Skimming off 0.5-1.0 grams off the piston crowns

Road-race engines and ancillaries - general discussion
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Brit01
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Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Wed Jun 29, 2011 4:05 pm

Hi members.

I am new on this forum and have been rebuilding my Alfa romeo 33 1.7 QV Series 2 (1988). It has 2 twin Dellorto downdraft carbs.

I've had to buy new camshafts, tappets (INA), new gears for the oil pump, new rings(Goetze) and big end shells.

2 of the pistons had some pitting on the crowns. They weighed between 384 grams - 388 grams.

I did not want to replace them with the small pits as I believe the expansion and cooling of the tiny crevices can cause more stress on the piston.

I sanded them down lightly, weighing them frequently making sure I wasn't taking off too much that may affect it's structural integrity.

After getting a smooth surface each piston had no more than 1 gram removed.

I'm aiming to get the surface very smooth to reduce the inevitable carbon build up, at least slow down the process.

Just to check with the experts that taking off a 1 gram off the crowns will not affect it's strength (maybe it was just 0.1mm or so). Otherwise I will try to find replacements. This is extremely difficult where I live here in Uruguay.

Thank you.


crown sanded 4.JPG
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Guy Croft
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Guy Croft » Wed Jun 29, 2011 5:34 pm

She'll be absolutley fine. Cast pistons are way thick.

G

Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Wed Jun 29, 2011 6:05 pm

She'll be absolutley fine. Cast pistons are way thick.
Thanks Guy. Yes I inspected the thickness and thought the same. How on earth is 0.1-0.2 mm off the crown going to make it weaker. There is a lot of metal underneath there.


What about in the concave dome? Just 1 piston left with very very fine pitting in the middle, maybe 0.05mm pitting or less.
I know this is the weaker part.
The other 3 are great.

Thanks.

Guy Croft
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Guy Croft » Wed Jun 29, 2011 6:49 pm

The amounts you are talking about are nothing really in terms of piston strength. Really determining what you can and cannot do is a question of experience (sometimes bitter when you get it wrong) and it's best to section a spare piston (?!) and find out what the true underhead profile is.

However the design you have will probably conform roughly to my photos below in that:

1. The most 'at-risk' bit of the piston under firing pressure * (and in the case of light detonation) is right in the center of the dish (or bowl) thus adquate thickness is vital there. One would design a piston (cast or forged) with peak firing pressure in mind + a safety factor of 2. Since cast material is at least half the tensile strength of any forging alloy cast pistons need to be way thicker and whereas a forged item on an atmo engine (normally aspirated) could easily tolerate 4mm thick there a cast one would need 7 or 8mm or in the case of the supercharged one I have drilled to illustrate this 9mm. You can certainly run 5mm thick on an atmo engine with a cast piston.
but...

2. ...in order to develop maximum bending strength the underhead of pistons of most types have an 'arch-shaped' cross section as you can readily see in my 2nd photo. This means the piston gets thicker toward the outer radius and esp if it has deep valve reliefs. It would be quite normal - if the center of the dish was 9mm - to see 12mm thickness (or more) under the crown. In other words massive! If the piston is not arched then one should be aware that the central region will not have as much support in bending stress and one should take maybe 6mm thick as the minimum.

(* In terms of maximum applied stress the weakest parts of a cast piston is always under the pin boss region or where that area connects with the ring land section and those regions can fracture a excessive rpm due to either stress in excess of tensile strength or fatigue).

A cursory survey with a digital calliper will hopefully indicate to you that the metal removal you have undertaken is functionally insignificant.

Hope this helps,

G
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showing the key features topside..
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Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Wed Jun 29, 2011 7:00 pm

This is an excellent forum Guy. You are quite an expert.

Thank you so much for this informative answer.

This stamp in the middle indicating the direction of rotation and width of the piston could indicate easily how much I'm taking off in the middle.

I wonder how deep these stamps are?

Guy Croft
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Guy Croft » Wed Jun 29, 2011 7:05 pm

Yup, thanks for that, once in a while I get something right.

Yes, stamping - significant stress raiser too...! Not that anyone in the auto industry cared about things like that in those days"!!

My how times have changed. We all use lazer etching these days.

G

Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Wed Jun 29, 2011 7:15 pm

Any idea how deep these stamps are Guy?

Could indicate how much I'm skimming off.
Surely no more than 0.1-0.15mm.

tmvolumex
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by tmvolumex » Thu Jun 30, 2011 6:33 pm

You may also want to investigate the cause of the pitting. Detonation is a possible cause.
Pits dont appear too bad, but it would be a shame to ruin a fresh engine if detonation is the cause.
Tom
GC_31

Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Thu Jun 30, 2011 6:49 pm

It's a design flaw on cylinder 3 on the Alfa boxers. The pistons are 24 years old so not bad really.

Cylinder 3 has the oil pressure sensor and filter and somehow disrupts the oil flow through there. Number 3 suffers.
Alfa romeo for you! Flaws or characteristics?

Slight pitting on another but over it's 24 years of use who knows how it's been run. Maybe from a previous owner.

In my time I've been very careful. I installed a Bosch wideband AFR sensor and gauge to monitor to mixture from the carbs. Nice piece of kit.

WhizzMan
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by WhizzMan » Thu Jun 30, 2011 7:32 pm

Ah! My kind of engine. Guess what my next project is going to be. I currently have a "to be rebuilt" 1.7 IE engine on the workshop floor, ready to be torn apart. In my experience, you can do a lot with the 1.7 engines without messing the pistons up. This also explains your "up and down" arrows and banks, still having a 4 cylinder. It's a boxer engine!
Book #348

Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Thu Jun 30, 2011 7:46 pm

It's a boxer engine!
They're nice to tear apart true.
And pretty strong engine due to the short crank and rods.
Great low centre of gravity and high revving, great overrun sound.

Once you manage the ART of balancing the twin carbs they reward you. If out of balance you're in trouble.

Brit01
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Re: Skimming off 0.5-1.0 grams off the piston crowns

Post by Brit01 » Sun Jul 03, 2011 5:56 pm

the pistons with rings and pin vary from 508 - 504 grams.

I tried for 20 mins grinding the inside of the pin wiht sparks flying off everywhere.

Nothing came off - maybe 1 miligram!!!

Think you need a lathe.

Don't think it's worth it for 4 grams difference no? Unless I'm racing at 7000 rpm.

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