So I Pulled My 26 Apart Today..

[Swiper the Fox]

another possibility was dirt entering the engine

I didn't go anywhere near it.

My guess is ring end gap too tight

[R31Nismoid]

Certainly looking like assembly at this stage

[femno]

You say you have unacceptable amount of piston slap , like how nuch clearence? What was the clearence ( piston to bore) when it was built? Was the ring gap meassured in the bore ? if yes what was the gap? Are the bores worn now? Can you still see the crosshatch hone ? How was th engine run in?

My guess is that the piston rings were broken in assebly or maybe you didn't have enough gap and they broke when they got hot and expended. I can't realy say without knowing the answers to the above . Don't forget when using forgies you have to have bigger piston to bore clearence than cast pistons .

Piston clearance and ring gap i will find out tomorrow.

I didnt measure myself the gaps and clearances when the engine was built, i can only ask the engine builder what he would of set them at...

Yes the bores are slightly worn now, no 6 is slightly scored from where the ring came past the piston and needs rebore. All bores are glazed with some patches of cross hatching.

Engine was run in on a dyno for its first kays, with running in oil for probly the first 1000, filter and oil changed within first half hour of running.

Will get pics of rings and measure top ring dimensions.

[femno]

Today i got pics of the conrod bearings and the rings, took measurements.

Current 2nd ring end gap: 50thou !

Current piston to bore clearance: a tight 9thou (4.5 either side) !

And something else:

The conrod bearings; the top bearing (the one with load on it) appears to have slight pitting, you can see the copper colour coming through - see pic. The lower bearing of the same set is fine, no copper. Apparently this would be caused by the 'banging' of detonation.

Lower bearing on right, top on left.

Yes the second ring has the step in it as its sposed to, so the shape of the top rings is purely from wear.

Also the ring lands look like this:

Measurements of the broken top ring:

[femno]

Anyone?

[wrxhoon]

I don't think you are measuring right, 9 thou is far to much clearence . Where do you measure the piston? piston to bore clearence should be no more then 4 thou for forgies and like 1.5-2 thou for stockers (cast).

50 thou ring gap is to bid as well, it should be like 15 thou max.

The bearings are shot, possibly due to detonation.

Start rebuilding and this time make sure whoever does it they measure everything and double check to make sure you don't have to built it again.

Today i got pics of the conrod bearings and the rings, took measurements.

Current 2nd ring end gap: 50thou !

Current piston to bore clearance: a tight 9thou (4.5 either side) !

And something else:

The conrod bearings; the top bearing (the one with load on it) appears to have slight pitting, you can see the copper colour coming through - see pic. The lower bearing of the same set is fine, no copper. Apparently this would be caused by the 'banging' of detonation.

Lower bearing on right, top on left.

Yes the second ring has the step in it as its sposed to, so the shape of the top rings is purely from wear.

Also the ring lands look like this:

Measurements of the broken top ring:

[proengines]

Bearings and ring lands reek of detonation.

[femno]

I don't think you are measuring right, 9 thou is far to much clearence . Where do you measure the piston? piston to bore clearence should be no more then 4 thou for forgies and like 1.5-2 thou for stockers (cast).

50 thou ring gap is to bid as well, it should be like 15 thou max.

The bearings are shot, possibly due to detonation.

Start rebuilding and this time make sure whoever does it they measure everything and double check to make sure you don't have to built it again.

No, trust me im measuring right. The pistons fit like a dick in a shirt sleeve. Im measuring near the outside of the block, not say next to the next bore if that makes any sense.

If i can fit a 9thou feeler down in between the piston and bore (far enough down past the ring lands, yes) this is accurate yes? same deal with the ring gaps. Rings in bore, push down with the piston a little to make it square, then measure. i had to combine a 35th and a 15th feeler...!

Possibly the reason for the massive piston clearance is the extreme heating of the piston; expanding compressing explanding and then compressing finally to a smaller size maybe? Is this a fair assumption?

Theres no way the bores can wear that much? ill measure dia of the pistons tomorrow.

Thanks guys keep it coming if you can please.

I really dont want to have to build a third...

[R31Nismoid]

The pistsons i wouldnt imagine would have "shrunk" as your thinking.

Sounds wrong in the first place

[GTRgeoff]

Here is the ACL RB26 page http://www.aclperformance.com.au/NissanRB2...rgedPistons.htm

5 thou is the clearance so the build was wrong from the start. Pistons won't shrink. There is no way the bore has worn that much if you can still see crosshatching.

That ring gap is horrendous.

Quite possibly the combination of the large clearances combined to induce excessive detonation through high quantities of oil in the combustion chamber. Oil is a detonation contributor.

I can offer a decent machinist/builder or two in Melbourne if you want, or just send it up to proengines for a good job. Word is he's tip top and has posted a detailed thread on here of one of his engine builds.

[proengines]

Thanks Geoff. What you really need to do is get an accurate measurement of everything. If the clearance is that large, you need to find whether it is the bore being too big or the piston collapsed. take it to someone who can measure it with a micrometer or bore gauge. If the bore is the right size, I'd imagine that the pistons have collapsed. They can do this if they are severely overheated. I haven't seen a set of pistons in years that have been made to the wrong size, they are made to give the correct clearance at the nominal size. The shape of the top ring land is a very good indicaton of detonation, the same with the hammering on the top conrod bearing shell.

The ring gap increases very quickly as things change, for every .001mm in bore size, you get .00314mm increase in ring gap (Pi x Diameter). The same deal as the outer face of the ring wears. If you measure the radial depth of the ring in different positions it will show the wear on the ring. They rarely wear as much right at the gap.

Take the parts back to whoever did the job and ask them to check it, if it ended up being a warranty job, they have first right of repair anyway. They may even help you out with fixing it.