6M crank?? I think I have a problem!

[veedubin]

It's now 9p EST and i have spent all day doing the rod bearings in my car. Cut to end then fill in..... It is now knocking as hard as a machine gun. so much for all that work. After removing the oil pan, I was greeted by a brand new oil pump.. thank you PO, and a crank that reads "6M" on the 4th? counter wieght so am I to assume this motor has a 6M crank??? anyways the bearings I ordered were for a true 7M motor. Fedral Mogul P/N:
6 - 1165CP Std size bearings. The bearings i took out were also stamepd with 1165CP Now my thinking is the same bearing.. right? well point is what the hell did i do wrong???? oh, while i was in there, i made a point of shimming the oil pump. so yea, nice oil pressure. At least at first start up.... after idling for about 5-7min till gauge got normal op temp.... the pressure went from 40psi to 20psi. shut the motor off, then started it back up after about 5 minutes.... then the knocking began..... wtf, i'm not even pissed about it just depressed. so any clue what i did wrong?? thanks in advance

 

[spoolint78]

did you check the clearances when installing?
and did you ytou take the crank to a machine shop to check it out and turn it if needed?

 

[Nomad707]

LOL FUCKING FUNNY AVATAR!!!!!!!!!!!!!!

 

[SLO-MK3]

Someone correct me if I'm wrong, but I think pre '89 7m engines had a 6m crank.

 

[2543arvin]

It wasnt just pre-89, mines an 89 and when I took it apart it had a 6m crank in it. They changed from the 6m cranks in the middle of 89, so some have 7m and some have 6m cranks.

 

[Piratetip]

My 87' has a 6M

 

[black91turbo]

I just completed my motor from an 87.....it had a 6m crank. My buddys was a 7M and we used the same exact bearings.... no problems here. You probably should have checked clearances like mentioned above^^^^

 

[suprarich]

It's now 9p EST and i have spent all day doing the rod bearings in my car. Cut to end then fill in..... It is now knocking as hard as a machine gun. so much for all that work. After removing the oil pan, I was greeted by a brand new oil pump.. thank you PO, and a crank that reads "6M" on the 4th? counter wieght so am I to assume this motor has a 6M crank??? anyways the bearings I ordered were for a true 7M motor. Fedral Mogul P/N:
6 - 1165CP Std size bearings. The bearings i took out were also stamepd with 1165CP Now my thinking is the same bearing.. right? well point is what the hell did i do wrong???? oh, while i was in there, i made a point of shimming the oil pump. so yea, nice oil pressure. At least at first start up.... after idling for about 5-7min till gauge got normal op temp.... the pressure went from 40psi to 20psi. shut the motor off, then started it back up after about 5 minutes.... then the knocking began..... wtf, i'm not even pissed about it just depressed. so any clue what i did wrong?? thanks in advance

You can not just swap out old bearings and install new bearings. The crank must be fitted to each bearing for proper oil clearence. Which means the motor and crank must be out of the car. Your stock motor will have up to 3 different size bearings, the new bearings you purchased will be all one size.
Anytime you have rod knock, the crank will have to be re-sized.

 

[MDCmotorsports]

LOL FUCKING FUNNY AVATAR!!!!!!!!!!!!!!

You're not helping here newb.

As for the crank problem...

The bearings will work for 7m/6m cranks.

Theres nothing wrong with having the 6m crank in the 7m engine.

A.) You needed to measure the clearences with a plastigauge.

B.) You needed to have the crank returned, sized, and polished

C.) You needed to let the car run ABOVE 800 rpms for the first 30 minutes of its new life. Any thing less than that and you run the risk of spinnning a bearing.

 

[Nick M]

It is now knocking as hard as a machine gun. so much for all that work. After removing the oil pan....so any clue what i did wrong?? thanks in advance

Don't take this the wrong way. What do you do for a living?

 

[ATL88Supra]

ouch thats just sad

 

[IJ.]

Sad but very common these motors even though old are a very precise assembly and need to utmost care to ensure all specs are correct.

What you could get away with on a SBC will turn a 7M into a pile of damaged parts within seconds.

 

[supra_ed]

Sad but very common these motors even though old are a very precise assembly and need to utmost care to ensure all specs are correct.

What you could get away with on a SBC will turn a 7M into a pile of damaged parts within seconds.

amen to that.
u have to let 7m idle above 800 for 1st 30 min after rebuild?? ive always let it idle till it gets to about 100deg, let it cool/ then let it idle till operating temp, let it cool/ then drive it easy for at least 1st 500 miles. never payed attention to the "above 800" thing.

 

[veedubin]

To the one who asked what i do for a living... I'm a diagnostic auto technician.... by no means an engine builder.

I did the bearings taking into consideration it is a long shot.... If it worked it worked If not it gives me an excuse to tell my wife i need to do a full rebuild :biglaugh: , that type of thing. I know my procedure was not up to par for most of you guys. Hell i'll be the first to admit that. But anyways not a concern right now. I'm looking at buying a new block already rebuilt, or sending mine to a machine shop for a full bottom end job. so yea that's were i'm at now.

I do want to thank you guys for all the replies.

Thanks

I'm pulling the motor right now to see the damage, i'll report with my findings... i think it spun a bearing or two.... oh well