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Ed,

I plan on purchasing a custom set of your designed Gibetec mod motor pistons . I have a question if you don't mind please sir.

From reading the thread you created a few years back it looks like the Gibtec's were designed with a center pin vs offset . I was wondering if you could elaborate on the advantages and dis-advantages if any on both styles please. My application is turbo
street/strip .

Second question kind of off topic would be on main bearings will the king MB5281SI matched with MB5219SI for the fully flanged thrust bearing setup work on the Teksid block ? The thread I read over was on a 05+ nemark style block .I assume all aluminum blocks use the same main bearings but that is just a assumption. This is my first 4.6L 4v rebuild so I have been reading alot of threads here on MFF and taking notes. Lots of good info for sure !

Thanks! Mike
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Mike,

When you offset a pin in a piston (other than OEM) one pin boss sidewall will be thinner than the other. See the pic below:



When that happens, the piston's weak spot or probable failure point is that thin pin boss wall. The pic below is a pin boss failure on a piston with an offset pin and one thin asymmetrical pin boss wall.


An OEM piston is forged with an offset pin boss that matches the pin offset so the pin boss walls are the same thickness on either side of the OEM piston's wrist pin. While you can do this when building billet pistons, the pregnant question is why do you want to spend the money to do this? When the answer comes back to accommodate an offset pin, the question arises again but is phrased slightly differently. It sounds like why do you want to use an offset pin.

The only credible response to that question is reduced piston slap at TDC. But that raises another question why do you want to reduce piston slap at TDC, after all it is only a cold start issue. Once the engine is warmed and running, it is inaudible. Detroit offsets the piston pins so new car customers do not badger the service department about the 'sounds' their engines make in the morning when they start them up. A properly sized piston with a reasonable 10W-30 oil will be all but inaudible when you start the engine unless you get crazy with PTW clearances.

Your bearing choices are correct and will serve you well in your build.

Something you should consider is the overbore on your cylinders.

#1 Never bore the cylinders before you have the pistons. Once you have the pistons, bore and hone each cylinder to target minimum PTW dimensions for it's specific piston, especially if it is for a street-driven car.

#2 If the engine is n/a, a 0.020 overbore can be used with caution. Maximum effort n/a engines should be treated similarly to a blown version of the same engine. For Mod Motors that means a standard or 0.005" to no more than 0.010" overbore. The OEM liners are just too thin. They will crack in service, damaging the aluminum liner support behind them, that is, locating the sleeve. After this happens, the block will begin to leak water into the oil pan. You can figure out pretty easily what happens next.

There is a good write-up in the TToC under Engine Build Threads titled "Why You Want to Use Standard-Size Bores". The write-up goes into all the why for and how come on the subject. It is worth your time to read.
 

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Thank Ed!

I ended up purchasing a Teksid block that was bored .005" over to 3.557" . I unfortunately at the time was unable to find a stock bore 3.552" in good condition in my area. So I reached out to Pro Weld performance and they were able to provide me with a fully machined and inspected 3.557" bore Teksid block . So what that being said I will have to order the Gibtec's based on that bore size. I will of course confirm the cylinder bore measurements before the piston order is placed.
Order your pistons 0.0065" over stock. That will give you room to hone the cylinders to fit. Order your rings for a 0.010" overbore. That will give you a similar file fit capability in your bore that a 0.005" over ring set will give you in a standard bore. The ring set should be an AP Steel top and a Napier cut second. The standard tension 0.010" over oil will become a high tension oil in a 0.0065" over cylinder just like the 0.005" over ring would have been in a standard bore. Top and second ring gap is 0.023" for boosted engines and 0.015" to 0.018" for n/a. Your oil rails should be OK but still check to be certain you have a 0.015" ring gap on the top and bottom rail.

In the future, measure all the bores in the target block, first. Find the largest bore. Determine what will be needed to clean up that bore. Add 0.001", and that should be the piston size you use. If the cleaned-up bore size is more than 0.010" oversize, discard the block and get a new block candidate. The exception to this is if you are going to sleeve the block. A sleeved block provides the best cylinder walls but will set you back about $2K. The good news is the liners are replaceable so if you damage one it is not the end of the road for the block. Always bore the cylinders after you have the pistons and size the bores to the low end of the PTW clearance. On a Mod motor that will look like 0.0035" PTW.
 

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Ed,

I guess I was under the assumption since the Gibtec's are Custom made to order I could just give them my bore size in my case that's 3.557"
and they would machine a piston set for me
with consideration to the PTW clearance needed based on the bore size i provided them. As mentioned above the block I purchased has already been inspected and was determined that .005" clean up was all that was needed.

Maybe you can help me understand as I am having a hard time wrapping my head around the idea of ordering a set of pistons before having the machine work done on the block . A example that comes to mind would be let's say I ordered a set of pistons based on the machine shop telling me that the block would only need to be machined .010" over but in fact when they performed the work on cylinders bores they ended up having to take it .015" over in order to get the bores corrected ( just throwing numbers out there) what would I then do with the pistons that I just paid for ?

Like I mentioned earlier this is my first full rebuild and I have alot to learn,hints the questions lol


Thanks!
Mike

Mike,

Neither machine shops nor piston manufacturers make products to precise and repeatable measurements all the time. Many times they do, and then they don't. Even Ford has multiple piston sizes, and bore sizes for the standard 3.552 bore used on these engines. They grade bore size and fit the pistons to the bores at assembly to achieve their desired PTW dimensions. You can buy multiple sets of pistons and duplicate what Ford does. The easier, equally if not more precise (and less expensive) way to the same end is what I have described to you.
 
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