piston skirt clearance

[brra1961]

I finally got my D4TE block back from the machine shop , took 5 months just to machine the block. I'm using AutoTec 4032  pistons 4.090 bore and there clearance spec. is .0028 - .0035
7 cylinders are  within ,but 1 cylinder is at .0055 measured at the top of the cylinder with the head torqued on and the bottom measures .0050  all the other cylinders have a similar .0005
larger bore at the top than bottom of cylinders.  My thought is to have 1 piston skirt coated.
I talked to Calico and they said a heavy CT3 coating which is a moly teflon base would add .002 to skirt diameter.  I've read some posts about coatings  being not as thick as promised or wearing off quickly, so my questions are :  Has anyone used this process  or any other process or coating company that you would recommend, and has the thickness held up over time. also is the .0005 larger bore at the top of the cylinders normal?  the engine is 4.250 stroke wont see over 5400 rpm and no power adders.

[blykins]

Did the shop use torque plates?

[1968galaxie]

The specific piston measures the same as the other 7?
Skirt coating could certainly be used to fix an issue. Calico is a very reputable company.

[brra1961]

yes torque plate used , I witnessed it installed at machine shop, the piston skirts all measure consistent  4.0870   I've measured  the cylinders with and without a head and gasket bolted on and the top of cylinder dimension does change significantly

[Barry_R]

That .002 seems like a notable "miss".  After double and triple checking measured dimensions I would have no problem using Calico.   They are good folks. 
That type of coating is likely designed to be sacrificial, wearing thinner in high load areas to conform to the bore.

[Joe-JDC]

Not to be too negative here, but I sent out a set of coated, forged JE pistons to have the skirts coated to take up .0005, twice, and when I got them back each time they measured exactly the same as shipped from JE.  That cost $$ plus shipping.  Just saying.  I was told they could coat them to add as much as .002", and I did not get what was promised.  Joe-JDC

[brra1961]

Thanks Barry and Joe , one of Joe's previous posts is what got me concerned about coating and wanted to hear of other experiences ,  .002 seems like a lot to make up with coating. Would I be better off honing the other cylinders up to the larger one and then getting new pistons ? or am I getting too concerned about clearance ? RaceTec said the only problem with .005 skirt clearance would be noise on cold start up, which I might not even hear because its going in a desert race truck with no mufflers

[My427stang]

Run it, .0055 sounds big by today's standards, and of course it may have a little piston slap, but I bet you won't even notice it.

[Joe-JDC]

By the way, those coated pistons were for my 375Y EMC engine with 12.8 compression, .0055 piston to wall clearance, and it made 613hp.  That bore is 3.862", so I don't think your 4.090" block with .0055 clearance is an issue.  Joe-JDC

[Ford428CJ]

Run it, .0055 sounds big by today's standards, and of course it may have a little piston slap, but I bet you won't even notice it.

  I fully agree! I’ve dealt with a customer one time where he had a machine shop do the same thing. Never even noticed it.

[frnkeore]

Joe, were your pistons 2618 or 4032?

[1968galaxie]

My old 390 built in the 80's has/had the TRW 2292F 0.030" pistons.
The skirt clearance was 0.009"
Ran that way for many, many hard run miles.
Had 5.14 traction lock rear - saw plenty of rpm on a daily basis - was my daily driver through university.
The 390 sat on an engine stand for 25 years - took it apart last year. The piston skirts look brand new!

[pbf777]

      Though it is practiced, utilize piston skirt coatings for what they are intended as: an anti-scuff treatment; not as a structural build surface to take up clearance.   Yes, it'll work for awhile, but if tested it's to soft to survive any really great period of time.   

      Scott.

[1968galaxie]

Run it!

From The Autotec 4032 piston instructions:

Applications Bore Range Min. Clearance
SB Applications 4.000-4.200 .0025-.0035+
BB Applications 4.200-4.600 .0035-.0045+

These are minimum values. You are at the minimum recommended on 7 cylinders.
I like to be a little above minimum recommendations. For me 0.0045" would be my happy place.
I would be honing the 7 other cylinders for more clearance - most certainly any that were below 0.035"

[machoneman]

My old 390 built in the 80's has/had the TRW 2292F 0.030" pistons.
The skirt clearance was 0.009"
Ran that way for many, many hard run miles.
Had 5.14 traction lock rear - saw plenty of rpm on a daily basis - was my daily driver through university.
The 390 sat on an engine stand for 25 years - took it apart last year. The piston skirts look brand new!

Had to laugh at your .009 clearance. Long ago, a now former T.A.D. racer ran .010 in his blown Donovan 417 blocked Hemi. I questioned why so big and he said anything less would backside the piston. This btw before the Teflon buttons went in the skirts. He had boxes and boxes of damaged pistons in his trailer too, ready to be sorted out someday for a few reusable ones.