Ring gap, new rings? Or new pistons?

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T

ThomasLRNL

Member
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53
Hello all,
I am redoing this 2.25 petrol, and I find that the piston rings are probably very worn out. Can someone tell me according to these pictures if new rings are needed; or a complete new piston? Thanks! Personally i think new rings, since on the piston themselves they have a gap too. Indicating that they need to be pushed further in to have no/a smaller gap. if these are fine/engine will run with them. Id like to know too! It doesnt have to be a performance monster i just want it running.

Please let me know!
1729072095257.png

1729072064899.png
 
Ok, i have measured the ring gap. This is around 1.8mm. So thats pretty much far out of spec. I dont have the tools readily available to measure the bore accurately.

According to these values, can any conclusion already be made?
 
Have you measured the gap as if the rings are compressed into the bore?
ie. put the ring in the bore and measure the gap, dont measure with it fitted on the piston
 
Last edited:
kermit_rr said:
Have you measured the gap as if the rings are compressed into the bore?
Click to expand...
yes, i put the rings into the bore as shown in the workshop manuals. Then I measured the gap. Consider my first picture of the original post as an example, just with some feeler guages in the gap! :)
 
ThomasLRNL said:
yes, i put the rings into the bore as shown in the workshop manuals. Then I measured the gap. Consider my first picture of the original post as an example, just with some feeler guages in the gap! :)
Click to expand...
sorry missed that!
do you have much piston slap? ie with the piston in place do you get much side-side movement?
 
kermit_rr said:
sorry missed that!
do you have much piston slap? ie with the piston in place do you get much side-side movement?
Click to expand...
OEH, good question; id need to check that. Would that be with all the rings installed and the bottom portion of the piston slapping/pivoting? Or with a "bare" piston, and see how much room there is from piston to cylinder wall along its entire side?
 
kermit_rr said:
With the rings on will make it easier.. there will be some movement but it shouldn't be excessive
Click to expand...




please see these videos of the piston slap from above and below. I can conlude that from the bottomside, there is little to no movement. On the top side, the piston seems to rock a little more with the pivot point being the piston rings.

Maybe im opportunistic, but doesnt that mean that there is room for bigger piston rings? Aka, the rings are worn? Ill be awaiting the opinion of you guys! Cheers!
 
The 2nd clip is what i meant. Normal i would feel by pressing into the top of the piston and seeing how much it moves, but that's when it's still attached to the crank.
That looks like quite a bit of movement to me, typically it's the bore that wears as the piston digs into it. You need to check for roundness. If you can put the piston back in at 90° and you get the same movement you know it's the cylinder and not the piston.
Too much movement becomes audible as 'piston slap'.
Why did you take it apart to start with?
 
@kermit_rr i will check, it is likely that its pretty round. Since i put it in already at 90 degrees, and i know that it moves similarly when in correct orientation.

This would mean the cilinder is at fault? what are the remedies to this?

I took the engine apart as a revision, i got it for a cheap price and would like to put it in my 109 to reduce noise as it now has a 2.5 NA. so i want it to be running and just basically working.
 
You can get it bored out and put oversized pistons in. I don't know a specific measurement of movement in the piston before it becomes too much. I guess you measure the ring gap and go from there using a new ring.
Others may know better.
 
Can you see or feel with your fingernail a ridge at the top of piston where the top piston ring stops this will show wear in bore
Have you tried putting the piston rings in bore using a piston to keep it square and placing it towards the bottom of cylinder ( no wear here) then measuring gap with feelers this should show if rings or bore worn or both
I took my block to an engineering shop to check my bores cost about £25 to compare with my own measurements ( both very close)
 
kermit_rr said:
You can get it bored out and put oversized pistons in. I don't know a specific measurement of movement in the piston before it becomes too much. I guess you measure the ring gap and go from there using a new ring.
Others may know better.
Click to expand...
You can measure the piston clearance in bore also as in the data sheet
IMG_3549.jpeg
 
Ah dangit, i really hoped i could avoid bringing it to a shop. Takes a nice cheap engine to a more expensive thing. ill use the trick of putting the rings at the bottom where there is no wear. Nice once.

There is a small ridge at the top, yes.

Any opinions on just rolling with it as it is right now? Any dangers? would like to know. Even though rebore and oversized might be better...
 
If you are in this far and it’s out of spec, who knows what it will run like.

But you have wasted the money on a gasket set and your time.

J
 
I could just feel a small ridge and I think that equated to less than 10 thou oversize on mine but I still got a rebore. But if you don’t want to I would measure what you can , completely strip down and check for visible wear , you can always do the minimum , put it back together , pressure test and make a decision then
 
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