Trying to determine Overall Drive Ratio

[Alumarine]

Hi there, I need some help determining the final drive ratio on my 3.0.
It has what I believe is a #1 upper and a MR/Alpha lower.
I took the top cover off.

The upper gear set is definitely a 20/24 as I can read the number 43-55766 on it (and I counted).

I marked the top of the drive shaft into quarters and (approx) 1.62 turns of it rotates the prop 1 revolution.

Thanks for any help.

 

[alldodge]

I would mark the shafts, put it in gear, then rotate the drive shaft 10 revolutions. After 10 revs it will be real easy to determine

 

[Bondo]

Ayuh,...... The over all gear ratio is changed in the upper unit,.....

Up until the Gen.IIs, ALL of the lower units were 1.65:1,......

 

[Alumarine]

I would mark the shafts, put it in gear, then rotate the drive shaft 10 revolutions. After 10 revs it will be real easy to determine

When you say driveshaft do you mean rotate the motor 10 times?

 

[alldodge]

Up until the Gen.IIs, ALL of the lower units were 1.65:1,......

I have to write that down some place I can find it later, :thumb:

 

[Alumarine]

Ayuh,...... The over all gear ratio is changed in the upper unit,.....

Up until the Gen.IIs, ALL of the lower units were 1.65:1,......

Ok, I checked parts diagrams for a few different drives prior to Gen II and they all seem to use 17/28 gears as standard.

 

[alldodge]

When you say driveshaft do you mean rotate the motor 10 times?

Could but I would remove the drive and rotate it, but looks like Bondo has you covered

 

[Bt Doctur]

[h=2]Mercruiser R-Style Sterndrives (1969 - 1983)[/h]

2.5 Liter	120	2.5/153	4	1.98	20-24	1969-1983
2.5 Liter	130	2.5/153	4	1.98	20-24	1969-1983
165	165	3.7/224	6	1.65	24-24	1969-1983
470	170 - 190	4.3/262	4	1.84	17-19	1969-1983
215 - 233	205 - 230	351	8	1.5	20-22	1969-1983
260	260	5.7/350	8	1.5	20-22	1969-1983

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

[h=2]Mercruiser Alpha 1 (1984 - 1990 Models)[/h]

2.5 Liter	120	2.5/153	4	1.98	20-24	1984-1990
3 Liter	130	3.0/181	4	1.98	20-24	1984-1990
165	165	3.7/224	4	1.84	17-19	1984-1990
175	175	4.3/262	6	1.84	17-19	1984-1990
180, 190, 470	180	3.7/224	4	1.84	17-19	1984-1990
200	200	5.0/305	8	1.65	24-24	1984-1990
205	205	4.3/262	6	1.84	17-19	1984-1990
230	230	5.0/305	8	1.5	20-22	1984-1990
260	260	5.7/350	8	1.5	20-22	1984-1990
350 Magnum	270	5.7/350	8	1.5	20-22	1984-1990

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

[h=2]Mercruiser Alpha 1 Gen 2 (1991 & Newer)[/h]

3 Liter	130	3.0/181	4	2.0	24-24	1998-2005
3 Liter	130	3.0/181	4	1.94	20-24	1991-2005
4.3 Liter	175	4.3/262	6	1.81	17-19	1991-2005
5.0 Liter	230	5.0/305	8	1.61	24-24	1991-200

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

 

[Bt Doctur]

[h=2]Mercruiser R-Style Sterndrives (1969 – 1983)[/h]

2.5 Liter	120	2.5/153	4	1.98	20-24	1969-1983
2.5 Liter	130	2.5/153	4	1.98	20-24	1969-1983
165	165	3.7/224	6	1.65	24-24	1969-1983
470	170 – 190	4.3/262	4	1.84	17-19	1969-1983
215 – 233	205 – 230	351	8	1.5	20-22	1969-1983
260	260	5.7/350	8	1.5	20-22	1969-1983

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

[h=2]Mercruiser Alpha 1 (1984 – 1990 Models)[/h]

2.5 Liter	120	2.5/153	4	1.98	20-24	1984-1990
3 Liter	130	3.0/181	4	1.98	20-24	1984-1990
165	165	3.7/224	4	1.84	17-19	1984-1990
175	175	4.3/262	6	1.84	17-19	1984-1990
180, 190, 470	180	3.7/224	4	1.84	17-19	1984-1990
200	200	5.0/305	8	1.65	24-24	1984-1990
205	205	4.3/262	6	1.84	17-19	1984-1990
230	230	5.0/305	8	1.5	20-22	1984-1990
260	260	5.7/350	8	1.5	20-22	1984-1990
350 Magnum	270	5.7/350	8	1.5	20-22	1984-1990

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

[h=2]Mercruiser Alpha 1 Gen 2 (1991 & Newer)[/h]

3 Liter	130	3.0/181	4	2.0	24-24	1998-2005
3 Liter	130	3.0/181	4	1.94	20-24	1991-2005
4.3 Liter	175	4.3/262	6	1.81	17-19	1991-2005
5.0 Liter	230	5.0/305	8	1.61	24-24	1991-2005
5.7 Liter	260	5.7/350	8	1.47	20-22	1991-2005

Engine HP Liters/CID Cylinders Gear Ratio Tooth Count Year

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[achris]

Easiest way to determine drive ratio, without stripping the drive and counting teeth, is to remove the spark plugs, have a helper (or wife/offspring) count the turns of the prop as you turn the engine. Obviously with it in forward gear... Count engine turns as you go, and when prop shaft turns reaches 10, divide the number of engine turns by 10 and you have the ratio...

Alternately, use DonS wonderful drive ratio tables in the stickies and just look it up... And yes, as the drives before the Gen II (What stupid name shall we give those? "Alpha style"?) most had 1.65:1 (the last ones had 1.62:1, but the difference is ignorable), we can assume that with a 20/24 tooth count in the top box, the overall ratio is 1.98:1 (or 1.94:1 for the later ones)... And that is the factory standard supply with a 3 litre engine.

Chris.......

 

[Alumarine]

Thanks guys. Once I track down a helper I'll rotate the motor and determine how many rev's of it for 10 turns of the prop.
It will probably take a week or 2 before that happens.