Hello guys, Some time ago i bought 81A already rebored and honed + ,020. Original Ford 4 rings pistons. I wonder if the wrist pins should be floating in pistons. The holes for pins are not perfect. Sometime on the one side of the piston, wrist pin is going in without big resistance..than going deeper to the other side and pin won't go without bigger force. Are they factory pressed in or floating. Could I make them floating by simply making holes in the pistons a little bigger . I've checked all of them (8) and they are more or less similar so it's not the problem of one piece.
those are floating pins. Note the groove for the retaining snap rings at the edge of the pin bore. The bores and the pins need to be perfectly clean and no burrs. They should slide in easily with a little oil.
Unfortunely they won't slide in easily and I think bores are not perfectly round. What ist wrist pin OD exactly?
take them to an automotive machine shop and have them honed slightly. You only want .0002"/.0003" clearance.
Pistons could be fine , wristpins could be reused from the original. Wristpins use to be available in different sizes . ( IIRC )
The instructions in an old Motors manual from flathead years specified a 'cold press fit with two thumbs'. Real precise! Just like the shim and spring scale pull measurement for 216 stovebolt pistons.
They are floating ,,,,,looks like the pins you have are kind of dingy with crap . The piston pin bores can be cleaned with a little Scotchbrite. Spin the pins in a lathe if you have access to one,,,,,,a little Scotchbrite on the pins and they will look new . You can also clean the pins very well by hand with a Scotchbrite pad,,,,,they don’t have to be perfect,,,just clean . Tommy
I was told if the wrist pins had the correct clearance in the rods that as the rods were turned so the pins were in the vertical position(straight up and down) they should fall out of the rods SLOWLY—-don’t know how to put a figure for that type of movement!! Flatheads Forever!!
Thank you guys, I think I need new wrist pins.Diamemter in the midle is slightly smaller. Cleaned both piston holes and pins. It's better but still sometime to loose , sometime to tight. New pins+ honing pistons wrist pin holes required.
The rods are meant to have a bushing inside that the wrist pin rides on. This can be reamed by the machine shop to perfectly match the wrist pins. Here's a pic of the rod bushing out of my 1939 81A flathead. NOTE, the Rod is a 91A rod, not sure which model your rods are, the number is cast on the side. ~Peter
Keep in mind,,,,,,the old way was not technically 100% floating . I believe it called out using two thumbs to insert the pin ? That is,,,if it was able to be inserted with thumb pressure,,,,the clearance was acceptable. And they kind of seat in a little after that ,,,,get looser. You’re probably good as is,,,,if you can insert the pins ,,,,they don’t have to be loose or able to spin in the pin bores . Maybe just a new set of pins is enough. Tommy
Agree with the cleanup methods above. But, the pin hole in that piston doesn't look good in the pic. Is it really rough have ridges like the pic shows? Or is that camera issues? If the pin hole is defective that needs to be fixed first. If reaming is needed you may need pins. Or if it is just a little issue maybe some careful work with 800-1000 wet or dry in only the rough areas.
I decided to order new pins (custom made ) some of them are thick wall....some not. Not good for the engine. Piston wrist pin holes will be slightly honed. Using reamer would be to dangerous. So far I'm preparing and checking the crankshaft. Do you guys know proper clearance and wear limit for main bearings? It's 81A crank. It's still STD with very little wear and perfectly round journals. I though drop in new bearings if I'm still in the "frames" of allowed wear.
Not sure right now but maybe yes...my idea is to use modern 3 piece low friction rigns instead of 1 piece originals. I have heard that with 4th ring piston will suffer from lack of oil... my solution for that could be intentionaly make more ring gap. In this case the piston will be stablilised and skirt better oiled ( just an idea)
All of the builders on The Fordbarn, Early V8 speak of eliminating the 4th ring. You are hip to the power gain of metric rings, good deal.
My advice, for what it's worth: Several things could cause your problem. Rod bushings reamed cockeyed, bent or twisted little rod end, crap and crud, out of round holes, etc. Buy new pistons. They come with fitted pins and lock rings and buy new rod bushings. Have a machine shop install and ream the bushings to specs. These things are critical if you want your engine to last. You won't regret it.
The problem is... i`ve bought this motor already freshly bored and honed with those pistons. So another set would need machine work ...maybe. I will cost more than just new pins.
May not be places in your area that can check things out. But maybe send the rods, pistons and pins to a shop that knows. Sure, gonna cost $$ for postage two ways, shop work, etc. But then you’ll know. Who’s to say buying another set will work as a drop ins?
Round straight well aligned holes. ( align honed ala Sunnen) Straight, round pins. Correct clearance. Tough to beat that combination. Some say a precision honed free running slip fit ( .0004" clearance) will last longer than the reamed "slight shrink fir" described in the FORD literature. I believe that. Cleaning the honing grit from a hone piston pin bore or con rod bushing ( or cylinder bore) is not a trivial "spray and air blast" process. An otherwise nice bearing surface with Grit embedded is a problem that no break-in can fix. Nor lots of oil changes.
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