I’m pulling a 3.8 out of an 85 Diplomat this weekend and wondered if anyone has any advice or tips to make this a painless experience. The front side looks simple with only a few bolts on the motor mounts but I can’t see the backside. How should I remove the coupler from the flywheel to the stern drive? Does this just slip off or is it bolted on? I read somewhere that engine alignment is critical to avoid vibration. What can I do to assure that I get this back in the original position? 
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Engine Removal Tips
- Thread starter RGraves
- Start date
I’m pulling a 3.8 out of an 85 Diplomat this weekend and wondered if anyone has any advice or tips to make this a painless experience. The front side looks simple with only a few bolts on the motor mounts but I can’t see the backside. How should I remove the coupler from the flywheel to the stern drive? Does this just slip off or is it bolted on? I read somewhere that engine alignment is critical to avoid vibration. What can I do to assure that I get this back in the original position? 
daren34
Member
rear bolts
after you take the drive off assuming you have a mercruiser of a omc cobra
there are 2 bolts on the flywheel housing
on a merc they have 3/4 heads the nuts go up in a cavity so no wrench is needed on the bottom there is a spring and a washer on each side between the flywheel housing and the transom plate
some people slide the motor forward off of the drive shaft but it will be very hard to put back in this way
on good thing is a checy v-8 will go in place of the chevy 3,8 you are taking out
good luck daren
after you take the drive off assuming you have a mercruiser of a omc cobra
there are 2 bolts on the flywheel housing
on a merc they have 3/4 heads the nuts go up in a cavity so no wrench is needed on the bottom there is a spring and a washer on each side between the flywheel housing and the transom plate
some people slide the motor forward off of the drive shaft but it will be very hard to put back in this way
on good thing is a checy v-8 will go in place of the chevy 3,8 you are taking out
good luck daren
Eaglenva
Member
3.8, is that an i/o - i think so. if outboard - disregard this post.
here is what i encountered with my Merc v8. it was my first time.
the front seemed more involved than the back due to brackets for powersteering, etc.
disconnect battery - place drive in forward all the way. remove outdrive (on alpha 1 it is 6 bolts facing aft. gently slid it out, weights about 70-80# so support it or be ready for the weight. once seal is broken should slide off easily, do not force.
take good pictures of all wiring and placement, write on paper to back up pics. disconnect all wires, hoses, fuel line and linkages. i dropped the alternator and powersteering as 1 unit and left it in the bilge so the power steering did not have to be drained.
once every thing like that is disco'd. hook engine up to lift but only apply a couple of lbs lift. remove the 2 bolts from front engine mounts that go into the stringer. there should be 2 big bolts as you look down at the back of the engine bell housing where it meets the support from the transom. take those out, there is a felt gasket between the bell mount and transom mount. one may be flatter than the other so remember which goes where. also note what order the parts from the rear mounts go, there are about 3-4 parts there. bolt, spring, spacer, felt, nut. lower nut will not turn as it slides into a slot that is supported.
at this point it should be ready to lift. be sure to have sufficient overhead support and do not get under it. lift slowly, there is likely something it will hang on or that you missed unhooking.
once out, mark mounts and which side they came from so they can go back to same spot. if it is a different block going in, does not matter... must be re-aligned anyway. when installing, be very careful on putting the outdrive back in. if possible borrow alignment tool from local shop. i told them what i was doing and they offered it before i could ask. dry fit w/o gaskets first, tore my first one. there is a little tab (slot) on the very bottom of the gimble ring that controls what gear you are in. that thing likes to turn. it must be aligned up (facing front to back) with the little tab on the outdrive (also must face front to back). actually had to have the wife hold it still while i slid the outdrive on.
it was not difficult to align. put grease on end, carefull slide in, pull back out. see where it is touching and adjust 1 turn (more or less as needed) on front mount (right or left / up or down depending). repeat process. at some point it will slide in about 1 more inch and it is done. took us longer to figure out it was only going in about 1 more inch than to get it adjusted. had it adjusted 4 times before we figure that one out. if touching bottom right, raise left. touch top left lower right, etc. after a bit we went more by feel. you can kinda tell.
have fun!!!!!
here is what i encountered with my Merc v8. it was my first time.
the front seemed more involved than the back due to brackets for powersteering, etc.
disconnect battery - place drive in forward all the way. remove outdrive (on alpha 1 it is 6 bolts facing aft. gently slid it out, weights about 70-80# so support it or be ready for the weight. once seal is broken should slide off easily, do not force.
take good pictures of all wiring and placement, write on paper to back up pics. disconnect all wires, hoses, fuel line and linkages. i dropped the alternator and powersteering as 1 unit and left it in the bilge so the power steering did not have to be drained.
once every thing like that is disco'd. hook engine up to lift but only apply a couple of lbs lift. remove the 2 bolts from front engine mounts that go into the stringer. there should be 2 big bolts as you look down at the back of the engine bell housing where it meets the support from the transom. take those out, there is a felt gasket between the bell mount and transom mount. one may be flatter than the other so remember which goes where. also note what order the parts from the rear mounts go, there are about 3-4 parts there. bolt, spring, spacer, felt, nut. lower nut will not turn as it slides into a slot that is supported.
at this point it should be ready to lift. be sure to have sufficient overhead support and do not get under it. lift slowly, there is likely something it will hang on or that you missed unhooking.
once out, mark mounts and which side they came from so they can go back to same spot. if it is a different block going in, does not matter... must be re-aligned anyway. when installing, be very careful on putting the outdrive back in. if possible borrow alignment tool from local shop. i told them what i was doing and they offered it before i could ask. dry fit w/o gaskets first, tore my first one. there is a little tab (slot) on the very bottom of the gimble ring that controls what gear you are in. that thing likes to turn. it must be aligned up (facing front to back) with the little tab on the outdrive (also must face front to back). actually had to have the wife hold it still while i slid the outdrive on.
it was not difficult to align. put grease on end, carefull slide in, pull back out. see where it is touching and adjust 1 turn (more or less as needed) on front mount (right or left / up or down depending). repeat process. at some point it will slide in about 1 more inch and it is done. took us longer to figure out it was only going in about 1 more inch than to get it adjusted. had it adjusted 4 times before we figure that one out. if touching bottom right, raise left. touch top left lower right, etc. after a bit we went more by feel. you can kinda tell.
have fun!!!!!
Eaglenva
Member
it may be cheaper to build a 350 due to volume of parts available. i would consider it, it may eat more gas than a 3.8 though. someone can tell you what else would need to be changed like exhaust - if you dont need to replace yours thats an extra 450 right off the top.
Thanks for all the good info!
According to the guy I bought it from the boat was only used once since the engine was rebuilt. Unfortunately it sat for about 8 years and when he got it back out the engine was locked up. Hopefully I can pop off the heads and a few bearing caps and see what the problem is. If I can get it running with a set of rings and gaskets I’ll stick with the 3.8. It should run about 50mph which is good for the small lakes where I live. Maybe a 350 will be a future plan. I work with the engineers in the GM performance parts division so building a nice stroker shouldn’t be a problem!
According to the guy I bought it from the boat was only used once since the engine was rebuilt. Unfortunately it sat for about 8 years and when he got it back out the engine was locked up. Hopefully I can pop off the heads and a few bearing caps and see what the problem is. If I can get it running with a set of rings and gaskets I’ll stick with the 3.8. It should run about 50mph which is good for the small lakes where I live. Maybe a 350 will be a future plan. I work with the engineers in the GM performance parts division so building a nice stroker shouldn’t be a problem!
Big Dave
Member
3.8 should be an omc stringer drive(big rubber boot where drive comes thru transom. I would get a manual.....the previous posts are for mercruiser(black drive) and may not apply. 4.8 could be either mercruiser or omc cobra..
I have a 86 Diplomat that came with a Mercruiser 3.7l 4cyl that was no good, and had a v8 from another project.....I love the v8 in this boat!!69.41mph on gps with the same prop used for skiing and wakeboarding...(wife says its too fast now, so no props to get the rpm back out of the sky)
I have a 86 Diplomat that came with a Mercruiser 3.7l 4cyl that was no good, and had a v8 from another project.....I love the v8 in this boat!!69.41mph on gps with the same prop used for skiing and wakeboarding...(wife says its too fast now, so no props to get the rpm back out of the sky)
Big Dave
Member
Brake fluid works great to loosen the rust on top of the pistons
presure test manifolds and risers, and you will probably find where the rust(water) came from. be prepared to replace them either way as they may be rusted thru even if not the cause of the problem
I found an online service manual and Confirmed that it’s a Mercruiser Alpha One. According to the manual, and the forum, the stern drive unit can be removed by simply unbolting the trim cylinders then the 6 locknuts on the stern drive. I broke the gasket loose and pulled the drive about ½” apart but it wont go any further. Am I missing something or could the shaft be rusted into the engine coupler?
Eaglenva
Member
note that the little rascal on mine loves to turn a little by itself during installation so be prepared for it to stop when you put it back on... you may have to get someone to slip thier hand in there and keep it aligned while installing outdrive later.
that part is heavy so be ready for the weight and watch that the prop doesnt catch your foot (if doing this by hand).
that part is heavy so be ready for the weight and watch that the prop doesnt catch your foot (if doing this by hand).
Big Dave
Member
I put the drive in forward and hold the prop with pressure against the gears. that is the only way mine will stay in gear long enough to get the drive back on...usually takes two people at least for me.
If the shifter was in forward before you took of the nuts, lift the drive and drop it a few times, and it should fall right off.
If the shifter was in forward before you took of the nuts, lift the drive and drop it a few times, and it should fall right off.