Separating the power head from the lower unit. '78 Merc 800

[1970Thunderbird]

Hello again, friends.

My engine lifting eye arrived today, so I immediately tried lifting the power head from the lower unit. I'd previously disconnected the 6 bolts that hold it to the lower unit, as well as wiring, hoses and other parts. I also removed the nuts from the two bolts up front, beneath the cowl that also hold the power head to the lower unit.

I started lifting, but they seem stuck together. The manual says to use an air hammer, or heat to help separate the two, but I am reluctant to do so, as I don't want to crack anything.

Could I be missing some hidden bolts? I've had the lower unit off before, so I know the drive shaft slips out easily enough. I'm wondering if I've missed some bolts.

If it comes to it, and I have to use an air hammer, where do I use it? Manual says on the 6 studs that join the power head to the lower unit. How can I ensure I don't damage anything?

You have my deepest thanks, as always.

 

[Gomer50]

Re: Separating the power head from the lower unit. '78 Merc 800

Make sure you unhooked the shift lever also inside the bottom cowling.You can see where the mid section and the power head come together small groove,I used a screw driver and a rubber malot to get them to come apart.The mid section and the powerhead are real good friends they have been together for a long time so getting them to split up sometimes takes a good convincing.Good Luck..

 

[Chris1956]

Re: Separating the power head from the lower unit. '78 Merc 800

Drop the gearcase first. You will not want to try to align the driveshaft when you reinstall the powerhead. it is too clumbsy.

there are two nuts under the front part of the motor that need to come off. they are about directly under the carbs. They only come off part way, so loosen them and lift the powerhead a bit and then take them off.

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

Chris,

I removed those two nuts. I'll probably remove the lower unit when the power head is at the machinist.

Gomer,

Do you mean the shift lever just beneath the throttle cable, or the other part beneath the carbs?

Thanks!

 

[supagidget]

Re: Separating the power head from the lower unit. '78 Merc 800

Is your driveshaft stuck in your crankshaft? Trying dropping the l/u and if it does not drop your d/s may be stuck.

 

[Gomer50]

Re: Separating the power head from the lower unit. '78 Merc 800

I agree on taking down the lower unit just to make sure the driveshaft is not frozen to the crankshaft.Concerning taking apart the shift shaft to pull the power head I took the bolt out here in the pic.

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

Gomer,

Thanks for the photo. Do you happen to know what size the nut is? Or is it even a nut? I can't get to it easily, so I'm not certain how it's attached.

Thanks

 

[Chris1956]

Re: Separating the power head from the lower unit. '78 Merc 800

All you really need to do is remove the nut that holds the shift cable to the shift slider. That should be a 3/8" or 7/16" socket. The shift rod will pull thru the rubber slide block and that will do it.

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

Chris,

I've already done that. I'll give it another shot, though. I'm a little bit wary of pulling on the cables too much, as I don't want to damage anything. I don't have enough space to get the rubber slide holders out of their holder near the front.

 

[Chris1956]

Re: Separating the power head from the lower unit. '78 Merc 800

The cables come out of the clamp easily. Just remove the clamp lock that swings up and out, to free the barrel nuts. After you remove the nut that holds the shift cable end, the shift arm is free to stay in the midsection.

You should have about 6-8 9/16" stainless lock nuts just below the lower cowling that hold the powerhead, plus the two in the front between the midsection and the steering arm.

Drop the gearcase first. Believe me it makes the job easier. Once all the nuts are off, rock the powerhead to break the gasket seal, to the midsection. Now use a hoist of some kind to pull it straight up.

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

Chris,

Perhaps I have a problem with the clamp lock. The bottom stays in place, so only the top moves. I'll see what I can do about that.

I've already removed all the nuts. You're correct, there were 6.

I'll definitely take your advice on the gearcase. I need to remove it anyway to adjust the reverse gear lock that prevents the motor from kicking up.

Excellent advice. This bad boy is coming off this afternoon!

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

All,

I got the power head separated from the lower unit and got it almost completely apart. The transfer port covers are still on. They wouldn't budge, even with tough tapping by a rubber mallet. Also, there's a piece bolted next to the cylinders that I left in place. It was beneath the exhaust cover plate.

All in all, I think it's ready to take to the machinist. The block looks almost sterile, it's so clean. I haven't seen any broken rings or cylinder gouging. But I'm sure the machinist will say for certain.

 

[Gomer50]

Re: Separating the power head from the lower unit. '78 Merc 800

So whats wrong with your power head to start with?

 

[1970Thunderbird]

Re: Separating the power head from the lower unit. '78 Merc 800

Gomer,

To be completely honest, I'm starting to wonder that myself. The reason I tore it down was I was getting 95 (cyl 1), 95 (2), 85 (3) and 65 (4) psi on my compression checks.

I was developing very little power under load. The motor would not smoothly progress from idle in forward to WOT in forward. You had to baby it, and even then, it put out no more than 2,500 rpm at WOT. It ran pretty well in idle.

Also, there's been a lot of oil in my test tank when I've run it. So, I though stuck or broken ring. Ruled out stuck by following a decarbon procedure. Still had the same compression readings.

Thus the tear down. I didn't give the rings a super-extensive examination, nor the pistons. I thought I felt some grooving on numb er three. I'll get a better look at them all tomorrow.

The bearings on my crankshaft do seem poor. They rattle a bit, so I'm wondering if that's part of it. And it could be that number 3 & 4 have just worn down, though I imagine I'd see the same throughout all the cylinders.

I'll give it a deeper exam tomorrow.