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Kameari street piston skirt /bore contact


Fairlineguy

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I’m using 87mm kameari street pistons
I had cause to remove the cylinder head after approx  150miles running in and was surprised to see

evidence that the lower skirt is contacting the cylinder wall.

Anyone else witnesses simulator wear ? If anyone is running slipper pistons and has a endoscope I’d be very interested in seeing a photo of your cylinder wall .

 

 

21B5672B-B502-4C9E-896E-3CA038A4967F.jpeg

C012E80E-50DA-4542-9DB4-CBC996FB1270.jpeg

7A21404C-26D4-4C16-9E6F-29AA1BB213D4.jpeg

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That is unusual. If there was something specific to the piston that would cause this, your machine shop would have been able to catch it during assembly. The piston is centered inside the bore via your rings. The piston can not contact the cylinder walls unless:

 

1) Piston installed is too large relative to your final bore. Machine shop would have checked that.

2) Your ring gap is incorrect, which is causing your piston to move around.

3) I have also seen skirt slap from damage due to detonation. Are you using forced induction? Did you experience anything that may indicate your engine was knocking?

 

I have Kameari forged racing pistons in a motor I am building, but the head is not finished yet so I have not run. 

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Can you catch the scratches with a fingernail?

 

I just pulled the head on my Rebello build and have the same markings . Wish I took a pic - but it’s all taped closed now . Dave allows a lot of room for the pistons to grow - like .006 difference . I have thousands of miles on this engine and have had the head of twice . Very early in the engines life it looked the same as now . I always let the engine warm up but I run it hard - A LOT :) 

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My experience is the same as @madkaw. On my 2.9, it looked like that at 200 miles and 10,000 miles.

 

Edit: I haven't had the 3.2 open again yet, but I expect less of it there. Longer rods and all that.

Edited by jonbill
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10 hours ago, Dat73z said:

@madkaw same observations here on my 3.2 Rebello. I'm only a couple thousand miles in boosted but I've been borescoping all the cylinders since it came off the dyno. 

I even sent a pic to Dave and he said to not worry about it 

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There are similar marks on my 3.0 rebello turbo after only a few thousand street miles. Weird.

 

I'm no engine expert but I have read that it can be caused by the pistons getting too hot possibly due to knock/detonation, expanding excessively, and rubbing on the cylinder bores. 

Edited by rossman
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20 hours ago, AydinZ71 said:

That is unusual. If there was something specific to the piston that would cause this, your machine shop would have been able to catch it during assembly. The piston is centered inside the bore via your rings. The piston can not contact the cylinder walls unless:

 

1) Piston installed is too large relative to your final bore. Machine shop would have checked that.

2) Your ring gap is incorrect, which is causing your piston to move around.

3) I have also seen skirt slap from damage due to detonation. Are you using forced induction? Did you experience anything that may indicate your engine was knocking?

 

I have Kameari forged racing pistons in a motor I am building, but the head is not finished yet so I have not run. 

So a bit more info 

I build the l28 last year with the Kameari pistons the second clear picture is of that engine at 150 mile when I had to have the head

back off. At the time I was a bit concerned to see evidence of skirt contact with the bore but I went on running the engine for a few thousand miles until for some unknown reason the bottom end bearing failed.

When stripping the engine for rebuild the bore scoring was really quite bad unsure if it could be cleaned up I took it to the machine shop who said it would need a fresh rebore .

Not wanting to buy another set of larger pistons they said that if I supplied another std block they could rebore it using the original pistons with new rings 

Anyway long story the fresh engine has now been totally rebuilt (bore and ring clearance are to spec.) Having now run the engine approx 150miles I decided out of curiosity  to look inside the bore with an endoscope (see picture one) there are the same signs of skirt contact with the rebuild engine .

I’m obviously now wondering if skirt/ bore contact is normal with slipper pistons 

 

 

 

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Edit - sorry I probably didn't read what you were saying correctly.  You meant that the oil film was breaking down and the skirts were scuffing the bore.  Anyway, the following still applies.  Short/small skirts have drawbacks.  Good luck.

 

Not sure why the focus is on the skirt contacting the bore.  Piston skirts do contact the bore during normal piston movement.  The reason for the small skirts is to reduce friction for better performance.  But it also allows more cocking of the piston in the bore so the wear patterns change.

 

Piston skirt design is a specific area of study.  Lots of stuff out there.  Performance improvements tend to have more expensive side effects.  You can build for longevity or performance, but those two things tend to conflict.

 

https://everythingaboutboats.org/piston-skirt-length/#:~:text=There is a direct relationship,rings and the cylinder bores.

 

https://auto.jepistons.com/blog/maximizing-horsepower-with-piston-skirt-shaping

 

https://www.sae.org/publications/technical-papers/content/970839/

Edited by NewZed
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On 4/1/2023 at 5:04 PM, NewZed said:

Imagine how these tiny skirts would run.  I think that they are coated to help with what you're seeing.

 

https://www.datsunspirit.com/product-page/super-lightweight-forged-pistons

 

image.png.7e65c1398a50901dc0852be26b8b2cac.png

 

I believe Datsun Spirit is an distributor for Kameari machined components. I'm curious if what you show here are the same pistons I recieved from Kameari direct. They look really familiar! Appear to be shortened piston height and slipper skirt. 

 

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So it seems slipper piston skirt contact isn’t unusual can’t say I’m convinced of longevity of them in road applications 

I’m using a 15/50 synthetic oil as recommended by the piston manufacturer but wondering if a zinc additive might be worth

adding ?

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