888 merc coupler fix

I'm new to iboats but have read postings here for a while.

I am about to replace a second coupler on a built 302 with a 888. Might be too much hp (around 300) for the aluminum coupler with small splines.
I know it could be alignment or lack of maintenance also.
I just don't want to ever have to pull the engine to change a third.

I saw an old thread where JasonB said "I know someone that has a built Ford smallblock that kept stripping stock couplers. He has lots of $$$ in that motor. I think he ended up having a Bravo couple machined to bolt to his engine." See mating a 351 to a mercruiser outdrive....
That post was 2006.
I'm waiting to hear from JasonB.

My other idea is to remake the flywheel cover with a removable cover that has enough room to take off the coupler. I would also reinforce to make up for loss of strength. Then I could leave the engine in and still change couplers.

Any other ideas?

Help save me from ever having to pull the motor to change a third I would appreciate it.


Thanks in advance.

Re: 888 merc coupler fix

I know it could be alignment or lack of maintenance also.

Click to expand...

Ayuh,.... It Ain't the HorsePower that's the Problem,...

I'd sooner think Bad alignment or a Lack of Grease....

Re: 888 merc coupler fix

I understand the possible causes. I just boosted the hp so I thought it might have added to the problem.

I just really really don't want to take the motor out a third time regardless of the cause. Unless its a motor problem.


Two couplers in thirteen years isn't that bad.

Re: 888 merc coupler fix

Using the same coupler on a 300 horsepower 351, four years now.

Re: 888 merc coupler fix

Thanks mkast.
At least that will help me rest easy about using a stock coupler.
What size boat/prop do you run that with?
A small boat might not put as much strain on the driveline.

Re: 888 merc coupler fix

The guys boat is a 1976 Hammond, 19.5.
If I remember correctly it uses a 21 propeller.

Re: 888 merc coupler fix

You better have some good money to get 300HP out of the 302. . . it pretty much has to be stroked. I doubt ou are ruining couplers because of power, it's because you aren't aligning the engine properly.

Re: 888 merc coupler fix

Motor never was on a dyno.
Just guessing based on the work I've done similar to a Mustang I had.
I may be over estimating... I'm sure those exhaust manifolds rob some power.
250 hp easy 300 hp might be a wish.
Regardless. Last coupler lasted 13 years +/- I would think the alignment was ok. Poor maintenance maybe. I didn't grease it every year.
I just did motor work over the winter then this summer it loses the coupler.
Coincidence?
Not just the horsepower but the boat is a 20' Slickcraft at about 3500 lbs
I am worried that it puts a lot of strain on the drive line from a hole shot. It launches hard for a good size boat. I was luanching it with 4 of us in the boat (700# +/-) when I lost the coupler.Coincidence?
If I put it back together I would like it to last.It sucks to have to blow "boating time" to pull the motor. Too much work for six bolts.

Everyone here says it will hold up.
So I will probably go with a stock coupler again and see what happens.
I still might modify the flywheel cover as insurance while I have it out.
Thanks

Re: 888 merc coupler fix

Another possibility is your transom flexing under load. May want to jump on the sterndrive a bit and see if you see much flexing.

Re: 888 merc coupler fix

I would think the alignment was ok. Poor maintenance maybe. I didn't grease it every year.

Click to expand...

Ayuh,... Did you, or have you Checked the Alignment,..?? Ever,..??

Re: 888 merc coupler fix

302slick said:

I am worried that it puts a lot of strain on the drive line from a hole shot.

Click to expand...

Try one of these. Sternjack.

Re: 888 merc coupler fix

The person I was referring to was running a 302/347 stroker highly modified in a 21' Glastron. He had gone as far as buying a $1500 set of stainless exhaust manifolds for it. He was putting out just over 300hp if I remember right. I'm wanting to think he had a Volvo drive, but I may be wrong as its been a few years. I think he went through 3 couplers in 3 weeks so he came up with the idea of taking a bravo coupler, filling in the bolt holes, and re-machining them to match the 302 flywheel. Last I heard from him, it was holding up well.

I've been tinkering with my 888/302 for 7 years now, though mine is much milder, only about 210-215hp guesstimated. It pushes my 19' Glastron to 49mph at 4150 RPM. I'm going with the others that I don't think HP is your root problem. When you did the work, I'm assuming you pulled the motor. This can hose your alignment. Pulling if for the first replacement may have hosed it worse. The tool to check is only $70. Cheap price to help make sure you don't have to keep buying new couplers. You should check any time the motor is moved or the drive is off. I would also check the bushings on the rear motor mounts as well. They may be worn out.

As much as I like my 302, unless you stroke it and or turn it past 6000rpm, it isn't a torque monster, esp at low RPM. Matters aren't helped if you still run the mercruiser log style manifolds. If you are hammering it hard enough to strip couplers with power, that #1 drive won't live long. If you are turning it past 5 grand, the drive won't last long that way either.

My gut is that you are going to find an alignment issue. The same style coupler and drive will live for years behind a 260hp 350 Chevy. The 351 model was rated at 233hp. Unless you have gone fairly radical, you probably aren't making much more torque than those.

Knock on monitor, my engine hasn't been out on 7 years as of yesterday. The drive comes off at least once a year for u-joint grease, bellows check, alignment check, and water pump at least every other year, although I find a yearly impeller kit to be cheap insurance as I find the cooling system on these motors to be marginal some days.

Re: 888 merc coupler fix

Engine -to-outdrive alignment affected by:
front motor mounting stringers rotted and causing the motor to flex
rear motor mount bushings bad, causing motor to flex
transom rotted causing motor /drive to flex
loose motor mount bolts/nuts
improper alignment
never removing the drive and greasing the input shaft

Re: 888 merc coupler fix

Yes, I checked the alignment when I greased it... But, I admited I've skipped a year here or there...just not enough time for all my projects. I won't skip ever again!!!

I'll go through and check transom flex and other listing by Bt Doctur.
I've never checked most of those.
This is one of the few boats I've ever worked on and I'm still learning.

Thanks ALL for being so willing to share you knowledge.

Re: 888 merc coupler fix

Old boats are definitly a learning experience..

Keep at it and you'll find the problem.

Re: 888 merc coupler fix

Bubba1235 said:

One more thing... Over time the splines on the drive shaft may begin to wear and you get a "sloppy" fit. Not good...

Click to expand...

Ayuh,... Agreed,... A Worn shaft will Chew through a coupler in No time...

The splines should have nice square flat tops...
And no Polished shoulders...

Re: 888 merc coupler fix

Took it apart... Drive shaft needs to be replaced also.
Made out of steel I never payed much attention to it.
Thanks again.