worn or the margin may be worn out. 0.0045" is ok for ptw. I live with that on my 81.5 mm motor with no problems.

 

late last year i developed some oil on the top of the 4th piston. over the winter i took the head off and replaced the valveseals, lapped the valves and re installed it. that brought the leakdown down from 10-11% to the 6-7% i have currently, but it didnt fix the oil on the top of the piston. the valves are all dry where the go into the guides, but i think there was a tiny bit of oil on the outside of the guides where the go up into the head. that a common place for them to leak? this is on the intake side btw, exhaust side is bone dry.

i do get a little piston slap with the .0045, but i usually dont drive the car till the coolant is up and oil is around 150* so im not loading it up when cold

 

have a nice day

 

Mini update. I hit the machine shop yesterday and didn't find anything overtly wrong. There was no more than. 0004 difference in bore between all cyls, and all of my intake and exhaust ports were able to hold the same amount of vacuum. No real signs of oil on the valves. A little confused still but im gonna get a fresh hone and take the valves out of the suspect cylinder and check them from there

 

i'm facing the same problem like u. and till today i have no idea what is the cause of the problem. my case is all 4 piston got oil on top of the piston.. hmm... maybe later i upload the picture here...

 

Got a call from the machine shop the other day. One of my intake valves in the questionable cylinder is bent. It was sealing well on the seat but the tweaked stem must have been destroying the seal in short order

 

Ahhh the smoking gun! Good find.

 

yeah i missed it when i had everything apart before replacing the seals over the winter. picking up the valve from the dealer in town tomorrow and running it to my machine shop after that. gonna be rainy this weekend hopefully i can get it together and put in

 

Did it not show up when you lapped the valve?

 

the valve still sealed, had a relatively even lapping surface. i did notice one of the 2 valves slid through the guide easier than the other, but i dismissed it since it still sealed and had good leakdown. for some reason i was paying closer attention to the exhaust valves. ive honestly only lapped my own valves a few times. in the past i used to send it out. in that sense ive got some noob status