Am in process of custom billet pistons (Ross) and am entertaining the pin offset to thrust side, .040, due to material (T-6-ish/2018 tempered) to reduce cold startup noise. This offset affects the TDC timing. Does it stand to reason that one should or could alter intake and/or exhaust cam timing? Ignition timing? Series 3 head mildly ported and polished, cams are stock Ser3 3.75 lift and duration. As of now am working in the 9:1 realm of CR and don’t expect to stray much from that facet of design. No split skirts, but skinny rings.
99% street use expected.
Thanks,
Huff 69 2+2
Frankly, your solution looks to be one in search of a problem.
In many, many Jag rebuilds, I never heard any bothersome piston noise.
My opinion? Leave it the hell alone.
What is Ross’ opinion?
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How many .000000% would the offset effect TDC ?
Pin offset is, or was, somewhat common to reduce skirt slap. Flipping the piston to improve the rod angle on the firing stroke, at the expense of noise and wear, has been around since the 50’s. It only makes sense if you are competing in a race class that requires all stock engine parts. It won’t produce any benefit you can feel, but it will be noisier. Smokey Yunick employed it in the early NASCAR years.
2618 (not 2018) aluminum, if it helps allay some of the evident confusion.
Huff
No confusion, here: still not convinced there’s a problem.
I have put some questions to Ross and will report back.
If one does the math the added distance of stroke is only approx .001", for a .040 offset if I calculated correctly.
Thx, Huff
Air-cooled VWs have offset pistons… and rods! Piston TDC was different than rod journal TDC.
Either I’m hallucinating or many/most OEM Jag pistons have offset pins as standard, and they are offset by 4-5 mm, not the 1 mm (0.040") proposed. I’ve never actually measured the offset but it’s clearly visible from the side or below.
That’s why the arrow or word ‘Front’ is stamped on the top - to ensure the pin is located nearer the thrust (inlet) side. All above from eyeball memory, but still…
I handed over my only remaining original piston and wrist pin to the fabber for rough reference (not unlike marking the limb to be worked on, before surgery) but can probably have them check if the WP is offset, as there is a “front” marking on that slug, IIRC.
Huff
4.2 69 2+2
Nothing to ‘check’ or have them do, except look at the sides of the piston for an obvious off-center pin. It’s not subtle so no chargeable check procedure required, just yes or no on the phone once they have the part in hand.
Interestingly, I do have a set of 4.2 pistons with central pins, but that’s because they have offset crowns to match the offset combustion chambers of the 4.2.
#2 and #5 are central domes and the others are offset, two one way and two the other. The custom pistons satisfy three different piston dome orientations with only two types of piston - central dome or offset. The offsets are just turned around during fitment to put the dome on either side as needed and because the pins are central it doesn’t matter.
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May be different now, but the last time I bought custom pistons (8 years ago), I think Ross required a minimum purchase of 8 pieces. I used Arias.
Also, noting Peter’s comments on the 4.2 cylinder offset relative to the combustion chamber, that was one of the reasons I stuck with a 3.4. I could use the same piston crown design in any bore location.
Wow! What CR are you running with that dome? Looks like 10:1! Carbs? Cams? Ignition/timing?
Thanks,
Huff
4.2 69 2+2
Shops says there is an offset but not how much. Who made your offset pistons? What type dome and CR?
As of now I am reading up on “squish and quench” the 2 elves that get less attention and I can’t tell if I should get the top-of-piston right up to the deck/underside of the gasket or not.
Currently my piston design is low dome w/flat-top with 9.25:1 CR and .015 from piston top to block deck. I may very well eat up some of that since i am going to be sporting a .051 gasket and grooming the squish to throw the mixture farther/faster toward the plug. Still need to work on understanding quench, tho.
We’ll see how the design develops. I have to say that I was surprised that Ross, who does/did a 3D scan of the head does not have a computer-simulation model for the chamber, utilizing the 3D image gleaned. I would think that could solve a lot of mystery,
In my reading I have also learned to understand some of the pros and cons of the hemi design ergo, such that I can only do so much without heavy-handed changes (i.e…add a second plug to the chamber, etc…) and it is not a high RPM engine, inherently.
Will share as development unfolds.
Thanks,
Huff
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Yup. Far from never seeing an XK offset I’ve never seen factory pistons without - although I’ve only done a couple that were not 4.2. I think the factory ones I’ve used/removed were all AE or Hepolite?
The 96mm offset dome pistons with central pin are Arias and the custom central pin pistons in my 4.8 stroker are by Diamond of Michigan. The rings only came in boxes of 4 piston sets and the pins came as a box of 8, so I have two of each spare, but they did make me only six pistons as required.
Thanks for the info.
Fab shop got back to me and measures .059 offset to my orig 4.2 piston/pin. They say that is a lot; as of now I am going with .040
Huff
1.5 mm sounds about right but I’d have guessed slightly more… It’s a lot, but the stroke and rods are very long by today’s standards so it all works fine.