Made a Real Rookie Mistake!

[chris-p]

Decided this week to rebuild a bunch of powerheads I had around that needed it. Got to the last one this weekend, for a 1958 35hp Johnson that had gone under while running a twisted a rod. Rebuilt it all, and USUALLY after I get the crank halves bolted together, I put my timing fixture on the crank and run it around a bunch of times to ensure it doesn’t catch and all is well.

Welllll, this one I got cocky and finished her up instead of testing it. Installed on motor, put in new water pump, cleaned rebuilt carb, did all the ignition, and was just about to set the points timing with the fixture when I realized the crank only rotates about 90 degrees or so back and forth!

I was just about to polish up the cowl and call her complete 🙂 SO discouraging on a Sunday morning to realize you just lost a couple days of work. I guess Ill have a coffee then start pulling everything apart again to see what the heck I did wrong.

[chris-p]

Had a coffee and then went back down. Im not sure what the heck is going on here.

She will rotate until the TOP cylinder just starts to expose the ports, then stop dead like hitting a wall.

Turn the crank the other way, then when the BOTTOM piston just starts to expose the ports, same thing, stops dead with a THUNK.

Almost like there is a bolt or something in the cylinder stopping it dead. My 5 year olds have done that to me before by the way.

This time exact same spot though, either way where it stops dead.

Any ideas before I pull this powerhead again what is going on? Ive stared at it long enough and have come up with zilch.

[thepetrolist]

Check the crank halves again. Make sure they are bolted correctly, that any shims (if used) don’t jam, and so on. Thats the only thing that comes to mind at this moment.. Need to think about that for a while and get back to you!

..edit: oh, and check the connecting rod halves too, same as with the crank

[Mumbles]

After all that work I think I’d have beers instead of jiggle juice!

A six horse did that to me once. One of the rod locks wasn’t folded over tight enough and was dragging on the cases.

Are you using the original pistons? Sounds like a skirt might be contacting the crank.

[chris-p]

No the crank, rods and pistons were in rough shape, so I pulled the pistons, rods and crank from another good ’58 35hp powerhead and used them. Installed new bearings and such, and new rings. Honed the cylinders.

Guess I should just pull the powerhead and open her back up. Was hoping for a miracle in which that wouldn’t have to happen.

[chris-p]

Well pulled her all back apart and determined the rod/piston assemblies are wrong? On the down stroke, the piston skirt hits the casting before it can continue on its up stroke.

The powerhead I pulled the pistons and rods from was a ’58 powerhead, or so the welch plug says? Perhaps it was a ”57 powerhead with a ’58 welch plug installed? I don’t know, but these pistons/rods definitely will NOT work in this powerhead. Are the ’58 rods longer than the ’57s, or perhaps the ’58 pistons are shorter in length? if so, that would explain why the piston hits the casting before it can continue on and around.

Strange.

[Mumbles]

That’s a tricky one Chris.

Do you still have the old parts to compare them to?

[chris-p]

Just dug them out of the parts bin.

Ill be, they are different! Just slightly, but the pistons are definitely different.

The thickness between the rings is thicker on the ’57 piston. Also, thickness from the top of the upper ring, to the top of the piston is thicker on the ’57 piston. Also has a touch more length on the skirt. Glancing at them, they look identical, until you hold them close side by side to compare. Funny as they have identical casting numbers inside them???

I cannot seem to see a difference between the rods, although they should be different as they have different part numbers, but cannot physically see any difference.

Ticked me off, but I guess I learned something today. Now to hunt for another powerhead to pull parts from!

[dan-in-tn]

I know it doesn’t help now, but keep in mind cast numbers don’t help much when deciding if a part will work. Many different parts are made from the same general casting. I am betting the pistons you used are from a late 35hp or replacement PH with 40hp pistons in it. I have swapped many of the 35hp internals from ’57-’59 and have not run into this problem, yet! I was hoping you would find a bearing/bolt lodged somewhere.
I make it a practice of turning the engine over each time I put in a piston & tie it to the crank. I am also usually doing V-6 motors so I have a lot more pistons to get into trouble with. I put the crank in first. I have made pieces from old blocks to hold the crank in place top and bottom while I flip the block over to install the pistons. Very much like a car engine. I load the rod bearings in the rod and cap for assembly. After they are put together hand tight, I roll the assembly to see if things are correct. That’s the way I do it! The more cylinders the more time you save from a tear down to find out what happened.
Sorry you had to go back in, but you always learn something when you have to go back!

Dan in TN

[chris-p]

Well all brand new pistons, rods, bearings, and needles arrived from Mr Ken Hall today, so the motor will get the full shebang and live to run for another 50 years.

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