piston to bore clearance

[Tcochran14]

Hello, long time reader first time poster here. I am in the process of building an engine for my 07 GT. I did tons of research, read everything i could find for months, build threads, videos, Sean Hylands book, everything I could find to prepare and make the best decisions the first go round. although I am in need of some advice. I’m running stock crank, manley rods, manley coated pistons, total seal rings with a vortech v2 bolted on the front, plan on 9-11 lbs. (I’ll spare all the build details) I visited several machine shops looking for one with a torque plate. Had them balance rotating assembly, align hone the mains, resurface the deck and hone the cylinders with a torque plate. I discussed with the shop .0035 piston to bore clearance like the manley instructions said. Picked up my block this week and upon measuring everything myself and according to the shops measurements piston clearance is coming in at .0055 across all cylinders. Machine shop says they have built several 3v’s with this clearance and it will be fine. This seems excessive to me. What do you guys think? Will these pistons slap like crazy? Prematurely wear out? I don’t know what do to. Looking for any advice here, I appreciate the wealth of info on this site, it is incredible.

 

[Badd GT]

I would hold them to the manufacturer specs..... especially if that was what you specifically instructed them to do.

 

[msvela448]

.0035 to .0040 is a good range for PTW... For a boosted 3v .0055 is a little loose in my opinion, but it will tolerate a little more heat from boost. I don't think you'll have slap.

You could always sell your pistons and order a set of custom Gibtec billet pistons to any size that you want and get the PTW exactly where you want it.

Ask for Nick at Gibtec if you decide to do that. https://gibtecpistons.com/

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[RED09GT]

It might be okay but I wouldn't be happy with it. Hopefully you have a 2618 piston.
Mod motors have a small bore so I'd be worried about ring stability and seal with that much PTB clearance. Running that much clearance on a 4.030" bore is different than running it on a motor with a 3.572" bore.

As mentioned, it will tolerate heat a bit better but there also introduces the risk for point detonation between the wall, the piston, and the top ring. Search for my KABOOM thread to see what that looks like.

 

[Midlife Crises]

Three to three and a half thousandths is what the clearance is supposed to be and they hogged it out to five and a half. I’d be pissed and would not run it like that. I believe oil control will suffer and pistons and rings will ware rapidly. Now your looking at purchasing pistons of the next oversize and boring the block to match.

 

[LarryJM]

The engine is already 2/3 used.

 

[86GT351]

I would hold them to the manufacturer specs..... especially if that was what you specifically instructed them to do.

This! We do this all the time. Not with Engines specifically. If a customer comes in and asks us to do a specific repair based on there own diagnosis or possibly an Auto Zone Code reader, we document specifically that this is a customer request and we can/will not be held responsible if it does not solve a concern. And the client still has to pay for the repairs. More specifically to what you are saying....We do a lot of custom wheel alignments because we have an inground rack allowing us to do all lowered cars, etc. If a customer comes in with there own specs we document accordingly even if we know it is going to wear tires prematurely. In your case you did not ask the machine shop for there opinion. You paid for a specific job and they did not do that. If they did the bore like requested and it did not work then there is no liability on them. Sounds like you might have a fight on your hands. Now the block will need to be replaced or sleeved and re-bored.

 

[Tcochran14]

I’m getting mixed results asking this on different platforms. some say it will be fine, others say they wouldn’t run it. machine shop says it’s perfectly okay, they are a little biased covering their own. Manley tech support says it will slap even when its warm. they may be a little biased as well lol
what does a set of gibtecs run price wise?
these manleys are 2618 so they may expand a little more correct?

 

[86GT351]

I’m getting mixed results asking this on different platforms. some say it will be fine, others say they wouldn’t run it. machine shop says it’s perfectly okay, they are a little biased covering their own. Manley tech support says it will slap even when its warm. they may be a little biased as well lol
what does a set of gibtecs run price wise?
these manleys are 2618 so they may expand a little more correct?

I just spoke with Tim at MPR in Boynton Beach Florida. He is a Mod Motor Expert. He states to send the pistons out to be coated to make up the space. If you run it at .055 it will knock.

 

[msvela448]

what does a set of gibtecs run price wise?

A set of Gibtec pistons will be about $1100 including Total Seal rings and wrist pins, and skirt coating (last time I checked). Keep in mind that they are 100% custom... Any dimensions you want, any compression you want, any ring height / spacing you want. You won't find a better piston for the money.

Photos below are a 4v version... But you get an idea of the skirt thickness, custom ring package / height / spacing, and overall quality... They are like jewelry.

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[msvela448]

Here's my 3v versions... Dish is for 9.5:1 compression. Note the custom valve reliefs.

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[Juice]

.001" to .0015" per inch of bore is what I recall from machine shop class.
Yes, that is on the lose side. .0035 is too tight IMO.
Type of piston matters some.
You had it all balanced, put that shit together. lol
Are you sure your measurement is dead on? Micrometer checked with standard? A tiny error could make the clearance .004". I would probably shoot for .004-.0045 clearance, and thats within .001".

 

[Midlife Crises]

There really isn’t a big question here. The piston manufacturer stated what the clearance should be. That’s what you should run. Also, if the machine shop that botched the bore job is the same one that did the balancing I wouldn’t trust that ether.

 

[Juice]

Did the machine shop have pistons in hand when honing/sizing the bores? Are the pistons marked as to which hole they should be installed into?

 

[Tcochran14]

yes the machine shop had this pistons when they honed it. This is the response I was given from the shop

good after noon…. I am the gentleman that machined your block…
I gave it an extra .0005” clearance than the normal .005” on your application because it’s boosted and I’ve done more than my share of them. I’ve machined 1000+hp turbo 4.6 blocks for many people and not single one had an issue with “piston slap”. If you have anymore questions feel free to call us again thanks again

should i just roll with it and trust the shop?

 

[msvela448]

Ask for references...lol... It might be interesting to talk to one of those customers.

I'm also going to go ahead and say that Manley has built more pistons than the machinist who did your block. Manley said .0035 was the recommended tolerance... But what do they know.

Sean Hyland has also built a few engines... Even wrote a book on it... But what does he know.?

Sounds like they aren't going to do anything for you.

Get those custom Gibtec pistons and set the PTW exactly where you want.

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[msvela448]

This looks about right.

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[RED09GT]

I gave it an extra .0005” clearance than the normal .005” on your application because it’s boosted and I’ve done more than my share of them.

Anyone see a piston that recommends 0.005" on a ford modular? This isn't a big block chevy.
I fear what he would he set bearing clearances to.

My diamond 2618 pistons recommended 0.003-0.0035".

 

[Midlife Crises]

I set my Wiseco pistons at a fat 0.003” clearance and haven’t had a bit of trouble.

 

[msvela448]

This is the spec sheet from a set of CP pistons made specifically for Saleen for the 302 Parnelli Jones and S302SC models (supercharged)... PTW is spec'd at .0020. I ran these pistons and had zero issues.

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