piston skirt clearance

[brra1961]

thanks for all the replies , 1968galaxie, I don't know what instructions your looking at, but the instructions that came with my pistons reads .0028 - .0035  min. clearance for bore range 4.001" - 4.250" .  some are at .0028 and another is at .0035 the rest are in between  except obviously the one .0055 . From my research it seems as Scott and others  suggest that coatings are for anti scuff  and not designed to decrease clearance , although RaceTec  suggested it.  Has anyone had experience knurling pistons ? would that be an option ? I'm waffling between running it as is  or  honing 7 cylinders up and getting new pistons and trying to sell the AutoTec ones I have that are 4.0870 dia. 1.325 ch. -22cc dish , 464 grams, 0.990 pin dia. 2.500 length  with 1mm,1mm 2mm rings , they were a custom order $1020 including shipping, if I decide to sell would discount some, anyone interested ?

[blykins]

Absolute no on the knurling....

I'd run it. 

[GerryP]

I too would just run it at .005.  You would be surprised how lose you can run a piston without any issues.  Just make sure you bevel or smooth the bottom of the bore to make sure no sharp edges in the bore.  Now, knurling was a pretty common practice when I first started working in a shop.  There isn't anything wrong with it as long as the machinist is reasonable about the process.  If you have some OCD working against you, you could knurl that one piston and it would be fine.  Again, it is a legitimate machining operation.  That we generally don't do it these days doesn't mean it is an obsolete practice.  Very few shops do small dollar rebuilds these days.  More bucks in a full on job.  Here's an article to give you some perspective:  https://www.enginebuildermag.com/2017/09/lost-art-knurling-pistons-takes-skill-guts/

[My427stang]

Also, no on knurling.  However 1980s and 1990s me did it on old cast pistons, I think it would go too far and then localize wear on the high spot anyway, which is no benefit for the clearance or ring seal as it wears in.  Keep in mind, you are talking a difference at less than a human hair from where you wish to  be

It is a minimum as said before, and it will truly do no harm.  Measure each piston, if there is one bigger, use it, and be sure you are measuring with the torque plate on

[pbf777]

    Yes. perhaps less than ideal, but in reality, and not judging your build, but as typically observed in many of these old FE builds there are other often greater compromises involved which are allowed to pass so I think I'd have to agree with as stated previously, just run it, it'll be O.K..    

    And as stated previously, there's a lot of latitude in what would be considered "acceptable" sums of clearance here; too tight is a real problem, but what might be felt as of a sloppy fitment, even if truly less than "ideal", can often be found to provide sufficiently in service function.  And in the case of the Ford FE, realize that the cylinder column is not very rigid and the bore dimension is not impressively stable, so don't try to make things overly exacting with tightness dimensions that belong to more modern engineering.  And as far as the .0055" specifically, that's not "so great" a clearance value.  Now I understand that the "acceptable" sum is predicated on the dimensional change relationship between the cylinder wall and the specific piston as designed, and in the environmental instance presented, but on average that sum (.0055") would not be so uncommon in this sort of engine (American V8 pushrod examples) for a forced induction or a nitrous engine application and in these examples generally when encountering piston failure it wasn't due to excessive clearance.
   
    And as for testimonials of extreme examples: we had a road-race customer who was dissatisfied with his then current engine program due to repeatedly failed engines and D.N.F.'s; and of the three engine's (scrap) he delivered we tried to salvage one complete unit as due to time constraints leading to the next race.  We ended up creating one engine (though there were many concerns) which proved successful, and even though we had replaced it later with new (better) engines, the customer just wouldn't let it go.  We probably refreshed the engine half a dozen or more times, without even changing the pistons (new engines were a different configuration), he used it for practice sessions and when his second car was being operated as a rental/loaner racer (new engines for the race), and if I recall we ended up out past .0011" to .0012" (on a 4.020" bore limit) and the pistons had several thousand miles of race time (@ 8000+ R.P.M.) on them..............and no failure!   

    Though now with the O.C.D. kicking in, the bore clearance differential would just bug me to no end!  So if one chooses to acknowledge that at the .0055" "it'll be O.K.", so perhaps then if only to make it look like someone knew what they were doing, should the other bores be made the same?   

    Scott.

[1968galaxie]

The 0.0028" to 0.0035" clearance specs are Minimum clearances!
I think you are missing the minimum part.
Just like bearing clearances, I never set to minimum.
I have seen many examples of damaged piston skirts from tight clearances.
As I mentioned a safe clearance for me would be 0.004+ not a bit less.
0.005" on one cylinder, not an issue at all.

[frnkeore]

If it were me, I would be very upset if I got a block back from the shop and the bore varied .0025, from what I asked. That's a lot and I would NEVER use them again, for anything!

If you research 4032, you'll find that one of the main differences with 2618, is that it it is more brittle, in comparison. The other, is that it does not expand at near the rate of 2618. You can get away with with more clearance and abuse with 2618.

Again, if it were me, I'd never feel comfortable with that one cylinder and would probably hear piston slap, every time I started it, where it was there or not and I think the OP, is indicating he feels the same.

So, if time and money is no issue, I would take it to a GOOD shop and and get a good job done.

I'm a old mechanic and have seen a lot of knurled pistons. I have nothing against doing that but, would really hate to do it on a brand new piston. Also, the knurling wears away over time.

Last, if that extra .002, really won't matter, call RaceTec and verify that.

[blykins]

There are also plenty of 4032 pistons that ask for more clearance and there are some 2618 pistons that want less.  Bore size, shape, and application has a lot to do with it.  Some of the BBF Mahle 4032 pistons want up to .005" of piston/wall clearance.  They also have some SBF 2618 pistons that setup at .0025".

At .0055" clearance, I doubt you'll ever hear anything.   I have had engines come back for freshen-up where we just honed a thou or so out to clean them up and put them right back together. In addition, Randy Gillis, lead sales/tech guy at Racetec (RIP), told me many times to run the 4032 at .005" on some of the applications that I was running.

As mentioned, other than the OCD part of it, I'd have no issue running it.  Just put a note in your build notes so that if you ever sell it, you'll look like you did your due diligence, measured everything, and made the executive decision.

[frnkeore]

The fact remains that 4032 is more brittle and it doesn't expand as much as 2618 plus, these are neither Mahle or SBF pistons. There is much more mass to these pistons, than the SBF.

I would diffidently check with RaceTec before installing that piston and he should have file fit rings for the additional .006+ end gap.

[cammerfe]

Back in the '60s, it was not uncommon to run the pistons available at the time, for performance applications, with as much as .008 wall clearance. When I put together the 427 that went into my one year old '67 Cougar XR-7 GT I believe I used C8AX-C pistons and set them at .008. The car was street-driven on a daily basis. We did have Sunoco 260 readily available and I used a home-made methanol/H2O fogger system to eliminate a bit of crackle under low speed load.
(Used an adjustable Hobbs switch to trigger the fogger.)

KS

[blykins]

The fact remains that 4032 is more brittle and it doesn't expand as much as 2618 plus, these are neither Mahle or SBF pistons. There is much more mass to these pistons, than the SBF.

I would diffidently check with RaceTec before installing that piston and he should have file fit rings for the additional .006+ end gap.

If you would read all of his posts, he did check with Racetec.........

I fully understand that these are neither SBF or Mahle pistons, but sometimes you have to do a little deductive reasoning......based on experience......

[brra1961]

Thanks for all the comments , I feel much better about running it as is. I'm not a engine builder or a machinist but have been a truck mechanic for the past 40 yrs. and thought it would be interesting to assemble this engine myself, thinking it would be fairly straight forward but from the beginning its been anything but. Was planning to use my old block but it sonic tested too thin in spots and looking for a acceptable replacement took months and a few road trips. The only good block I could find was a D4TE with the extra main webbing that had already been machined. It looked great ,sonic tested good but had some pitting in one cylinder, thought I could just have a sleeve put in and didn't realize would have to bore and deck it again, 5 months at the machine shop , got it back and installing the camshaft, had to shave two cam bearings to .007 clearance  to get the cam to rotate freely. Then measuring the piston skirt clearance on one cylinder .002 larger than the rest . The main clearances all measured .0025 - .0032 so that was nice but now as I'm mocking up a piston and rod on cylinder no. 4 , it measures .007 in the hole and when I used the same piston and rod in no. 1 it was .005 in the hole.  Is this acceptable ? The machinist is a one man show been doing it for 50 yrs. very FE experienced and a really nice guy, he said he square decked it, maybe he's having vision problems.  Also, when I torque the main caps to even 70 ft. lbs. my camshaft becomes noticeably harder to rotate. Is this normal ? It makes sense to me that if the heads torqued down distorts the cylinder bores, that the mains torqued would distort the cam tunnel. I think I'll install new cam bearings and make a cam cutter out of an old camshaft as Brent suggested and see if that helps. Should fitting the camshaft be done with the mains torqued ?  I'm not looking for perfection , I realize this is a 50 yr. old truck block, just want to know what is acceptable. Even with all these issues, I am enjoying learning this process and feel very fortunate to have this forum with so many good people with so much engine knowledge and so willing to help.

[1967 XR7 GT]

I, ran a Set of BME pistons, recommended clearance was .010".
After a time I took it apart, the skirts looked like the same as when I put them in. I was pleasantly surprised.

[cjshaker]

I'll let others chime in on the cam tightening when mains are torqued, I've never come across that issue. But otherwise, all those specs are better than probably the majority of the engines that came out of the factory back in the day. Unless it was a high-end build, I'd let 'er fly. There's been tens of thousands of engines rebuilt with way worse tolerances, we just happen to be in an age where anything less than perfect is now considered unacceptable by many.

If the machinist has been doing this for 50 years, there's a chance he's using much older machining equipment, so those tolerances might not be completely his 'fault'. Not saying that's the case, just that a lot of guys can't afford the expensive newer equipment to keep up with modern expectations, unless they are trying to cater to performance builds specifically. A guy that is on the verge of retirement, as someone who's been doing it for 50 years is likely to be, is not going to shell out tens of thousands of dollars with no chance of recouping those costs.

[blykins]

Thanks for all the comments , I feel much better about running it as is. I'm not a engine builder or a machinist but have been a truck mechanic for the past 40 yrs. and thought it would be interesting to assemble this engine myself, thinking it would be fairly straight forward but from the beginning its been anything but. Was planning to use my old block but it sonic tested too thin in spots and looking for a acceptable replacement took months and a few road trips. The only good block I could find was a D4TE with the extra main webbing that had already been machined. It looked great ,sonic tested good but had some pitting in one cylinder, thought I could just have a sleeve put in and didn't realize would have to bore and deck it again, 5 months at the machine shop , got it back and installing the camshaft, had to shave two cam bearings to .007 clearance  to get the cam to rotate freely. Then measuring the piston skirt clearance on one cylinder .002 larger than the rest . The main clearances all measured .0025 - .0032 so that was nice but now as I'm mocking up a piston and rod on cylinder no. 4 , it measures .007 in the hole and when I used the same piston and rod in no. 1 it was .005 in the hole.  Is this acceptable ? The machinist is a one man show been doing it for 50 yrs. very FE experienced and a really nice guy, he said he square decked it, maybe he's having vision problems.  Also, when I torque the main caps to even 70 ft. lbs. my camshaft becomes noticeably harder to rotate. Is this normal ? It makes sense to me that if the heads torqued down distorts the cylinder bores, that the mains torqued would distort the cam tunnel. I think I'll install new cam bearings and make a cam cutter out of an old camshaft as Brent suggested and see if that helps. Should fitting the camshaft be done with the mains torqued ?  I'm not looking for perfection , I realize this is a 50 yr. old truck block, just want to know what is acceptable. Even with all these issues, I am enjoying learning this process and feel very fortunate to have this forum with so many good people with so much engine knowledge and so willing to help.

Doesn't make sense that the mains affect the cam.  I've never seen that before.

.002" between ends of one deck isn't really a deal breaker.  Probably just had the block out of level a hair.  Doesn't take much over the length of the deck. 

I don't really care for .007" cam/bearing clearance, but it would most likely run ok.