Restoring an 88 DT40

[mike__b]

I've made a few posts on here about this motor, so I decided to start a thread for my restoration. That is, assuming I choose to go forward.

So far, I bought a DT40TC for almost nothing. The owner didn't know anything about it, except it was attached to the boat he wanted. It seemed to turn over easily with the pull starter. Electrical is a bit hit or miss in the main harness, the engine cover was home to some mice for a long time. I've checked all the relays, and they work. The trim motor makes a noise, but doesn't operate. Knowing it wouldn't be accurate, I attempted a compression check, and it was almost non-existent. I'm not sure if that is because the engine is cold and hasn't run in forever, the engine is beyond repair, if the gauge I rented was bad, or if I didn't have it seated properly.

So far, I've disassembled the starter relay box, the CDI box and removed those, and removed the heads to inspect inside. The head gasket is far gone, and it was obvious that water has gotten on several head bolts by the rust. So I'm not completely sure it didn't have a blown head gasket at some point. I didn't see any rust down in the engine so far. There are some visible "lines" in the same spot down each cylinder wall. They both come from the corner of the exhaust outlet. Rubbing my finger around the cylinder wall, it is incredibly smooth, with the exception being that line running form the exhaust port. It has a friction coefficient not unlike a piece of notebook paper. To uneducated me, that doesn't seem like much wear. Nothing was gouged at all. There also seemed to be a large pool of blue/green oil in the engine, but I haven't opened up behind the carb yet to inspect. That is my next task.

Any advise or suggestions on how to proceed would be appreciated. I'm trying to determine if this is fixable before ordering new parts.

Line from exhaust port corner. About 1cm wide.

Other visible lines which I can't feel at all. Blue/green oil is visible in the intake port.

 

[99yam40]

Re: Restoring an 88 DT40

I would open up the motor and inspect the pistons,crank, and bearings

 

[stylesabu]

Re: Restoring an 88 DT40

want to buy a running dt 75 w/factory manual, I also have a back up motor dt 75 for it

 

[mike__b]

Re: Restoring an 88 DT40

So an update on where I am. Haven't had much time to work on this motor, but I found a few hours this week. Removed all the ancillary components of the motor. Carb, starters, electrical, etc are all off. All that remains is the flywheel, stator, then I'm lifting the motor out.

The intake on this thing looked brand new.

Currently I'm working on removing the flywheel. I've got is soaking in pb blaster, using a harmonic puller, impact wrench, etc. All the normal things to remove a stubborn flywheel. So far, no budge. Right now, I'm just being patient, and trying every day for a little while to get it to release.

Here is what it looks like today:

 

[mike__b]

Re: Restoring an 88 DT40

Well, the motor is disassembled, and I'll be taking it to a machine shop on Tuesday. If anyone has an opinion on the wear marks on the pistons, let me know. Most of the wear is on the exhaust port side (photos below). I'm also going to ask the opinion of the guys at the machine shop, have them measure everything, etc.
I don't notice any rust on the larger bearings, and I'm not real inclined to replace the larger ones. I'll look closer at the wrist pin bearings when I remove the piston. Otherwise, I have seen not rust inside the unit except to say that there seems to be some light rust from condensation on the upper crankshaft. rubbing my finger on it, I couldn't feel anything, but I can see the stain. The bearings still seem shiny on the crank end of the connecting rods. I'm going to ask the machine shop to dump the whole crank in an ultrasonic bath.

I'd appreciate any feedback. As long as the cyls are in good condition, the debate will be whether or not to replace rings only or to replace pistons entirely.

top

bottom

disassembled

 

[99yam40]

Re: Restoring an 88 DT40

I would replace the pistons if they look like that, the exhaust port gets the hottest with the exhaust gasses flowing out of the cylinder.
high octane fuel that is slower burning can be still burning as it leaves the cylinder could cause that, retarded timing could also, or just over all overheating of the motor.
Just my thoughts

 

[mike__b]

Re: Restoring an 88 DT40

Any advise on evaluating the condition of the bearings? I think the outside of them needs cleaning. Where they mated with the casing, they are clean, but they are a slightly different color where they were exposed. All spin very smooth, the upper bearing is the only one I can "hear" spinning. I'm not sure if that is due to any dirt that got in while disassembling, or if always did that. All are tight, laterally.

I measured my cylinders and pistons with a caliper I have. I got the following results:
Top Cylinder: 78.95 mm
Top Piston: 78.88 mm
Top difference: .07 mm
Bottom Cylinder: 78.94 mm
Bottom Piston: 78.69 mm
Bottom difference: .25 mm

I know a machine shop can better measure them, that's just what I could do using a caliper. The pistons were very difficult to measure with what I have...