Slant Six Forum

One hydro motor piston
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Author:  Ed Mullen [ Mon Sep 02, 2019 10:53 am ]
Post subject:  One hydro motor piston

I need one piston for my 87 225 van.

Additional needs may come to light as I get more parts clean.

(So far, piston is the only thing that doesn't come in a basic rebuild kit and expansion plug set that needs anything beyond a freshening at the machine shop.)

Author:  Joshie225 [ Mon Sep 02, 2019 11:32 am ]
Post subject:  Re: One hydro motor piston

$18.20 plus tax and shipping from

And, all the OE pistons except the dished low-compression export model are the same. You could take a nice piston from a '64 170 and it is the same dimensions as the pistons in your '87.

Author:  DadTruck [ Mon Sep 02, 2019 6:52 pm ]
Post subject:  Re: One hydro motor piston

are you putting the stock sized pistons back in? No over bore?

Author:  Ed Mullen [ Tue Sep 03, 2019 7:47 am ]
Post subject:  Re: One hydro motor piston

$18.20 plus tax and shipping from
I got so hung up on the fact they only sell the first piston listed in multiples of 6. that I did not even check for others that may be sold individually. Thanks.

are you putting the stock sized pistons back in? No over bore?
I think so. Cylinder walls in good shape. minimal ridge. I did not check taper yet. (mostly just cleaning stuff so I can put it away until I have $ for machine shop, freeing my garage and bench space for other cheaper stuff--painting engine tin/mounts/brackets/bolt-on chassis parts, rebuild transmission, change out ball joints and bushings in control arms etc.) I'm assuming stock.

The piston I'm replacing had weird grey stuff I could not clean off the top. After giving up on reusing it, I aggressively scrubbed it off (partially). It was covering little pits and gouges.

I think something got in that cylinder, and got all mashed up. Whatever it was filled the imperfections like Bondo.

No damage to either valve or the underside of the head in that area. That cylinder had 155 PSI at pre-autopsy compression check. (same as most cylinders apart from 145, and 175 PSI outliers).

Author:  Dart270 [ Tue Sep 03, 2019 8:21 am ]
Post subject:  Re: One hydro motor piston

Might be pitting of the piston from detonation. 155 psi is good, so I might just use it as is. If the cranking psi numbers were so good, I am curious why you took it apart? One replacement piston should work well too, of course. I assume you will replace all the rings with good quality replacements? Are you honing it, or having a shop do it?

Thanks and all the best,


Author:  Ed Mullen [ Wed Sep 04, 2019 10:44 am ]
Post subject:  Re: One hydro motor piston

It wasn't torn down for compression, or anything to do with that piston.

It was making a low RPM knock (pronounced when starting/turning over by hand).

On top of that, it was stalling under load (to the extent it stopped moving under it's own power).

It burned oil, and leaked oil from every gasket.

Van needed a bunch of 'take it off the road for a long time' work.

I had meant to check oil pressure before removing from van, by the time I was ready to pull it, it wouldn't idle well enough to do so.

Compression was performed with it out of van.

Knock was gone after I removed transmission, so I am guessing TC bolts (although I don't recall any seeming necessarily loose).

Engine is just a small part of why I took it off the road. I intend to do a full resto-mod, and rebuild 'original' transmission (it was in it when I got it. It is the same type: 2" low/reverse band w/ETCLU. My dad had surrendered identical original as a trans shop core)

Machine shop is going to hone cylinders, and probably other stuff (valve guides, true mating surfaces--They suggested line-boring, which seems like overkill, but I have time to figure out what all I want done.)


Author:  Dart270 [ Thu Sep 05, 2019 2:17 am ]
Post subject:  Re: One hydro motor piston

Sounds like good plans and decisions. I don't see a need to line bore/hone it, unless the crank/block were substantially damaged from a spun bearing or something else.


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