Piston Rings Gap 3.0 Mercruiser

M

mb7

Seaman Apprentice
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Mar 11, 2009
Messages
30
I am rebuilding my engine and I got a new set of pistons (two off the old one had pieces broken)
I had a set of STD rings (new) and the gap was 0.041 (biggest)
I know on the manual the gap must be 0.010 to 0.020, so I order 030 oversize rings

My questions is:

Should I keep the gap 0.011 with the new rings? Too low?

Can I adjust this new rings to a - 0.020 (grinding) to match the gad on the others cylinder bores? ( they are all different)

The cylinder walls are perfect I am just honing them so the rings will sit better
1986 Mercruiser 3.0
 
Bondo

Bondo

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Re: Piston Rings Gap 3.0 Mercruiser

mb7 said:
I am rebuilding my engine and I got a new set of pistons (two off the old one had pieces broken)
I had a set of STD rings (new) and the gap was 0.041 (biggest)
I know on the manual the gap must be 0.010 to 0.020, so I order 030 oversize rings
My questions is:

Should I keep the gap 0.011 with the new rings? Too low?

Can I adjust this new rings to a - 0.020 (grinding) to match the gad on the others cylinder bores? ( they are all different)

The cylinder walls are perfect I am just honing them so the rings will sit better
1986 Mercruiser 3.0
Click to expand...

Ayuh,... Ya can't put Oversized rings on Standard sized pistons....

They're .030" over Diameter, not just their length...
 
D

daydreamer1252

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Aug 7, 2007
Messages
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Re: Piston Rings Gap 3.0 Mercruiser

It's been a long time since I did engines 101 but.....if the end gap is TWICE what is specified the cylinder bores are NOT PERFECT.....They are oversized and if you want it to work well and for a long time you will need to have the cylinders bored and fitted with properly sized pistons.

In the olden days.....many engines had slightly oversized pistons available like .002 oversized. I don't know if this engine is one or not....if it is it may give you a less expensive alternative that is also not as GOOD as a proper solution......as in all things you get what you pay for....Good luck..
 
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mb7

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Re: Piston Rings Gap 3.0 Mercruiser

Thanks for the reply


The rings I got was STD size and the and these rings was given me the 0.041 gap, but this the weird part, when I merger the original rings gap drops to 0.021
I am guessing the new ones must be the wrong set. The brand is Enginetech part no. R40054-STD

The new pistons I have is 1491HT Silvolite requirers 20 to 40% more gap on the first ring (Hypereutectic. Mercruiser Marine Engine. 3.200' Dia. D Head Recess .130" Deep (14cc Cup). Not Destroked.)
 
Bondo

Bondo

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Re: Piston Rings Gap 3.0 Mercruiser

mb7 said:
Thanks for the reply


The rings I got was STD size and the and these rings was given me the 0.041 gap, but this the weird part, when I merger the original rings gap drops to 0.021
I am guessing the new ones must be the wrong set. The brand is Enginetech part no. R40054-STD

The new pistons I have is 1491HT Silvolite requirers 20 to 40% more gap on the first ring (Hypereutectic. Mercruiser Marine Engine. 3.200' Dia. D Head Recess .130" Deep (14cc Cup). Not Destroked.)
Click to expand...

Ayuh,... Those numbers don't add up either...
That motor has a 4" bore, not 3.200"...
And,..
I've Never, Ever seen D-dished pistons in 1 either...
 
Don S

Don S

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62,321
Re: Piston Rings Gap 3.0 Mercruiser

The cylinder walls are perfect I am just honing them so the rings will sit better
1986 Mercruiser 3.0


The new pistons I have is 1491HT Silvolite requirers 20 to 40% more gap on the first ring (Hypereutectic. Mercruiser Marine Engine. 3.200' Dia. D Head Recess .130" Deep (14cc Cup). Not Destroked.)
Click to expand...

The 3.20" pistons are for the 120 (2.5L) engine, not the 140 (3.0L). Which do you have really ??????
 
M

mb7

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Re: Piston Rings Gap 3.0 Mercruiser

I have a 3.0 mercruiser 1986

3.200" is the "D" shape recess on the top of the piston
The piston is made for:
"L4-181
3.0L
4.0000(IN)
101.6(MM)
CHEVY ENGINE
1982-89 Hypereutectic. Mercruiser marine engine.
COMP HT: 1.625 PIN DIA: .9273 Offset
1491HT
STD. or oversizes

Rings sizes 2-5/64,
1-3/16"


According SilvoLite piston catalog page 79
The piston inside my engine has a "D" shape
My Question is?
Do they make a Standard ring with oversize OD like 0.010 or 0.020 over (not a oversize ID)......

I heard a something about a "Service Ring"
 
Bondo

Bondo

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Re: Piston Rings Gap 3.0 Mercruiser

My Question is?
Do they make a Standard ring with oversize OD like 0.010 or 0.020 over (not a oversize ID)......
Click to expand...

Ayuh,... It appears yer asking the same basic question in different ways, Hoping somebody's gonna give you the answer you WANT...
Well,...
It Ain't gonna Happen....

From everything yer saying, the bores of the block are over-worn...
The block needs to be bored oversize, 'n new pistons, 'n rings fitted...

You can't just put oversized rings on standard sized pistons to make up for the wear...
 
F

fishrdan

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Messages
6,989
Re: Piston Rings Gap 3.0 Mercruiser

I went down that road once, tried to piece-meal an engine together (never again). Sure it ran, but wasn't perfect. A huge disappointment and I wished I had spent the extra money to do it right, a complete rebuild.

Have you mic'ed the cylinder bores to see if they are within spec? Personally, I'd haul the block down to the machine shop and have them bore it to the next size to clean it up, then install a new set of over-sized pistons and rings.
 
M

mb7

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Messages
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Re: Piston Rings Gap 3.0 Mercruiser

Thanks Guys,

No short cuts...... I got it ......

I have the oversize rings (0.30) so I just need a set of pistons 30 over
I have the block apart so I just need to find a good machine shop around here

What is the diference betwen the "D" shape and the "flat" piston head?
 
Fishermark

Fishermark

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Joined
Oct 19, 2003
Messages
5,617
Re: Piston Rings Gap 3.0 Mercruiser

It seems to me that you ought to have the machine shop measure your bore before you go ordering anything new. Forget the ring gap for the time being. Verify your actual bore diameter and go from there. If you do need to order pistons, you might find that the machine shop can get you the pistons and rings for as good - or better - price than you can get yourself. Plus you will know you have the right parts. You might even find you simply have the wrong rings, and there may be nothing wrong with your current cylinders and pistons.
 
M

mb7

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Messages
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Re: Piston Rings Gap 3.0 Mercruiser

I will bring everything I have to the machine shop and lets start from there

Thanks for all the advices
 
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