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mahle piston failure

General engine tech -- Drag Racing to Circle Track

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Protech Racing
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Re: mahle piston failure

Post by Protech Racing » Tue Nov 05, 2019 1:05 pm

Long term detonation . If the ring gaps are all lined up, it has run for a while with light detonation.

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Re: mahle piston failure

Post by exhaustgases » Tue Nov 05, 2019 7:22 pm

The material looks like its been pretty hot. Looks like a heat stress crack in a thinner area.

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Re: mahle piston failure

Post by Krooser » Tue Nov 05, 2019 8:41 pm

Are Mahle's made from 4032 forgings?
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Re: mahle piston failure

Post by smeg » Wed Nov 06, 2019 5:41 pm

Protech Racing wrote:
Tue Nov 05, 2019 1:05 pm
Long term detonation . If the ring gaps are all lined up, it has run for a while with light detonation.
I have seen this rings lined up before, what are the reasons behind this please?

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Re: mahle piston failure

Post by Tuner » Wed Nov 06, 2019 8:27 pm

smeg wrote:
Wed Nov 06, 2019 5:41 pm
Protech Racing wrote:
Tue Nov 05, 2019 1:05 pm
Long term detonation . If the ring gaps are all lined up, it has run for a while with light detonation.
I have seen this rings lined up before, what are the reasons behind this please?
I saw that several times in the 454 in my flatbottom after several hours of run time with several races with several minutes of WOT in each race. It was apart several times in one summer of racing because of cam lobe failures. I think the rod angle thrust pushes the cylinders out of round and the rings just conform to the slight oval shape. The gaps of every ring, both compression and oil scrapers were in line with the center line of the cylinder, 90 deg. to the crank axis. What I don't remember is which, CW or CCW side. rats. #-o but they were up on one side and down on the other and all perfectly lined up. They were installed by the GM book picture, all in different clock positions. Fuel was Avgas and there was no detonation.

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Re: mahle piston failure

Post by smeg » Wed Nov 06, 2019 9:13 pm

Thanks for that, I am going to have to do some thinking on this one for sure.
Makes me wonder if you had a gapless top ring combo, would the gaps in the spacer ring line up??...

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Re: mahle piston failure

Post by Ericnova » Thu Nov 07, 2019 7:16 am

smeg wrote:
Wed Nov 06, 2019 9:13 pm
Thanks for that, I am going to have to do some thinking on this one for sure.
Makes me wonder if you had a gapless top ring combo, would the gaps in the spacer ring line up??...
Spacer rings have a "pip" mark (punched, divot on top side and peaked bump on the opposite side) that hooks in the blank ring land area over the wrist pin opening. The spacer ring cannot turn and align the gap with the pin bore opening. I'm assuming when you said "spacer rings" you were talking about for the oil ring group.

If you meant the gaps in a 2 piece gapless compression ring, they seems to stay opposite of each other.

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Re: mahle piston failure

Post by Dan Timberlake » Sat Nov 09, 2019 1:29 pm

In the 70s aftermarket forged BBC pistons had top ring grooves that were pretty close to the valve notches.
When fly cutting the pockets for bigger valves and more V-P clearance it was not unheard of to break thru to the ring groove.
Folks ran them that way fairly successfully, at least in drag racing.

The first tip-off was the valve pocket cutting a scallop thru the top land.
https://i.ebayimg.com/images/g/gDEAAOSw ... -l1600.jpg
https://i.ebayimg.com/images/g/b7YAAOSw ... s-l640.jpg

Looks like Even some modern pistons have the intake valve pocket create the tell-tale scallop thru the top land.
http://www.2040-parts.com/_content/item ... 02/001.jpg
https://static.summitracing.com/global/ ... ?rep=False
Probably not much material between the pocket and the back wall of the ring groove in a small region.
So if those new BBC pistons require deeper/larger/re-angled notches breaking thru to the ring groove is a possibility.
===========.
The OP's pistons look to have OEM finished valve clearance trench.

Are they hypereutectic?
Any signs of ring end gaps butting ?

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Re: mahle piston failure

Post by cjperformance » Sat Nov 09, 2019 6:32 pm

Dan Timberlake wrote:
Sat Nov 09, 2019 1:29 pm
In the 70s aftermarket forged BBC pistons had top ring grooves that were pretty close to the valve notches.
When fly cutting the pockets for bigger valves and more V-P clearance it was not unheard of to break thru to the ring groove.
Folks ran them that way fairly successfully, at least in drag racing.

The first tip-off was the valve pocket cutting a scallop thru the top land.
https://i.ebayimg.com/images/g/gDEAAOSw ... -l1600.jpg
https://i.ebayimg.com/images/g/b7YAAOSw ... s-l640.jpg

Looks like Even some modern pistons have the intake valve pocket create the tell-tale scallop thru the top land.
http://www.2040-parts.com/_content/item ... 02/001.jpg
https://static.summitracing.com/global/ ... ?rep=False
Probably not much material between the pocket and the back wall of the ring groove in a small region.
So if those new BBC pistons require deeper/larger/re-angled notches breaking thru to the ring groove is a possibility.
===========.
The OP's pistons look to have OEM finished valve clearance trench.

Are they hypereutectic?
Any signs of ring end gaps butting ?
Ford cleveland pistons, ford 385 series bigblock (canted valve) have always had a valve cut nipping into the area above the top ring land, no big deal at all.
Stroker windsor with short pistons and big valves/intake lift end up with valve reliefs that come very close to the deepest area of the top ring groove and again no problem. That piston has simply suffered poor tune/excessive heat , inadequate top ring end gap, have a look at the bores as well, they always show ring butting in the form of extra wear and scuff right at the top of the bore/ring travel.
Craig.

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Re: mahle piston failure

Post by Dave Koehler » Sat Nov 09, 2019 10:05 pm

Anyone ever consider that the ring gaps come close to lining up in operation and appear lined up when you pull it apart as the rings expand in both directions?
Leakdown test with the rings lined up on purpose?
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