70 hp rod question

[sasko1980]

I have a question about crankshaft connecting rods.

I have a 2 stroke yamaha 70 BETOL (usa 70 TLR)and on this motor connecting rods are pressed in the crankshaft, all others 75,80,90 hp and on have a removable connecting rods.
I'm know that probably you'll say -buy a new crankshaft

My question is : Is it possible to mount a removable connecting rod on this 70hp crankshaft? Will the removable connecting rods from example 75 80 or 90 hp fit the 70 crankshaft?

Why a 70 hp yamaha didnt deserve to have a removable rods?

[TownsendsFJR1300]

No solid answer to your question BUT,

You'd have to find a two piece rod that is the same as your one piece.

A bunch of the smaller engines, Yamaha uses a one piece rod.

Now, yours CAN be rebuilt. There's a place in NC 99Yam and Rodbolt used (uses).

99Yam or RB will chime in with the name of the company.


If your outside of the US, shipping would be $

[99yam40]

the rods are replacable, but you need to find a machine shop that has the proper tools/jigs to press apart the crank and put back together properly.

there are more motors than just that 70 that this the way they are put together.
I had to send off my C40 crank shaft to have it rebuilt due to main bearings going bad.
the shop I used was in NC not Florida, it was the only one I could find

[sasko1980]

No solid answer to your question BUT,

You'd have to find a two piece rod that is the same as your one piece.

I was thinking that one of you came across a two piece rod that could fit. Do you think that a two piece rod from yamaha 75 could have the same dimensions as 70hpor maybe a 90 hp rod???

On my 70 the crank was rebuild and the previous owner had it done. All new 3 main bearings and the top one was replaced , and the new rods all three with their rod bearings, and the new pistons oversize were put in

The problem occured after aprox 10 hours of use, started to hear a clack in the head, and when i opened the block saw a middle cyl scorred, and the rod bearing destroyed.

Maybe a bad rod bearing or pour quality.

As for a scorred cyl , my idea ia to rebore it and put in a oversize one maybe a .030 or .040
i know that all three have to be the same size.

Any ideas or thoughts will help

Thx for your answers

[99yam40]

I do think putting parts not made for your motor in it will not end up well.
sticking with the proper parts is the way to go.
bearings normally go bad due to lack of oil, maybe he did not fix what caused it to fail the 1st time and it just repeated the failure

[TownsendsFJR1300]

You may want to find a shop manual for one of those engines and check the rod spec's and compare to yours.

Something to consider, if that lower crank bearing failed, you very likely have damage to the crank pin, IE, it has to come apart anyway.

Also, between the scoring of the cylinder and bearing failure, you very likely had a lubrication issue in that cylinder which could be from a clogged carb (I don't know if pre-mix or factory oiling system).

[sasko1980]

The 1 st time the upper piston created a problem because autolube system failed. There was a cloged out of oil tank and the oil didnt came to the first cyl. After that the autolube system was bypassed and a premix 50:1 is used after that.

So, thru the reeds the crankshaft had to have lubrification. Yamalube oil is used for premix.

The only thing that i can see from crankshaft is that the first rod has a axial play as in the manual, but other two rods have a greater axial play. Maybe when it was preesed in they didnt checked that.

Could a greater axial play be the reason why the rod bearing was destroyed??

[TownsendsFJR1300]

^^, the crank could have very well sustained some damage the first time.

Now you have a scored cylinder.

Again, if the carb was clogged for that cylinder, no fuel (or less than required), no lube either....

[sasko1980]

I was looking for a rods/rod bearings/pin from yamaha parts but the only i found was a whole crankshaft assy.

On ebay the rods/rod bearings/pins are listed and the go about from 40 to 60 $.
They are china made, so there is a question about quality??

Any advice??

[99yam40]

where are you located?
it cost me $400 to get a rebuilt crank for my C40, but had to have a good core to turn in

[sasko1980]

Another continent, europe, adriatic sea, croatia.

I thought to buy a new crankshaft from yamaha in my country.

Then im left with a scorred cyl. Also im thinking of a new sleeve to be preesed in, or to rebore to fit a 0.040 piston inside. Then i have a upper and lower on 2.854 and middle 2.874.
Potential problem or not ???

[panasonic]

The 1 st time the upper piston created a problem because autolube system failed. There was a cloged out of oil tank and the oil didnt came to the first cyl. After that the autolube system was bypassed and a premix 50:1 is used after that.

So, thru the reeds the crankshaft had to have lubrification. Yamalube oil is used for premix.

The only thing that i can see from crankshaft is that the first rod has a axial play as in the manual, but other two rods have a greater axial play. Maybe when it was preesed in they didnt checked that.

Could a greater axial play be the reason why the rod bearing was destroyed??

So you know the reason for the first failure, auto lube quit. The second failure in my opinion is the crank was not correctly repaired.

I have seen what is involved in pressing a crank apart, reassembling and checking the run out and alignment of the crank shaft. If all this is not done with much precision, we are talking about +/- a few thousands of an inch or less, then the crank WILL shake itself to pieces in short order.

I do machine work all the time and been in shops that do this, very precise work. The more cylinders on the crank, the more difficult it is to repair.

As for putting bolt on rods on that crank, the answer is 99% no. Here's why:

The crank you have, with the one piece rods is balanced with those "particular" rods installed. If you happened to find another two piece bolt on rod that had the same EXACT big end, small end, length dimension's. These would be a different weight. The bolt on rods are usually heavier constructed on the bottom end as there has to be more material there for the bolt holes. Your crank is not balanced for those rods and I highly doubt it could be balanced for them. It was designed for one piece rods.

They use one piece rods as they are lighter and reduces the overall weight of the crank to allow it to spin up quicker. Also some say that one piece rods are stronger.

I would find a reputable shop to have yours fixed or try and find a new power head.

Best of Luck

[99yam40]

being you pre mix and having a scored #2 piston/cylinder and a bearing go bad, I would think the carb was not dumping enough fuel and oil into that center cylinder and it overheated(running lean) along with lack of oil got bearing and cylinder /piston.

makes me wonder if the jets in a pre mix motor are a different size than an oil injected motor due to the specific gravity of the fuel changes when you mix oil into the gas.

[TownsendsFJR1300]

being you pre mix and having a scored #2 piston/cylinder and a bearing go bad, I would think the carb was not dumping enough fuel and oil into that center cylinder and it overheated(running lean) along with lack of oil got bearing and cylinder /piston.

makes me wonder if the jets in a pre mix motor are a different size than an oil injected motor due to the specific gravity of the fuel changes when you mix oil into the gas.

Following that train of thought, he should have had damage in the rest of the engine as well, which didn't happen.

I would strongly suspect(as posted above), that carb got clogged up, leaned out(less fuel AND OIL) and broke.

Panasonic, excellent info re the crank and specifically the rod(s).

[99yam40]

I was not trying to say that the jet sizes were his problem, That thought just came to me for some reason and I posted it