Piston slap…best way to repair

[tcoop]

As in what is the best way to repair. On my 78 the #3 piston melted on me and put a hole in the case. So I found and engine and almost all the parts from a 79 special and put the engine in the 78. (The 79 frame was cut up to the point that repairing it was beyond my skill level). Go the engine in. Rebuilt the carbs using the best parts from both racks (The bodies from the 79 were better). Put the carbs in, resolved the kinked fuel lines charged the week battery (It need to be replaced) and she fired up!! A nasty banging sound was coming from the #1 cylinder, checked for an exhaust leak but found none. All four cylinders were firing. Other than the piston slap it seems to be a strong running engine.

What is the best process to replace the bearings? Do I need to pull the engine and split the case or pull the head and jugs to come at it from the top?

[crazy steve]

...pull the engine and split the case...

If it's a loud banging, you may have a bad rod bearing. You'll need to check the rod big-end for roundness, as well as the crankshaft. Plus pull the head/cylinders. Not good....

[tcoop]

Looks like I'll need to get a gasket kit and work on this bike in the spring (Currently working on my daughters LG). Good thing I have other bikes to ride while this one is down.

[fredintoon]

Hi Ty,
sounds like you are deep in the soft and brown.
Alas, new rod bearings, pistons and rings are eff nigh unobtainable.
With luck a complete teardown of both motors should yield up enough good parts to
Frankenstein a runner which will be a load of work but at least it's doable.
Or search for yet another motor?

[ViperRon]

Unfortunately Steve is probably correct but I want to ask again because of the terminology you are using. Is it piston slap which is an issue with the skirt of the piston slapping the cylinder wall? Is it Rod knock a deeper motor sound mainly heard worse with a change in load on the engine?

I ask because there is a big difference in what problems you may expect to encounter. You may want to pull the head and cylinder off first and look to see if you have a cracked or damaged piston broken rings etc. You can check the side of the piston for wear and check down in the cylinder. Try rocking the piston and see if there is a difference in the slack you feel between that piston and the others at the wrist pin.

The difference is that if it is only piston slap that the rod may not be damaged and you may not have a bad Crankshaft. With the work involved splitting the case you may want to check the top side first and see what you are dealing with.

With the cost and time involved In engine rebuild its best to do a complete assement before you decide to fix the engine as you can often buy complete engines on EBAY much cheaper than the sum of their parts. The other issue is you will have useable parts from your current engine to sell on ebay to make up for the cost.

[tcoop]

I had planned to use parts from my old motor to repair this one. My wife nix the idea (at least for now) of getting another motor (funds are tight and with the age of these motors she is afraid of the same thing happening)

It is possible that I have miss diagnosed the problem. There is a definite knock that sound like it is coming from the head, louder that valve taping noise. It is less noticeable with higher rpms. I did adjust the valve shims before trying to start the bike (exhaust were all in spec and intake had to change all the shims). I have not ridden the bike since changing the engine so I don’t know if it is worse under load.

[fredintoon]

- - - It is possible that I have miss diagnosed the problem. There is a definite knock that sound like it is coming from the head, louder that valve taping noise. It is less noticeable with higher rpms. - - - I have not ridden the bike since changing the engine so I don’t know if it is worse under load.

Hi Ty,
one time I tried a test run to see if the knocking noise would go away.
Turned a relatively easy rod end bearing shell replacement into a 100 thousandths undersize crank regrind and getting the rod end direct filled with babbit metal as the fattest available shell was - .030".
And where do you get babbit poured when it ain't 1955 no more?

[crazy steve]

...And where do you get babbit poured when it ain't 1955 no more?

I wonder how many here have even heard of poured babbit bearings in a motor....

[DAVINCI]

Yes, Steve, some of us ARE that old.

[cywelchjr]

I very faintly remember back helping my dad. That said, if the sound is coming from the top end, that may be either piston slap, which may quiet down as the engine warms up, or wrist pin knock, both of which are IMHO far better than rod knock, as it should be easier to frankenstien good parts from the other engine to fix that. In my experience rod knock on these engines clearly is in the lower end, not in the top end, top end is much less likely to be rod knock.

[petejw]

usually u can hear a bearing knock thru
the crankshaft on the timing cover side,
when u replaced the pistons, were they numbered
did u put them back into there original cylinders,
if they were mixed up that could cause top end problems.

[natemoen]

I wonder how many here have even heard of poured babbit bearings in a motor....

I understand it!

[BA80]

And where do you get babbit poured when it ain't 1955 no more?

http://www.milwaukeebearing.com/bearings.asp

[natemoen]

http://www.milwaukeebearing.com/bearings.asp

I live pretty close to that place.

[cywelchjr]

http://www.milwaukeebearing.com/bearings.asp

I wonder if we could get any of the unobtainium bearings for our engines made by these guys at anything resembling a reasonable price? If they use CNC machining processes for bearing creation largest cost could be setup for the first run. Imagine being able to get brand NEW crank bearings. I know that we found a source for rings that requires only having one of the ring grooves cut slightly deeper, this would make things so much better, because this is one of the things that are so hard to replace.