3.2-3.4 mystery pistons

[infraredcalvin]

Can somebody help identify these Mahle 98mm 11.3:1 pistons. Look to be set up for single plug? Can’t find any visible P/N or match the domes to what’s currently available. I need some rings as many are frozen to the piston due to long term hibernation.

TIA

[Henry Schmidt]

Wrist pin size will tell you if they are 3.2 (22mm pin) or 3.4 (23mm pin).
From there you can calculate dome volume. Were I to guess I would think closer to 10.4 :1 than 11.3.

[infraredcalvin]

Thanks for the quick reply, I believe there are 23mm , cp was measured at 11.3…. Some weird things upon disassembly, including 2mm worth of base gaskets and double chain housing gaskets. Do these look like single plug domes to you? Engine is twin plug making low power…. Looks like a lot of nice parts, but wrong combo…

[Peter M]

The current 98mm Mahle pistons use a 1.2, 1.2 & 3.0 ring set. Maybe your pistons are the same?

They certainly have more crown on them than the ones currently sold by Mahle and measured at 10.4:1 in my twin plug 3.4. The single plug versions have a large flat bevelled crown. Google Moritz for images.

However surely you could reuse the current rings as I've noticed you have matched the pistons and cylinders? Assuming a couple things spec out OK of course.

However, the challenge will be to free up the rings without damaging the ring lands that would render the pistons useless new rings or not.

Looking forward to hearing how it all goes.

[stownsen914]

If the engine is still assembled enough to do so, put a piston/cylinder back on and measure the deck height. The prior builder may have used shims to lower compression. That can be OK, but I've heard many builders say to keep deck height in lower end of the usual recommended .040-.060" range.

[dannobee]

As for the thick base shims, verify the crank stroke and rod length.

[gled49]

The pistons look like earlier production as late production valve pockets were not as isolated. These valve pockets have been cut a little. These have 22mm pins. Do the cylinders have groves cut for CE rings.

[Henry Schmidt]

Originally Posted by infraredcalvin

Thanks for the quick reply, I believe there are 23mm , cp was measured at 11.3…. Some weird things upon disassembly, including 2mm worth of base gaskets and double chain housing gaskets. Do these look like single plug domes to you? Engine is twin plug making low power…. Looks like a lot of nice parts, but wrong combo…

How did you measure the compression ratio?

[Bira]

They do look like high-compression single-plug pistons, possibly custom or older race-spec Mahle units. If there’s no part number, you might try measuring the ring grooves and contacting JE or Total Seal with specs to match rings.

[stownsen914]

For what it's worth, those look similar to the Mahle 100mm 3.5L pistons I pulled out of my engine. They measured around 11:1.

[silverlock]

As a point of comparison, these are the Mahle PP98-014 10.2:1 98mm pistons in my 3.2SS:

[infraredcalvin]

Originally Posted by Henry Schmidt

How did you measure the compression ratio?

My mech cc’d the volume before tearing apart. Long story short, I woke up a garage find sitting 10+ years after what appears to be a diy rebuild. Looked to have a lot of nice parts. But never made any power (165 hp at the wheels), chalked up to bad combo. Ran well enough to leave alone for last 2 years, but had a fuel pump and reg go haywire and flooded carbs, till they popped and caught fire, melted the 2ndary venturis. It’s now a shop trying to figure out what I have so I can build it back right. Was trying not to crack the case, but finding a few of these unknowns such as the base gaskets it’s time to open her up to see what’s actually inside.

[infraredcalvin]

Originally Posted by Bira

They do look like high-compression single-plug pistons, possibly custom or older race-spec Mahle units. If there’s no part number, you might try measuring the ring grooves and contacting JE or Total Seal with specs to match rings.

I think we’re going with new JE pistons for an updated dome to work better with the twin plug, and more importantly to control the compression without having to stack gaskets. This will ultimately be a DE/canyon carver car.

[PeteKz]

Agree with opening the case. You are into it this far, and if you don't split the case, you will always wonder what's inside. When you do, take lots of pictures and measurements.

However, be careful with the main and rod bearing shells, and keep them in order. There have been some bad runs of bearings. If the ones that are in the case now are within spec, and look clean, then reuse them. Bearing shells are not "wear items" in the traditional sense. Only replace them if out of spec or visually damaged.

[mikedsilva]

Originally Posted by PeteKz

Agree with opening the case. You are into it this far, and if you don't split the case, you will always wonder what's inside. When you do, take lots of pictures and measurements.

However, be careful with the main and rod bearing shells, and keep them in order. There have been some bad runs of bearings. If the ones that are in the case now are within spec, and look clean, then reuse them. Bearing shells are not "wear items" in the traditional sense. Only replace them if out of spec or visually damaged.

Pete
if I didn't read one of your posts where you recommend to re-use old bearings, I'd be disappointed!

[Henry Schmidt]

In this thread there has been a mention of "twin plug pistons" This term doesn't make sense. There is no special "dome shape" for twin plug ignition.
Twin plug ignitions will certainly help with flame propagation when dealing with high domes that split the chamber but in general, twin plugging works well even with low compression pistons. We build them all the time. The key is to simply match the ignition curve with the cylinder pressures. In general, low compression engines can run increase advance curve. Where 26-28 degrees might be appropriate for an 11.5:1 compression, 32-34 degrees might be appropriate for a 9.5:1 engine.
Those pistons would lead me to believe that they are something unique. Generally you would see larger valve pockets to accommodate more aggressive , high lift cams.

[Henry Schmidt]

Reusing old bearings is an unnecessary gamble. Bearing are held in place by compression and when you relax the bearings and journals measurements change.
Although some times "new parts don't work", an engine builder worth his salt should be able to ascertain the quality and usability of any part used in his rebuild.
I have a friend who was notorious at the repurposing of bearings.
His favorite saving: "better than new, because they're tested".
He was even seen spraying used parts with Cosmoline before placing them on the shelf.

[infraredcalvin]

Originally Posted by Henry Schmidt

In this thread there has been a mention of "twin plug pistons" This term doesn't make sense. There is no special "dome shape" for twin plug ignition.
…
Those pistons would lead me to believe that they are something unique. Generally you would see larger valve pockets to accommodate more aggressive , high lift cams.

Bingo! Thank you Henry for binging this up, i was hoping to get your insight.

It has been explained to me that there is a piston dome “optimal for twin plug”. This “optimized” dome has a flatter area, sometimes even a pocket in the middle. This flat area provides a central area for fuel/air mixture to evenly propagate providing more even combustion… particularly in the case where only one plug fires.

Of course I cant find it now, but when i was trying to research these pistons, I swear I saw a vendor or two with descriptions differentiating weather they were twin or single plug pistons.

When comparing to other 3.2 to 3.4 Mahle pistons (the shop had some sitting there from another motor being built) mine the tops were much different, mine have a more pronounced dome whereas the others were flat. In addition the wrist pin sleeve on the piston (the part of the piston that holds the wrist pin?) is cast lower than the other Mahle piston, probably the reason for higher compression necessitating the previous builder to stack gaskets to lower the compression.

Bottom line (cost and uniqueness), I wouldn’t mind identifying my existing pistons for new rings and reuse them, i just don't want a high strung engine necessitating race gas. I suppose we could send one to total seal and get some custom rings fitted…

[Henry Schmidt]

Originally Posted by Henry Schmidt

In this thread there has been a mention of "twin plug pistons" This term doesn't make sense. There is no special "dome shape" for twin plug ignition.

Of course for every "rule" there is/are exceptions. Somewhere in the back of my muddled brain I remembered some pistons we made to accommodate a tenuous twin plug situation.
The projected tip spark plug created an interference that needed attention. Options were surface gap plugs or creative machining. Surface gap proved to be too cold.