How do you tell if a 1970 35 HP Chrysler?s rod bearings are OK

[wickware]

How do you tell if a 1970 35 HP Chrysler?s rod bearings are OK (times slack/play that would cause a low knock)? I fired up today prepping for some fishing and heard a low knock when it coughed. I was not use to the cough as much associated with the low knock in this cool North Tx weather. Is there a rotation degree test to check for wear? Naturally above idle and the cough, I d/n hear the low knock, in my opinion. Would a stretched fuel pump diaphragm cause both matters to be worse? Thanks For Any Info! jow

 

[Frank Acampora]

Re: How do you tell if a 1970 35 HP Chrysler?s rod bearings are OK

Consider the rod bearings to be roller bearings with the crankpin as one race and the rod as the other race. all parts are fully hardened. Thus as with regular roller bearings, they don't wear--they fail. Any wear at all due to inadequate lubrication will quickly escalate into failure. I have ruined a crank when the rollers did wear and fail. It happened fast.

Now, as with all mass produced parts, there is a tolerance on all parts, and if the tolerances gang-up, then there is the possibility of loose fits. At speed, the load on the rods will be in one direction only and you will not hear anything. In a backfire or spit, you may hear a knock. However, the most common symptom of loose rod bearings is a "squeal" under heavy acceleration as the rollers slide along the crankpin rather than roll. I have not personally experienced this in any engines I have had.

Normally, there will be a couple of degrees on either side of top dead center where the piston will not move because of normal tolerances. That is why the engines are set at a number--I forget-- like .025 inches at 36 degrees with a special tool to find real top dead center.

Thus, if you are seeing way more that a couple of degrees, then you might want to look at either the small end wrist pin, rod, and needles or the big end rods, rollers, and crankpin. You would need a micrometer to measure these though.

 

[wickware]

Re: How do you tell if a 1970 35 HP Chrysler?s rod bearings are OK

Thanks Frank! I should have a dial indicator in my STUFF. Would it measure the movement as well as a mic or caliper with a dial I know I have? I know the feel due to just discover the feel and play in my mid 80s chainsaw at the crank while at TDC just below thw carb. I retired it for parts vs letting it throw the rod.

 

[pnwboat]

Re: How do you tell if a 1970 35 HP Chrysler?s rod bearings are OK

The marine outboard machine shop that I take my blocks to have some old timers that seem to be pretty knowledgeable. They work on all brands of outboards. They've advised me in the past to also look at the color of the crankshaft journals and the roller bearings (rod and crank). If the steel has any "blue" color to them, then they've most likely been overheated. May be due to lack of lubrication or excessive heat. Even if the parts measure to factory specs, they recommended replacing. If the parts have been heated enough to turn them "blue", then the integrity of the metal has been compromised. Don't know how accurate this is but I've followed their advice and never had a problem.