1954 Bigtwin 25 help please (photos)

kodibass

Master Chief Petty Officer
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I have been wanting to re-ring this old 25 auqusonic for a while now mod# 25916-1193 this motor ran crappy, hard to start, compression was 60 top & 95 bottom, pulled head & covers and found seized rings, tore motor apart and only 1 ring per piston was not seized, ordered rings & gaskets from Seaway marine, I lightly honed the cylinder walls which looked ok, not scored, pitted etc. I used a fine flexhone for this I soaked the pistons in berrymans chem dip to clean ring grooves, I put the new rings on lined up ends to the pins and installed, the pistons are so tight that they will not move in the cylinders unless tapped with a wooden rod and rubber mallet, Prior to install I coated walls and piston with 2cycle oil. When I put the rings on they almost dropped over the piston, they needed very little spreading to get them in the grooves, the looked a bit big to me kinda like they were spread to much or something, But I used a ring compression tool and they will go in but As i said they are much to tight, the P/N for the rings is 378431? These are vintage rings & the packages are stained and very hard to read the # but that's my best guess. I have done a few rebuild on outboards mostly small, 9.9 15. 9.5 6,4, etc and I don't remember the rings being this sloppy and loose after being placed in the grooves. Any help much appreciated. I'm feeling very giving at this moment in time and I'm thinking Davy Jones might be needing a new anchor.
 

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Chinewalker

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Re: 1954 Bigtwin 25 help please (photos)

Take the rings off the piston and slide the rings into the bore. Check for the gap at the ring ends. Should only be a few thousandths of an inch.
 

HighTrim

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Jun 21, 2007
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Re: 1954 Bigtwin 25 help please (photos)

Yeah havent we talked about these rings before?

The superseeded part number for the rings you bought is 382825.

The part number for the rings for your engine is 302533 superseeded to 378421

That is not to say that the rings you have are wrong, Im not sure offhand what they are for. Best bet as mentioned in the last thread is to measure them and see what is what.

I can tell you that you should not need anything mechanical to get the pistons to move in their bore. Measure the clearances, the ring end gap, the gap between piston and bore, the groove clearance, and the cylinder bore.

All these clearances work together and need to be within spec.
 

Joe Reeves

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Feb 24, 2002
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Re: 1954 Bigtwin 25 help please (photos)

I have been wanting to re-ring this old 1954 25 auqusonic mod# 25916-1193. I put the new rings on lined up ends to the pins and installed, the pistons are so tight that they will not move in the cylinders unless tapped with a wooden rod and rubber mallet, the P/N for the rings is 378431?

The rings you have purchased (378431-382825) are for the following engines: 1960 75hp, 1961/68 75-85hp, 1969 85hp, 1964/68 90-100hp, 1971/72 100hp (to name a few). Obviously these rings are not suitable for your 1954 25hp model.

The rings needed for the 1951/1955 25hp models are (as HighTrim states) 378421 (Standard Bore) or 378422 (.020 O/S Bore)
 

F_R

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28,226
Re: 1954 Bigtwin 25 help please (photos)

There seems to be something very wrong with this scenario. A 25hp is 2-7/8" bore, and a 75hp is 3-3/8" bore. It would be totally impossible to get 3-3/8" rings into a 2-7/8" bore. You need to do as suggested, and try the rings alone in the bore and check the end gap. That is just Standard Operating Procedure to begin with
 

Joe Reeves

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Re: 1954 Bigtwin 25 help please (photos)

F_R.... In total agreement with you. However I was just going by the part numbers supplied above. Quite impossible to insert a 382825 ring into a 25hp standard bore without one hellava overlap :)

Wonder if somehow, although the package may very well be imprinted with the 378421 part number (std bore), the rings inside are actually 378422 rings (.020 O/S bore)?

At any rate, it should be no problem in moving the installed piston with rings installed up/down within the cylinder.

If I remember correctly, with the ring installed in the cylinder, the gap between the ends should be between .0070 Minimum and .0170 Maximum.
 

HighTrim

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Jun 21, 2007
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Re: 1954 Bigtwin 25 help please (photos)

If I remember correctly, with the ring installed in the cylinder, the gap between the ends should be between .0070 Minimum and .0170 Maximum.

Correct, I have found most of the rings I have done on RDs are in the .010 area.
 

1946Zephyr

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Re: 1954 Bigtwin 25 help please (photos)

Those pistons look pretty decent. I think that motor will run nice, once things are checked out okay. Luckily, no scoring. Pistons aren't very cheap for those old Big Twins anymore.:eek:
 

HighTrim

Supreme Mariner
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Re: 1954 Bigtwin 25 help please (photos)

Those pistons look pretty decent. I think that motor will run nice, once things are checked out okay. Luckily, no scoring. Pistons aren't very cheap for those old Big Twins anymore.:eek:

Exactly, which is why it drives me nuts to hear guys running them on 50:1 or leaner. Powerheads and pistons will eventually run out, making it harder for our kids and grandkids to keep the hobby going.
 

nwcove

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Re: 1954 Bigtwin 25 help please (photos)

i run my 24:1 motors at 16:1. even run my lone 50:1 motor at 16:1 on occasion. no issues other than drips of unburnt oil, and dirty plugs. plugs are cheaper than pistons!!!
 

1946Zephyr

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Re: 1954 Bigtwin 25 help please (photos)

A good healthy motor will burn the extra oil just fine and it won't phase it. You're right. fouled plugs are cheaper and easier to replace than a scored piston.
 

kodibass

Master Chief Petty Officer
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Apr 10, 2010
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865
Re: 1954 Bigtwin 25 help please (photos)

Sorry to all, yes we have talked about these rings before. so I do apologize for the above book. & of course the first thing I did after my last post was to get out my johnson 10th and reread the whole page about measuring end gap. but true to my geezer status I leave that information out of the above whine. If I measured correctly the gap is .011 .012 will push the ring, and a 10 feels a bit loose .011 goes thru about like setting points would feel. I also forgot to mention I broke one of the new rings taking them back off the piston, got in a rush/ frustrated & snap. the old rings all came of in sections so I don't have a complete ring to compare these against, but I'm thinking these are not correct, I cleaned the grooves good, honed the cylinder for about 10 up and down strokes of the flex hone, cleaned everything with gas, blew with air oiled all parts with 2 cyl oil........parts is parts:)
 
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