Piston/wall clearance issue 2618 alloy

[dhidaka]

Customer brought in a set of new but old Venolia pistons (2618 alloy) for a street turbo application on air cooled motorcycle. Inline 6 cylinder Honda big bore 72mm (2.835"). Instructions say .005-.007 piston/wall clearance but that seems excessive. Called Venolia guy and he said .0045-.005. (Venolia website says .005-.007) Other than that info he was not very helpful (ie not friendly so "Boo" to Venolia). Cylinders are now bored to piston size (before honing) and ring gap is .007. If I hone (with torque plates) to .005 the ring end gap is .023 (.0082 per inch of bore). I know other brands of motorcycle pistons even for drag racing set the clearance to close to .002-.0025 Piston domes and skirts are currently being machined for a little less compression and coated at Rebco. What would be a proper clearance for 2618 piston for this application? Rings seem to be cast iron. Where can I find oversize nodular rings?

[FC-Pilot]

I have run about 8 sets of Venolia pistons and all of them called for more clearance than any other manufacturer. I never had any issues with them, but the bigger clearance numbers had me a little concerned though. That is all I have for you, wish I had more info.

Paul

[bigjoe1]

The problem is not the 2618 alu. It is that Venolia does not know how to shape the piston correctly. I saw this over 30 years ago, when I opened up the wall clearance on a serious drag race engine, it PICKED UP OVER 30 HP. Most guys will never figure it out.



JOE SHERMAN RACING

[SupStk]

Joe is right. Don't be afraid of opening up the clearance with Venolias. When I was using them I'd start at .010 and they worked good. Some engines had over .015 after several freshings and they still hit good numbers on the dyno, although ring life wasn't the best.

[wyrmrider]

what I was going to say- it's the skirt cams Venolia used- do not run them tight
way different now with the cnc multiple axis skirt cam profiles

[modok]

I agree.
piston clearance is not JUST ONE SPOT.
What size is it just under the oil ring? How much smaller at 45 degrees?

[Dan Timberlake]

What is the piston clearance in a "nominal" bore ?

[ProPower engines]

Customer brought in a set of new but old Venolia pistons (2618 alloy) for a street turbo application on air cooled motorcycle. Inline 6 cylinder Honda big bore 72mm (2.835"). Instructions say .005-.007 piston/wall clearance but that seems excessive. Called Venolia guy and he said .0045-.005. (Venolia website says .005-.007) Other than that info he was not very helpful (ie not friendly so "Boo" to Venolia). Cylinders are now bored to piston size (before honing) and ring gap is .007. If I hone (with torque plates) to .005 the ring end gap is .023 (.0082 per inch of bore). I know other brands of motorcycle pistons even for drag racing set the clearance to close to .002-.0025 Piston domes and skirts are currently being machined for a little less compression and coated at Rebco. What would be a proper clearance for 2618 piston for this application? Rings seem to be cast iron. Where can I find oversize nodular rings?

I would call the boys at Total Seal. It seems to me you may want a tool steel top ring for durability.
Also you can get a ring set better suited to your end gap requirements

[PackardV8]

It was sort of the way of the day. ForgedTrue was a big name back when and some of their pistons were heavier than the hubs of hell and required .009" even on small bore diameters.

[Jagdpanzer]

ForgedTrue was one of leading custom piston manufacturer back in the period between WWII and early 70's. Many of the top teams used them. However, they seemed to have lost the bubble after Art Sparks cashed out and "investors" took over. The ForgedTrue brand name was in pretty bad shape by the time Bill Miller acquired it in the early 80's. In the meantime Bill has done a great job bringing it back, albeit under his BME company label.

[PackardV8]

ForgedTrue was one of leading custom piston manufacturer back in the period between WWII and early 70's. Many of the top teams used them. However, they seemed to have lost the bubble after Art Sparks cashed out and "investors" took over. The ForgedTrue brand name was in pretty bad shape by the time Bill Miller acquired it in the early 80's. In the meantime Bill has done a great job bringing it back, albeit under his BME company label.

For true, Art Sparks made the best forged pistons of that time. But I was there back in the day and very little of what was leading custom edge would be acceptable today. How many of us want to run .010" piston-to-wall clearance? Pistons, rings, cam profiles, valve springs; all are greatly improved.

FWIW, we've just had a discussion here in another thread of how Venolia, also a great piston company name, didn't keep up with improving technology and their pistons, good then, aren't what we'd choose to run today.

[DCal]

I agree with Jack Vines. Pistons had about a 25 years stagnancy during which time 5 companies closed their doors, and I can name 3 more that are less than satisfied with their sales. The advent of Cad Cam is what advanced piston quality and design. Those that didn't want to step up are having to step out.