94mm HC Piston Identification

tubbylad

Hello collective.
I hope you can help me identify these high compression pistons.

Not seen a segmented oil control ring like this either.

Supposedly from Ducati however I cant find any details and hand written "instruction" don't strike me as factory...

Considering using them for my rebuild but would have them teflon coated on the skirts first.

Thanks,
Martin

Mr.R

94mm pistons were OEM on 888 and 916's but there's a difference in the deck height.
Do you know what deck height you have?
On another point...
Skimming the 888 or 916 heads by 1mm results in a raised compression ratio (using standard flat top pistons) slightly higher than dropping in lumpy after market items.
The "Lumpy" pistons are usually only an advantage when fitting them alongside wild/higher performance cams.

Steve R

brad black

are the skirts grooved? they look like ram to me. i have a set in 92mm. i think i've read bruce meyers talking about them breaking, but don't recall now.

tubbylad

They are grooved, yes.
Is Ram the manufacturer then?
Perhaps Bruce you can share your experience with these if they are indeed the same...

distance from top of the piston flat (not the hump) to the top of the gudgeon pin is 0.015" more on the new piston vs old flat top - rough vernier measurement.

BABA

These are Gio.Ca.Moto pistons kit.
I have got the same on my 888. They are lighter than strada/SP/Racing OEM pistons.

tubbylad

Thanks for the feedback.
Kind of makes sense with the hand written Italian instructions.
For how many miles have they been in the bike?
Does it get "used" or is it just paraded on sunny days?

You are correct, they are 12% lighter as an assembly vs flattop (piston, rings, gudgeon pin)

Thanks.

Mr.R

Measurement from the top of the pin to the squish land (the flat part NOT the height of the crown) on the pistons:-
888 = 18.50mm
916 = 17.50mm
Both of these being Ducati 4 valve 94mm items.

Most aftermarket/race pistons weigh less than the OEM 851/888 items.

Steve R

BABA

tubbylad wroteThanks for the feedback. Kind of makes sense with the hand written Italian instructions. For how many miles have they been in the bike? Does it get "used" or is it just paraded on sunny days? You are correct, they are 12% lighter as an assembly vs flattop (piston, rings, gudgeon pin) Thanks.

Around 15000 km since I have restored the bike (engine rebuilt and tuned).
I don't ride a lot but it works very well (not just because of the pistons of course 😁) and the engine takes (easily) its 11000 rpm (limited by the eeprom) if I want to push it high (well sometimes... because this is a road bike, not a race bike, so I m not often revving it so high on the road).

outtacontrolla

Mr.R wrote94mm pistons were OEM on 888 and 916's but there's a difference in the deck height. Do you know what deck height you have? On another point... Skimming the 888 or 916 heads by 1mm results in a raised compression ratio (using standard flat top pistons) slightly higher than dropping in lumpy after market items. The "Lumpy" pistons are usually only an advantage when fitting them alongside wild/higher performance cams. Steve R

Don't skim the heads ,unless you have shrouded valves - remove metal from the barrels if needed , you can replace with thicker base gaskets if it doesn't work !

marvin

I believe Brad is correct. I had a set of those years ago. Bruce suggested that I not use them.

P.S. I bought a big bore kit. I was told that they were Gia-Ca-Moto (perhaps made by RAM ???). I was told that the barrels were OK but throw the pistons away. So, I bought Pistal's. A less expensive alternative might be Woosner pistons. I don't know if they are quality pistons or not.?? A friend put them in his 996 track bike a few years ago and they have been OK. But then he's not revving the snot out of it !

Mr.R

Wossner pistons are top quality kit, I have them in my 970cc Bevel twin.
I have PISTAL'S in my 955cc DESMOQUATRO motor.

As for skimming the barrels instead of the heads??
I'll have to agree to disagree with Steve Hillary aka OUTACONTROLLA...
If you take 1mm off the barrels it's likely that the pistons at TDC will be close to the top of the barrels, if not actually above them?
Skimming the heads won't alter the deck height of the pistons but shortening the barrels certainly will, possibly then needing to modify the pistons.
It's 100% OK to skim the 851/888/916 heads by 1mm without the need to sink the (standard 32/28, 33/29mm) valves.
I set the squish clearance on the 92,94 and 96mm DESMOQUATRO motors to 0.8mm, to be on the safe side.
Anything more than 1.00mm isn't going to achieve what a squish should and any less (0.75mm or under) is running the risk of the pistons hitting the heads at high rev's.

Not wanting to start an argument here, rather just stating what I know works

Steve R.

marvin

I tend to agree with Steve R

brad black

the old fibre head gaskets were about 1.3mm thick, so you needed the piston 0.3 - 0.4mm up above the top of the cylinder to get the squish up. std the piston was pretty much level with the top of the cylinder. leave the original 0.3 - 0.4mm base gasket out and you were about right.

the later steel shim head gaskets are thinner, so if you are using old cylinders and pistons you generally need to lift the cylinder to effectively lengthen it to get the correct piston to head (squish) clearance. later engines tended to have 0.6mm base gaskets too, so cylinder lengths varied as well.

the issue i have with shortening cylinders or skimming heads is that you bring the cams closer to the timing shaft, which can lead to belt tension (lack of) issues. so you need to pursue methods there to bring the tension back up, like the gold or silver belt rollers that are larger as used on later engines. even leaving the base gaskets out can give issues with otherwise all std parts. we saw some all std bikes not able to get the correct belt tension over the years.

the purpose of skimming the head is to reduce the chamber volume and thereby increasing the compression. machining the cylinder doesn't change the compression from where it would be if you were setting the squish anyway, the squish face on the head surface limits where you put the piston.

imo the 748 head is a great starting point over an 851/888/916 head if the valve sizes suit what you're doing. lot smaller chamber. i even think the 748 ports work better at 850cc level than the 851 ports do - 94mm bore kitted 748 make very nice torque and power (and compression) for the capacity.