Saga of the 30 hp rebuild

[Harker]

Well..I have been procrastinating about sending my block out to be bored for new pistons. Meanwhile, I just picked up a brand spankin new bare block with intake manifold for little above what machining would cost. Is there any prep that is needed on a new block before I put it together? There is a light surface rust(dust) in the cylinders that will come off with an oily rag...does it need honed? Thanks...Lee

 

[Dhadley]

Re: Saga of the 30 hp rebuild

Does it measure within specs? Sometimes a brand new block is a little bit tight and a finish hone is called for.

 

[Harker]

Re: Saga of the 30 hp rebuild

Don't know, I'll have to find the stuff to measure it with..wonder if the NAPA store would be able to measure it?

 

[OBJ]

Re: Saga of the 30 hp rebuild

A good machinist could measure it for you Harker.

 

[jimalsk]

Re: Saga of the 30 hp rebuild

Harker take to a good machine shop with the specs and I'm sure they will do the mike up for you. Make sure the little bit of rust hasn't pitted the cylinder walls.

 

[Harker]

Re: Saga of the 30 hp rebuild

The manual says 3"..then has a basic range of 2.? and 3.?..Can an inexpensive cylinder hone be used..what type of grit? The cylinders have the cross hatching marks in them. How sharp should the ports feel? The old pistons( without rings) slip in the bores with the same clearance as the old block (maybe just a tad snugger).

 

[Mark42]

Re: Saga of the 30 hp rebuild

The hash marks on the bore seem to indicate that honing has been done. I would call the supplier of the block and ask them exactly what has been done to the block, what needs to be done, then take it from there.

 

[Dhadley]

Re: Saga of the 30 hp rebuild

Have it measured with a dial bore gauge at a marine machine shop. Thats the proper way.

 

[Harker]

Re: Saga of the 30 hp rebuild

The marinas around here are very small and send the machine work out of the area..they also wanted to chrge me a 1/2 hour labor to measure...I offerd 15 bucks but no go..for about 30 bucks (I hope) I can get an inside cylinder micrometer off ebay....I also have to get a torque wrench. I own a lumberyard and get a kick out of DIY'ers so I know you real wrenches are laughing at me for being a DIY mechanic..like I always say..fix it till it's broke.

 

[Harker]

Re: Saga of the 30 hp rebuild

What are the best pistons? Sierra or Wiseco?

 

[G DANE]

Re: Saga of the 30 hp rebuild

Well I'm sure noone here is laughing at anyone. Most of us are DIY's ourselves, and have learned thru the years. If your block really is new, you can tell from the outher of it that it was never assembled. A slight layer of surface rust in bores ( and that is slight ) can be removed with one of those cheap automotive honetools from wall-mart, doing a slight hone. Wont hurt anything. If this block is new it will be bored for standard OMC pistone.

 

[Dhadley]

Re: Saga of the 30 hp rebuild

Again, a new block is sometimes tighter than what you'd like. No matter what brand piston you use, no matter if its new / used / standard / bored, the first thing you have to know is the finished bore size. We're talking about down to 1/2 of a thousand of an inch. <br /><br />You've not mentioned the year of the motor but you did mention a 3" bore. If thats correct you want the finished bore size to be no less than 3.0005 but certainly no more than 3.001. <br /><br />If its too tight, the piston will scuff and then youll be looking at the piston manufacturer for help. At that point you'll need something in writing from the machine shop as to the finished bore size.

 

[Harker]

Re: Saga of the 30 hp rebuild

Dhadley,It's a military Johnson 2 cylinder AML35D. The numbers place says 96 35hp..the place I bought it from said 92 30hp. It had the dewatering device and I am rebuilding (attempting) to civilian specs. I have two Johnson factory manuals, one is for 89-92 and the other is for 95. The earlier manuals pictures of the engines matches more closely to my motor..carb etc..The new bare block is new,unpainted and no gasket marks anywhere..it is a gold anodized color. I ordered a set of inside bore measurement tools that look like T's, I have a regular micrometer so when the tools get here I can get it measured. Thanks for all the help..I read this forum daily and have learned a TON from you guys..and have amazed friends by using info gleaned from this page to help them with their motor problems. Thanks again..Lee

 

[Dhadley]

Re: Saga of the 30 hp rebuild

Ah, good info!<br /><br />Some of those military motors used a piston made of different material and had a different expansion rate. That one may not but the point is to make real sure you measure the bore. It may indeed be tight. <br /><br />Be careful with those snap gauges. Its easy to mismeasure when transfering from the cylinder to the mic.<br /><br />Good luck!

 

[Harker]

Re: Saga of the 30 hp rebuild

Gathering up stuff to get started..I know to get new rodbolts..what about crankbolts? can those and other bolts be re-used? Just bought two torque wrenches and have an inside bore micrometer on the way. I have to order some gelseal,pistons w/rings and pins and lower crank bearing..anything I am forgetting? Thanks.

 

[BillP]

Re: Saga of the 30 hp rebuild

DeHadley,<br />Are the military engines the same as commercial ones? If so, the 30 is probably a decompressed 35. The heads have spacers to lower compression so they can run down to 67 octane.

 

[Dhadley]

Re: Saga of the 30 hp rebuild

I'm not sure if they are exactly the same or not but I dont think so. They may be very close though. Some of the heads on the larger engines have combustion chambers that look like the inside of Hoss Cartwright's hat.

 

[CFronzek]

Re: Saga of the 30 hp rebuild

Harker,<br />Proper bore measurement needs to be compared with proper piston skirt measurement. There must be sufficient clearence or the pistons will seize. Two-stroke motors require tighter piston fits than four-stroke motors but will run if the fit is a little loose. <br />Brand new cylinder bores should be round. Pistons, whether new or used, are slightly oval shaped and can be tapered from top to bottom. Measuring a piston properly requires knowing where to put the mic. <br />Knowing that everything fits properly sure reduces the sweat factor the first time you start the motor. <br />Charlie

 

[rodbolt]

Re: Saga of the 30 hp rebuild

hello<br /> be aware that most military motors with the water drain have 1/2" slots milled down the pistons to relive water from the wristpin area. these pistons are custom made and not commercially availible and should not be used on a pleasure motor cause it tends to shorten piston life.

 

[umblecumbuz]

Re: Saga of the 30 hp rebuild

Load of good advice in thesee posts.<br /><br />I love what What Charlie has just said - it puts a different perspective on things:<br /><br />

Two-stroke motors require tighter piston fits than four-stroke motors but will run if the fit is a little loose.

You're looking at thirty year old technology there. Motors were built with less precision than today. We used to use dial gauges and micrometers then, just like Rodbolt correctly advises - but we also assembled small two-strokes with feeler gauges and vernier calipers! <br /><br />And we used to hone bores using an old ringless piston on a con-rod, wrapped in wet-n-dry, working up the bore with a twisting motion. They always went well, but much of the assembly and finishing work was down to experience.<br /><br />Maybe among your yard customers there's an old-timer who was a marine or auto mechanic. If so, sweet-talk him and you might have found yourself a diamond.<br /><br />I know this isn't really helpful, but it just shows that these small motors were really tough, and you can sometimes get away with a lot.<br /><br />Ciao