Old School Skirt Clearance

[FElony]

I have several sets of the old 2291/2292 and similar 427/428-number TRW forged pistons. What is an acceptable clearance range for strip-only use?

[Joe-JDC]

The clearance should have been built into the piston from TRW, but measure it 1" down on the skirt.  Most TRW performance pistons used .0045" to .0075" clearance depending on the builder's preferences.  Joe-JDC

[fekbmax]

I use to run the old 12- 1 TRW shelf pistons quite often in .030 and .060 over 390 engines strictly for drag racing. Always ran .006 - .007 clearance.  Even knurled a couple of sets of loose pistons (.008 -.009 ) and never had a prob with them.
Of course big tube open headers, a fat lopey cam and a gear drive wine, you probably wouldn't hear allot of piston rock..  .. lol..
good luck ..

[FElony]

OK, thanks much guys!

[TomP]

You can get away with up to .010", my 427 is about that with L2298F's.

[66FAIRLANE]

I put my street 390 2291F's in at .0045.

[My427stang]

Especially on the big bore ones, you can go real loose, they grow like crazy

I ran a set in my Mustang when it was a 433 that had a couple at .010, couldn't hear them and didn't do anything funky.  I think the desired spec is .007-ish though

Big think to look at is ring lands though, if they are used and hammered closed from detonation or loose from wear, it won't last too long.

[Falcon67]

The 302 here started life with 4.030 TRW powerforged units running .0035.  Tight IMHO, but no problems over the years.

[FElony]

Especially on the big bore ones, you can go real loose, they grow like crazy

I ran a set in my Mustang when it was a 433 that had a couple at .010, couldn't hear them and didn't do anything funky.  I think the desired spec is .007-ish though

Big think to look at is ring lands though, if they are used and hammered closed from detonation or loose from wear, it won't last too long.

I was reading a non-Ford forum (I do that sometimes) wherein some dood with a max effort Ratiferous car with Racetech slugs couldn't figure out why his ET's were slowing up after several checks of the innards. Turns out his lands had developed a taper, giving him a case of the flutters. Makes me wonder if I should pick some ball end gauges for checking go-no go. Yes? Which?

[My427stang]

To be honest, I haven't done much in a long time, parts got cheap and not many rering jobs come by anymore.

However, I used to take a new ring and a feeler gauge, typically a crushed land would be readily apparent, and if was looser than .004-ish plus a new ring, we'd say they were loose.

I'd think the ball end gauges would be even more effective and much quicker to check around the whole piston

[MeanGene]

The old heavy TRW's did grow a bunch when hot, I've run both the flattops and the MR small domes at .010 (at least), oil control a little iffy and sounded like a grenade war going off inside until  it got warm (as noted, just uncork it). Ran pretty good and very low turning effort (7 lbs for the shortblock), but the skirts would start cracking on the MR's after a while. They were pretty happy at .007

[Richard F]

.007

[FElony]

OK, now let me ask the same question concerning original OEM cast pistons. How loose can you go on those during a refresh/overhaul? I have several engines to evaluate. Many are still standard bore, and I need to figure at what point of taper cleanup would it be better to switch to a forged piston, if you know what I mean. Anybody here run loose originals without failure?