Bore/hone and break in for DT75

[Dropshot Fool]

Hey all,

Gotta say thanks to everyone posting and replying on here. I've learned a lot already!

The story goes: I recently bought my first boat that has an '86 DT75 on it. I had it out a few times and something seemed off about it. Low and behold I cooked it since the oil injector was failing to to send oil to the top 2 cylinders.

I'm a CNC machinist by trade so I'm just going to bore the cylinders out, hone them and put the first oversize pistons in. I think I have most of it figured out but could use some reassurance and maybe advice here.

What I think I know:
The service manual says that the bore is to be sized at finish hone to .004-.005" over the diameter of the piston as measured above the skirt. This will give the rings their proper gap (.008-.016"). I can test this by putting the ring in the honed cylinder slightly above the ports and ensuring the gap is correct with a feeler gauge.

My questions are: After I complete the the rebuild, what is the proper way to break it in? Should I mix my oil (not using the oil injection anymore) a little rich? I'm using the next hottest spark plug in it already.
When the rings finish seating themselves to the new bore, do I need to tear it all down again to clean out the filings that will be created from the rings seating? Will these just collect right at the head or get blown out the exhaust port? I was told that I'll need to tear it down to the crankcase again and change the oil out.

That's about all I can think to ask now. Thank ya's!!

[Solarman]

The first 12 hours 25:1 oil ratio, and not over 3000rpms, for the next 4 hours, not over 4000rpms, and from then on not over 5500rpms. If your motor goes over 5500rpms (5650 max), add another 1 inch pitch/ or comparable cup to lower rpms by 100-150rpms, so that max rpms with normal load is back to 5500rpms.

After your final breaking phase, before changing props, change oil mixture back to the normal 50:1 ratio. After running this mixture then you need to check rpms to see if prop needs to be changed.

After about 50-75 hours of run time (after break-in period), check the compression between cylinders (no more than 10% difference between any cylinder allowed).

No, you do not need to tear motor down again to clean anything. Any finite particular matter from break-in will go to exhaust port with the extra oil, then out the prop. (Unless you hear bad noises, and don't trust the sounds)

Good luck. Post back when able.

[mphelle8vld]

Good advice, wanted to add that the oil injection rarely fails but gets a lot of blame. It's more likely that the motor was running lean from dirty carbs or over-heating from a worn water pump impeller.

Not sure if you're familiar with 2strks but make sure to radius the edges of the ports after boring so as not to snag a ring. (give it a google)

[Dropshot Fool]

Solarman,
Thanks for the break in procedure! I plan on oiling the cylinder walls and rings before I insert the new pistons into the cylinders. Should I also turn it over a few times by hand with the plugs out before I fire it for the first time?

Also, I don't have the optional tachometer listed in the service manual and can't seem to find it anywhere (34200-95500). My understanding is that I can use any tachometer as long as it was designed to run with the same number of poles as my stator has? If this is true, would you know how many poles my stator has or a compatible tachometer?

Mphelle,

I read in the service manual to break the edges of the ports, but didn't know exactly why. Makes sense now. Also, I see that the oil injection pump is gear driven directly off the motor and don't think it was the actual failure. The oil lines for the top 2 carbs were empty for the first half an inch at the carb and those are the two cylinders that scored. My guess would be that either the oil filter is plugged or that the check valves failed. I plan on mixing the oil for the duration of the break in and will check the oil flow rate while running it on muffs once the break in is done. In addition to the oil lines being emptied above the carbs, I also noted that timing appears to be advanced beyond what the service manual states. Do you think this could have been set this way to make up for a loss of compression on the top 2 cylinders? Thank you!

[mphelle8vld]

Unless you know the complete service history, it's a good idea to check compression, service water pump, and run pre-mix while you evaluate your new (to you) purchase.

I wouldn't recommend using the hotter plug and the timing needs to set according to the manual. How much was the timing off? It could also be a reason for the torched pistons.

[Dropshot Fool]

Found what failed. Is it worth rebuilding? Pictures attached.

I finished tearing it down today and this is what I discovered: Inside the retainer for the oil injection pump driven gear, a piece of brass broke and galled to the wall and to the driven gear, stopping it's rotation. This in turn wiped out the teeth of the drive gear. In addition or maybe as a cause to the oil pump failure, the lower bearing cage let go and the bearings are freewheeling in the race. The the next higher bearing and the piston bearing at the top don't sound perfectly smooth either, I'm assuming from the lower being gone. Only the second bearing from the top does. I'm not sure if it's even worth rebuilding at this point and have no idea how to split the crankshaft and reassemble correctly? I'm working on uploading pictures. Thanks all.

[mphelle8vld]

What a bummer. How long did you run it until you had problems?

[Dropshot Fool]

One day.

Would you know if the crankshaft pins are splined? I have access to a hydraulic press and can cut a relief in a block of steel to hold it square to press them out and back together, but if they're not splined for alignment I don't want to deal with it. Thanks.

[mphelle8vld]

The pins are not keyed in any way, shops use jigs and black magic to press them together into alignment.

Where are you located?

[Dropshot Fool]

I'm located near Albany, NY......but it's time to just thank you for your assistance and write it off as a glorious anchor. I took another gander at the crankshaft after my last reply and saw that: the lower bearing retaining pin sheared allowing it to spin freely in the journal (bearing cup?) which in turn eroded it. Boring and honing the cylinders for oversize pistons seemed like a good challenge, but I'm having a hard time justifying the cost to rebuild or rework this thing from the block up. I've found almost all of the parts, including a brand new crankshaft and block, but the cost is fast approaching 3k which is about 1/3 the way to just repowering the boat.
Thanks again.

[mphelle8vld]

A real shame for this to happen at the height of the season, if you are a gambling man, you could make an offer on this powerhead.

Motor Only from A Silver Suzuki DT 75 Outboard Motor | eBay

[Dropshot Fool]

I ran a search on all of Craigslist and looked everywhere I could think to across the Internet for all my options. It seems bearing failure is the most common reason these engines die, so.....I think I'll pass on it.

[Solarman]

Sorry to hear what you've found. You're better off looking for a replacement.

Good luck in your search.