Grind & polish the bearing half's or the crankshaft?

[Harbour View]

Connecting Rod bearing half failure.
In replacement, it seems that the connecting rod is too stiff when fitted to crank shaft. Do I need to Grind and polish the bearing half's or the crankshaft?

(This is a MYCOM RECO N6B Reciproc Compressor)

[tbirdtbird]

Bearing cap half failure and the crank journal is OK??
Brass shims if you must

[Harbour View]

Yes, the crank journal seems okay.

[Magoo]

Micrometer check each surface, then assemble and crush test clearances with out rotating anything. next blue each surface and check for high spots after one or two crank revolutions.
You are convinced that the problem is lower end.
Are you sure that you are fitting standard shells, or undersized shells or oversized shell bearings. Check part numbers per catalogue measurements.

[Harbour View]

I am fitting undersized bearing halves. What is the ideal clearance between the bearing halves and the crank journal?

[Magoo]

Check the manual from compressor manufacturer, it will clearly state optimum clearances for under sized shells.

[Harbour View]

The manual only gives the clearance when the bearing halves should be replaced. No reference is made to a optimum clearance.

[Magoo]

Well there you go, the tolerance max and min are no different if shells are undersized/ oversized/ or standard.

[Harbour View]

The manual says "connecting rod bearing halves should be replaced if the clearance between the bearings and crankshaft exceeds 0.30mm".
I'm replacing the old connecting rod bearing halves because they appear to be fatigued and have flakes coming off the surface.
I ordered undersized bearing halves and when fitted there is no lateral play. The connecting rods are very stiff and rigid.
Should I grind and polish the bearing halves or the crankshaft?
I fear grinding the crankshaft as it has been done before, I don't want to be in a position where I have to replace the crankshaft or to shorten the life of the crankshaft.
So, if I grind and polish the bearing halves (connecting rod) to achieve the necessary lateral play of the rods, what should be the amount to be taken off the bearing halves when grinding them?

[lowcool]

just curious.whats the clearance with the new standard bearings

[Harbour View]

There seems to be no clearance since the connecting rod is so stiff when fitted to the crankshaft and there is no lateral play.
So I need to grind the bearing halves, but I'm not sure how much.
But if I know that standard optimum clearance between the rod and the crank journal, I guess thats how much I'll grind.

[lowcool]

if grinding the smaller bearing halves to stop nipping up would this not be making small halves oversize.what happens with standard ones?i would expect the bearings to wear before the crank does.

[tbirdtbird]

If lateral play is the issue why not use a feeler gauge to determine what the lateral play is with the connecting rod against the crank? And use that as a guide for what that clearance should be for the rod cap. I think there is confusion over whether you are referring to side-to-side thrust play or rotational play meaning the space between the rod and the crank journal where the oil film is.

[charlie n]

You probably have the wrong bearing halves. Check your email. I sent you an old set of measurements and wear limits.
I would never grind bearing halves. The Crankshaft can usually be ground once to the manufacturer's undersize recommendation. If you get the crank ground, be very careful that the regrind shop keeps the radius cuts where the journal meets the counterweights. I've seen cranks destroyed by poor regrind jobs where a sharp square cut increases stress concentration which eventually snaps the crankshaft.

[Harbour View]

I have found that the Con Rod is oval. Will I have to grind the rod to remedy this?
Also, please tell me everything I should watch out for when grinding the Crankshaft.

[cadwaladr]

I have found that the Con Rod is oval. Will I have to grind the rod to remedy this?
Also, please tell me everything I should watch out for when grinding the Crankshaft.

change the con rod or rods if it has more,does the crank measure ok.

[Magoo]

If you have flacking indications then it is either too tight a clearance or lack oil lubrication.
Oval conrod shells. Where are your parts coming from, after market or genuine parts. After market parts are cheap/ not when they are sub standard quality and create the situation you now have.
I would bet that the after market parts do not have a warranty. Good luck.

[Harbour View]

I am replacing the connecting rod bearings with OEM parts. I have also found that both my Connecting rod and the crank shaft is oval shape. Is it that my only remedy lies in grinding the crankshaft and the connecting Rods? If so, please tell me all necessary steps I need to take in grinding the Connecting rods and the crankshaft. Please help me.

[charlie n]

I've never hear of anyone grinding Con Rods. They probably have to be replaced. The crank shaft must be ground by a professional regrinder. You can probably find a machine shop which rebuilds automobile engines to do this. The journals have to be ground to Mycom's specification for the under size bearings.

[RANGER1]

Harbour view
Never machine big end shells or shim .

Flacky big end shells or small pieces of whitemetal loosing bond I have found when
there is a long time between sevice intervals .

[ZeroTolerance]

Always grind the crankshaft, never the bearings, but don't do it unless you have the right equipment or take it to a machine shop. Just like an engine. Bearings and journals never get ground. Only the crank

[rickyccong]

Crankshaft must be grounded if oval-shaped by professional Engineering company, After grounded the crankshaft apply bearing blue to top and bottom halves of the white metal bearing and tighten up and rotate the crankshaft. Check for bearing blue high spot area and scrap off with bearing scraper, as such require to repeat until the bearing blue cover all most the whole white metal surface.
Place a lead wire on the bearing surface and tighten both bearing halves and remove the bearing and measure the lead wire thickness with micrometer than you get the correct clearance between the bearing and the journal.