TorqueJunkie

Correct piston size?

13 posts in this topic

Posted (edited) · Report post

So I started hearing weird knocking noises from lower part of engine (roadster)... Good power, no smoke.

During inspection, some pieces of piston were found in oil sump.

Now I have the engine taken apart. All bearings are like new, all working surfaces are mirror like, no apparent damage, or excessive wear whatsoever. Cylinder is fine, no scratches.

However one of pistons is shattered in pieces...

1. Any idea how this could happen? There were no codes for detonation, also no mechanical signs of it

2. What size pistons should be ordered? These seem quite loose: cylinders are 66.5mm, and pistons 65.9mm.

3. Second picture is of a valve, taken with endoscope, through spark plug hole. Should I replace the valves, or can they be ground down together with head inserts?

distr.jpg

151209_191903.jpg

Edited by TorqueJunkie

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Nasty!  The top surface of the piston is OK?

 

I'd just go for a rebuilt long block and save the hassle.  That fit does sound extraordinarily loose, as though it was rebuilt poorly previously.  That could explain the skirt failure.

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Posted (edited) · Report post

Top surface of all pistons has some oily carbon deposits, but after that was cleaned off, they are like new.

After googling around I see that 66.5 mm is a standard size. No idea who, why and how managed to find and install 0.5mm downsized pistons.

 

About rebuilt long block, where can I get it at fair price?

 

Also, what clearance should be in bearings? Will have to check if those are correct size...

Edited by TorqueJunkie

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Try one of the smart specialists in England, they probably have long block exchange programs.

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Why is this piston kit so much (3x) cheaper?

http://www.ebay.co.uk/itm/201882512540

 

I have already gave the job to local mechanic and just guiding him with smart-specific stuff. I'm not from UK...

 

As for bearing clearances, the internet says generally 0.2mm is fine. Is this right?

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i dont like the wording of the listing

 

they show 3 pistons listed with title that says "set"...meaning most people would expect 3 complete piston and ring sets (with pins)...

 

yet they describe it as a ring set in description which leads people to believe maybe its just one set of rings

 

then they go along and post pin sizes

 

id stay away from a seller like that unless you message them for a clear description of what they are actually selling

 

 

plus the picture clearly shows pistons are 65.48mm so why list them as 65.60?

 

 

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Where are you located?  Your user info says London UK.  

 

I would stick with OE or better, not cheap pattern parts from discount sellers, made out of bubble gum in China.

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Posted (edited) · Report post

So I risked it. Now I know it's only one piston, with rings and pin. Contacted the seller about it and he was like 'without being rude, you are naive'. Well, I was naive, expecting it to be as pictured, and he is being rude as well.

Anyway there is a problem though: the pin does not have any mechanism to hold it in place. My original pistons have a spring ring, however this one does not, and pin can be easily moved around. I can see how this will damage the cylinder very quickly:

Or is this normal?? How do I fix it in place?

Edited by TorqueJunkie

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that is odd

 

maybe its designed to be press fit into the rod instead of floating with clips

 

cant say i like that idea 

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I've heard about press fit pins, but there's nothing about press here... It just floats around in there.

I hope I just don't know something, but looks like it's going back.

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Now I get what you meant. Yes, my piston pin is of "floating type" - held by retainer clips on both sides.

The one I received is of press fit type - heat pressed in to the rod and is somehow being lubricated properly to work inside piston walls.

The main problem is that pin diameters are different. Mine is 17mm, and this new one is 16mm. Very weird, as 17mm is hard to find....

 

If anyone has a leftover roadster piston with 17mm pin, I'll buy it off you... Let me know.

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Do I need new rings? The gaps are, from top: 0.4mm, 0.6mm, 0.8mm (oil).

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Never rebuild an engine with old rings!!

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