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Discussion Starter · #1 ·
Hello guys, i'm rebuilding my 19S engine (1969, from a rekord C, same engine as in the GT) with 20S standard pistons (95mm) and i came across this "situation":
The pistons that came from a 2.0 engine had very light traces of wear on the skirt from bottom to top and more pronounced traces on the top side of the piston, above the piston rings.
I corrected most of the light traces on the skirt using P1000 sand paper and water, cleaned the pistons, greased them with 10w30(what i have around, not what i will be using when fire up the engine) inserted them into the cylinder (without segments), torqued them to the crankshaft and started rotating it with a 17mm wrench.
I noticed that the bottom side of the skirt rubs on the cylinder, despite the extra-oil i have added. Rotated the engine 20-30 times and then removed the pistons for inspection. I've seen that the pistons rub in almost the same place, the bottom side of the skirt.
What do i have to do about this situation? Is it possible that the same thing that caused the pistons to start rubbing in the previous engine to cause the rub again? (although i don't know the previous cause)


I should specify that the crankshaft has been rectified for +0,25mm roller bearings, both main bearings and rods. The crankshaft is the original one that was in the 20S donor engine (the rods are also original, haven't separated them from the pistons).

The main bearing cap bolts were torqued at 90Nm (40 to 90 in 10Nm steps, yes, i have time:) ) and the rod caps were torqued at 70Nm (again, 10Nm steps). Have i used the right torque for the bolts?
 

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So you are using old pistons from one engine in an old block of a different engine? This is a bad idea. The cylinders must be bored to the pistons, not just slap some pistons in and hope they work. The pistons are likely worn out and will never fit the cylinder correctly. You need to bore the cylinders and fit new pistons properly.
 

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Discussion Starter · #3 ·
The cylinders are bored and honed. Took the engine block and pistons to the machine shop and told the guy: i want to fit 2.0 pistons into a 1.9 engine, stroke is the same, difference in bore is 2mm (in diameter). Here's the block, here are the pistons i want to put in, here's the crankshaft for overhauling.

Seems i forgot to specify this, although i tried to keep in mind the above words untill the end of the main post.
 

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If the piston rings or the wrist pins were worn, you could get some scoring on the sides as the piston would not be held in the center of the cylinder. Did you try it with new rings? Is the scoring the same on all 4 pistons?
...inserted them into the cylinder (without segments)
what do you mean by "segments"?
 

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The pistons that came from a 2.0 engine had very light traces of wear on the skirt from bottom to top and more pronounced traces on the top side of the piston, above the piston rings.
There is a chance that the pistons are no longer 'true', they have probably been overheated at some time in their previous life. I wonder if your machinist measured the pistons in the correct location when he bored the block to size. They should be measured approximately 13 mm from the bottom of the piston skirts, on the thrust faces.

The main bearing cap bolts were torqued at 90Nm (40 to 90 in 10Nm steps, yes, i have time:) ) and the rod caps were torqued at 70Nm (again, 10Nm steps). Have i used the right torque for the bolts?
No, unfortunately. Main caps are supposed to be torqued to 72 ft lbs, or about 97 nm. You need to tighten them up a bit.

The rod bolts are supposed to be torqued to 36 ft lbs, which is 48 nm. You have severely over-torqued the rod caps by tightening to 70 nm (about 51 ft lbs). The rod bolts are already damaged (stretched) and should NOT be used! They will likely fail. You may also have distorted the rods themselves, so the big ends may need to be re-sized to avoid rod bearing damage.

HTH,
Bob
 

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Discussion Starter · #6 ·
what do you mean by "segments"?
segments=piston rings, the term just went off my head and wrote the first translation that passed my mind.

The scratches (believe that's what you ment by scoring) are not the same on all pistons. They vary from one to another, but they are pretty close.

I haven't tried the pistons with the new rings. I had too many troubles with this engine so i tried to play it safe.
RallyBob said:
The rod bolts are already damaged (stretched) and should NOT be used! They will likely fail. You may also have distorted the rods themselves, so the big ends may need to be re-sized to avoid rod bearing damage.
Bad news for me, stupidity must be payed (like usually). When I opened the rod caps, i remember using alot of force, comparable with the one required to open the crankshaft caps.

You are right about the distorted rods, i have noticed uneven wear on the rod bearing after i re-opened the caps. I was thinking that i have done another stupid thing (overtightening the caps, in this case), i just needed someone to confirm that to me.

The good part is that i have at least a spare set, from the original 1.9 pistons (forged ones:) ) but i was afraid to use them from the beginning because of the "disaster" i found in the engine i'm rebuilding. How can i make sure that the rods are not damaged? They don't seem bent or damaged..

About the dissaster inside the engine: 1.2mm of ashes from burned oil - oil soot?? not sure how it translates - inside the engine, water combined with oil and gas due to damaged head gasket, worn to the extreme piston rings and bearings, bearings had thin metal foils underneath them, timing chain was stretched, water pump housing had some bits missing from it - got a replacement aluminium housing. More scary pics with an engine with 0.4mm wear inside the cylinders that was still somehow running with the original STD pistons can be found in my flickr gallery.

The cylinder head from this engine was also in bad shape, i managed to partially restore it (still needs new valve guides, i intend to do custom ones and fit 2.0 valve gaskets, not the silly rubber rings). After i'll fix this "thing" with the pistons i'll take care of it. BTW, is the 95Nm torque value for the cylinder head bolts torque the correct ?

Surprisingly, the only 2 things from this engine that seem to be in good shape are the pistons (admired them like a work of art, best damn looking ones i have ever seen) and the camshaft.

RallyBob said:
I wonder if your machinist measured the pistons in the correct location when he bored the block to size. They should be measured approximately 13 mm from the bottom of the piston skirts, on the thrust faces.
He measured them in the right place (at least 3 of them, i was there when he did that), but i tend to doubt his measurement. I'll borrow a 10cm micrometer and measure all the pistons again.

I have noticed when cleaning the pistons that some of them had very light traces of valves in the valve shallows. Don't know if that damaged the pistons or not (from what i can see, they didn't), but i think that's the reason why i found an old-style 19S cylinder head bolted on the 2.0 engine.
I believe i'll get some sugestions about thinking of changing the pistons as well, but unfortunately it's not that easy, since they are pretty expensive at the moment and i kinda' have to finish up the entire car by the end of june (simultaneous with finishing up the garage, geting an engine crane, an air compressor with painting accesories and a mig welder)
 

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How can i make sure that the rods are not damaged? They don't seem bent or damaged..
A few things you can check or have checked.

Once the rods are disconnected from the pistons, they can be checked to be sure they are not bent or twisted by laying them on a surface plate (flat granite block or piece of glass even). They should not 'rock' side to side or diagonally. If they do they are probably twisted or bent.

A machinist should be able to measure the center-to-center length of the rods....128 mm. If they are shorter than this number they have perhaps been resized before (material ground from cap and honed to size).

And a good machine shop should have magna-flux equipment to check for fatigue cracks on the rods too.

BTW, is the 95Nm torque value for the cylinder head bolts torque the correct ?
Yes, 95-97 nm is good for the cylinder head too.

Bob
 

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Discussion Starter · #8 ·
Thanks for the tips Bob, i'll try those as soon as possible.
The only problem around "here" is that you can not find a good machine shop. Even harder to believe is that i'm searching for awhile for a shop that can dynamiclly balance a crankshaft or a flywheel (actually i've found one or two, but in the other corner of the country, 600-700km away)
 
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