My son plans to rebuild the lower end of his '87 f150 300 this spring. The engine has 170,000+ miles and you can hear piston slap.

Can someone tell me how the wrist pin is assembled in the piston? Specifically, does this Ford engine use the spring clips on each end of the wrist pin that snap into grooves in the piston head (so the wrist pin pivots in the top of the rod and is stationary within the piston itself), or is there a "pinch bolt" at the top of the rod that clamps the wrist pin tight within the rod so the angular movement of the pin is between the wrist pin ends and the piston?

In short, does the wrist pin normally rotate relative to the rod or to the piston as the crank turns?

Thanks in advance for your help.

 

I'm pretty sure you will find that it is a pressed "interference fit" where the pin is bound tightly in the small end of the rod due to the rod hole being smaller than the diameter of the wrist pin. can't imagine them doing it any other way since Ford has been using this (cheap) method since the beginning of time.

 

Thanks for the response, Bern.

Your answer raises a couple of other questions, though. What, other than the press fit, keeps the wrist pin from sliding sideways in the piston and out one side enough that it's scored the cylinder wall the length of the stroke? Are we liable to find such a problem in one or more cylinders, given the high mileage on this truck? Is this a common problem?

We have no indications (sounds, etc) that we have a problem, just some piston "slap" going on that we want to correct before we break a piston skirt.

This engine has never had the lower end apart (as far as I know, but we bought it used at 115,000 miles). Assuming there's no cylinder wall damage, what are the chances that we can just re-ring it with a standard set and put it back together again? Does anybody know what the maximum "out-of-round" tolerance is that's allowable on the cylinder diameter, and also the maximum cylinder taper tolerance over the displacement length? I'd prefer not to have to re-bore and go to oversized rings and pistons, if I can get by without having to do it.

Also, are the connecting rod bolts the "stretch-to-yield" type, so they should be replaced rather than re-using the existing ones?

TIA for any answers.

 

Hi Joe, here’s my 2 Cents.

I don't think slipped wrist pins are a common problem on Fords (someone will correct me if i'm wrong). I know of one occurrence on a 351W but that was in a motor that had been rebuilt. Usually the pins are pressed out of the rod using a press ram. To replace the pin the small end of the rod is heated in a rod oven then placed in a waiting piston and the pin just drops in.

With that low mileage and if the motor was cared for, I would guess that you would be at the limit on taper, and could go with a re-ring. I re-ringed a neglected '68 240 with massive bore wear (required a ridge reamer to get the pistons out!). That motor ran good and didn't use oil. surprised the heck out of me! It did however have some loud piston slap. It ran like a clock until the cab mount rust made the truck un-drivable.

Get a Haynes manual for your truck it will help with some of your questions and may even have the out of round/taper spec that you seek. Even better find a set of Ford shop manuals. I got mine on ebay. They cover everything.

I would begin the process with a thorough set of diagnostics (school of hard knocks talking here) including compression test and leakdown test to determine your engine's condition prior to tearing it down. I once pulled and disassembled a 351W trying to trace down a loud knock. I suspected a busted piston skirt. Turned out to be a chunk of carbon crap that broke loose from the top of an intake valve! Wow, what a waste of time. I did get to clean off the valves and it did run much better afterwards. Moral of the story, they have good chemicals and processes these days that can save you a lot of time and money. Consider BG products and services. I do now.

As far as the rod bolts go, I imagine that they are not the stretch to yield type but someone will correct me, i'm sure. You could always change to a stronger (ARP) bolt but you will have to pay machine costs related to resizing the big end after pressing the bolts out and in.

Enjoy your I6!