Saga of the 30 hp rebuild

[Dhadley]

Re: Saga of the 30 hp rebuild

Be very careful about the piston fit. Different 2 stroke pistons have different expansion rates. To just say any 2 stroke piston must be fit "tighter" than a 4 stroke may lead to disaster. <br /><br />The finished bore size must be held to a specific size. As the fuel changes engineers change the shape of pistons. As new materials are tested and proven the make-up of the piston will change. Even new ring material will change things. <br /><br />If the finished bore size is held to a specific size then the manufacturer of the piston will regulate the clearance, depending on material and shape, for their specific piston under a set of given circumstances.

 

[R.Johnson]

Re: Saga of the 30 hp rebuild

As Dhadley said! a dial bore guage is the best way to measure, and is a little more fool-proof for the average person, but keep in mind, it has to be checked against a known standard to make sure it's zeroed. Here's a method you can use if you are measureing with a telescoping guage. Put a piston ring down in the cylinder, and push it down with the piston skirt, this will make it square with the bore, and act as a guide for the guage to keep it from getting cocked. Check the bore at top, center, and bottom. Also turn the guage at 90 degrees to make sure the bore is round. When you lock the guage, see if you can sweep it side to side in the bore. If there is a slight sweep, lets' say 10 degrees, or so, this is normal, and have but a slight affect on the finish reading. There is a formula for this, but I can't recall it at this moment, but it is something like 3, or 4 decimal places. Hope this makes sense, others may have more to add.

 

[Harker]

Re: Saga of the 30 hp rebuild

Let me give a bit more info..the motor originally is a millitary motor with the "pump jet" lower. The number is AML35D. I WAS going to bore the original block and put civilian pistons in it and do away with the "de-watering" setup.This motor ran super rich and gagged out the wife and grandkids. I bought a BRAND NEW bare block that is identical in all appearances.Numbers on the crate are,796165, old 432540, new 433533. To this block I will put brand new pistons and rings (without the flats milled) new lower crank bearing and the original other parts,crank,rods,carb, electrics, etc. I traded a guy a decoy for a new set of Boyeson reeds also. It must sound harder than it actually is to do this..I HAD confidence in my mechanical abilities when I started..now I am wondering if I should just part the damn thing out and get a new motor...

 

[Dhadley]

Re: Saga of the 30 hp rebuild

You'll be just fine. You just want to make sure of the bore size so you dont stick a piston when it first fires up. You just need to be aware that some new blocks are produced a little tight because most folks want to do a final hone right before assembly.

 

[Harker]

Re: Saga of the 30 hp rebuild

Where can I find the seals and o rings for the crank? Seems those items and a center main bearing are scarce as hens teeth.

 

[Harker]

Re: Saga of the 30 hp rebuild

bump

 

[G DANE]

Re: Saga of the 30 hp rebuild

Bearings and stuff are available at any JohnnyRude dealer and Siearra too I think.

 

[Harker]

Re: Saga of the 30 hp rebuild

Went to NAPA and picked up a cylinder hone..is there a special oil I should use? What RPM's should I use the hone at? Thanks....Lee

 

[Harker]

Re: Saga of the 30 hp rebuild

bump

 

[rodbolt]

Re: Saga of the 30 hp rebuild

hello<br /> most blocks are finnished before sale. you just need to check it, sometimes they are tight but not always. <br /> the center mains sometimes are only OMC dealer stuff.<br /> is yours a stamped steel shell or a cracked shell bearing? the two blocks wont interchange as the saddles are different. they look almost identical but the crank, mains and blocks are different.<br /> the big difference is in the center main saddle in the block. make sure the old block and the new block use the same style center main.<br /> dont quit now your almost there <br /> now is where the fun starts <br /> take your pistons,block and service manual to any machine shop and ask them if all is the the specs in the manual. if your using a telescoping or "snap" gauge on a new bore then taking 6 measurements is usless. a new bore wont be egged nor tapered, center the gauge and tighten the knurled end lightly and rock it back. dont rock it twice or it will change. it takes a feel so check it several times. I held plus or minus.001 for years on 8"bores with a snap gauge.<br /> Dial bores are nice but a snap will suffice if enough patience and practice is avalible.

 

[Harker]

Re: Saga of the 30 hp rebuild

Hmmmm....the one that came out of the original had a caged roller with a split sleeve over it. Will I be able to tell by trying it in the new block? I have the crank and parts out in the shop and the new block in the basement. I'll get all the parts in one spot tomorrow and see what I see I guess. Pistons haven't arrived yet...ever use GLM parts? I still want to hone the new cylinders as they have a tad of crud in them from sitting for years..any special oil I should use for honing? I'm tempted to use vegetable oil. I picked up a Starret inside cylinder micrometer with rods and with the 3-4" rod in it, it won't go down into the cylinder( I did the ring thing pushed down with the piston)...but it won't go down the old block either..about the same..this thing is sure an education!

 

[G DANE]

Re: Saga of the 30 hp rebuild

The two types of center main is following:<br /><br />Up to 1976, bearing had a casted two part outher race, held together by a spring ring. 1977 and later had a sheet metal outher, made by two arrow shaped parts. This cranck has larger diameter center journal. Be sure to put them in as manual shows in pictures, no big deal. It IS possible to mesure cylinders by pushing a ring down, and mesuring ring gap, yours will be perfectly round. If there is rust in bores it will have to be honed away first to get a true reading.

 

[Harker]

Re: Saga of the 30 hp rebuild

G.Dane, mine has the "arrow" shaped parts. I was going to measure the ring end gap when the new rings and pistons get here too. What type of oil do you use to hone with?

 

[G DANE]

Re: Saga of the 30 hp rebuild

On the blocks made for the arrow shaped bearing shells, the bore diameter for the bearings are the same, top and center. I always hone with ordinary automotive oil.

 

[Harker]

Re: Saga of the 30 hp rebuild

Honed the cylinders to clean them up a bit. The ports "feel" sharp, but no sharper than before...should I try and round the edges of them a bit? Would polishing the intake and exaust passages be beneficial?