Piston clearance.

[voodoo]

After boring my 89 triple 65, I will end up with .0055" final piston clearance.
Is this a good number?

 

[Dhadley]

Re: Piston clearance.

Where and how did you measure the piston? Many of todays pistons are cam ground, tapered and barrel shaped. Measuring can be pretty confusing.

What was the finished bore size (after final honing and with the block at room temp)?

 

[voodoo]

Re: Piston clearance.

Thanks for the reponse.

I am not doing it.
It is a good friend of mine who does countless numbers of motorcycles, snowmobiles and othet two strokes is.
It has not been bored yet.
We took 3.1875 and added .0305 (.030 oversize + .005 like you said, to come up with a final bore of 3.2180.
He then measured the piston ( I am not sure of where but he was familiar with the terms you use and measured accordingly)
and found that would leave the .0055 final gap.
He told me most of his clearances are between .003 and .008 (air cooled), so that puts me in the middle.
Seloc does have spec of .0055 to .0045.
Should I go smaller?

One more thing, would a nasty 35 mph rock bottom contact slap a piston enough to crack it?

 

[Dhadley]

Re: Piston clearance.

3.2180 is right on. With todays fuel I wouldn't go any smaller. Remember, the load factors on an outboard piston are completely different than on a snowmobile, motorcycle, ATV etc.

Where did the piston crack? I'm not sure that an abrupt stop would cause it all by itself but it certainly couldn't be good for it.

I think you're right on with your machining. Now, what's the top rpm of the motor? Do you spend a lot of time in the cruising range? Make sure to use only Champion plugs and 87 octane.

 

[voodoo]

Re: Piston clearance.

Piston cracked from just above the wrist pin to below the bottom of the skirt.
The motor didn't stop, nor did the boat hit the rock.
It just sent the lower unit skyward and blew off the skeg.
I got to keep going and not die from embarassment.
Always champion, always mixed synthetic oil 50:1, 87 octane stabilized if i am not going to use it.
With a full load I run at 5100 rpm, with a light load I run at 5800.

I try not to run at below 5000.

 

[Dhadley]

Re: Piston clearance.

Sounds like it's set up OK. Let's see what it does after it's broke in and ready. Wouldn't hurt to get the r's up a little. That loaded rpm is a bit low but I know there's not a lot we can do about load.

 

[voodoo]

Re: Piston clearance.

Unfortunately it is usually loaded, and it is only going to get worse.
I am running a 13 1/4 X 17 now.
Would I be ok going to a 15 and feathering back at 6000 if the load is light?
The boat isn't exactly a speed demon anyway, best I have seen is about 35 mph in perfect conditions.
I can pick up a rebuilt prop when I pick up my lower unit from the welding shop.:/

 

[Dhadley]

Re: Piston clearance.

Let's wait on another prop. Let's see what happens when the fresh powerhead is broke in.