Premature crank failure?

[Watty]

Hi guys,

I recently had a piston in my 701 go on me from running the engine too lean.

I had the cylinders bored to 82mm with Pro-x pistons and ported at the same time.

After putting the cylinders on, I tested the movement by turning the coupler by hand, and I noticed when each piston is at tdc and also at it's lowest point, I get a little pop kind of noise. I can get the noise to stop by putting some pressure on the tops of the pistons, so I thought it must be the little end bearings.

So, I order the bearing, install them and put the cylinders back on but the problem is that the noise is still there? I was under the timpression that there should be no noise at all when turning the motor over without the head on, apart from the obvious rings against the bore.

This gets interesting as I have a pretty much brand new Hotrods crank in the engine, which has done somewhere between 15-20 hours. I suspect that the big end bearings are the culprits, and presume that the engine is not safe to run in this state, can anyone confirm?

Where should I be looking to find out where the problems may have started? I'm not 100% on the big end bearings being the cause of the noise, but knowing my luck it will be.

Any help/advice would be great, as I don't, and probably shouldn't run the engine that way it is.

Thanks,

Leigh

 

[sflsurfrider]

reeds possibly?

 

[Big Kahuna]

Charlie, what kind of response is that?


hey Wattage, PM Jr Magoo, explain this to him, he uses Hot Rodz Cranks.

 

[sflsurfrider]

Charlie, what kind of response is that?

sometimes the reeds will make a popping noise when rotating the engine by hand. just a thought.

 

[freestylegeek]

I may be going out on a limb here, so bear with me.

The sound 'may' be coming from your con-rod centering itself on the piston pin. When it reaches TDC, pretty much all the vertical forces are gone (including the weight of the piston, since the rings can hold it in place). When everything eaqualizes, if there is the slightest misalignment pressure on the rod, it will slide over.

I was going to suggest that the rings are just popping back and forth from the bottom edge to the top edge of their groove, but that still happens if you push on the piston. (That's what my motor did the last time I rebuilt it, and it scared the crap out of me - until I realized what was happening).

Anyway, it's just a theory.

Paul might be able to tell you what's going on.

 

[Watty]

No worries guys, thanks for the info.

I actually thought the reeds may be making th noise as well, as I just fitted a Riva manifold with V-Force II reeds.

I'll have to try and get a video up of it. I'm assuming the worst at the moment, because thats just my luck!

I'll see if Paul can comment on this thread.

Thanks again!

Leigh

 

[Jr.]

There seems to be many posibilities with this........... Lets start at the top.
Crank. It's entirely possible even a new crank is bad, very rare but possible. the crank needs to be removed from the cases & fully inspected.
normaly the big end bearings fail first. check the crank with a dial indicator & see what runout you are dealing with. also with feeler gauges, check the side to side play in the rods, also compare the top/bottom rod readings. Like what geek said, the rod is self centering, but if it's a extreme movement, a failure is immenent. look thu the rod oil port & inspect the inner needle bearings & cages, look for discoloration of the bearing & thrust washers. if you see heat marks, change the crank.

look at the piston to wall clearance. no more than .0055"

The reeds will make a little noise like Charlie suggested. but you should be able to distinguish that from crank noise.
remove the rings from the pistons & re assmble, (leave the head off)
remove the reed block so you can see the crank thu the intakes.
rotate & see what is happening & try to locate your noise.
9-10 times, there is some small point in the assembly process that is your issue. you are doing the right thing by finding the problem & fixing , rather than just running it.

let me knw how you make out

ski ya, paul

 

[Watty]

Thanks for the reply Paul.

I just pulled the intake off and the noise is definately coming from the bottom end. When I rotate the crank, and press sideways on the big end bearings, the noise changes somewhat but doesn't stop as I can't get as much pressure on the bearings as I can when pushing on the pistons.

Either way, the crank is coming out tonight for inspection. I'm going to start this build from scratch I think from the bottom up.

Thanks,

Leigh

 

[waxhead]

did you have a detonation with this motor before
because the pushing back onthe pistons will tend to make the bearings work harder than normal

 

[Watty]

Jared,

Yep, this is the engine that I took the corner off the piston. No deto before that though as far as I'm aware as I pulled the head regularly to check. I did find out earlier this week when I took the 38's off that they aren't a matched set! No wonder I was having troubles tuning the thing!