Main bearing housing tolerances
 

Hey all, I'm in the process of reassembling my ls vtec turbo build. Tore down initial build running naturally aspirated after about 5k miles due to low oil pressure. Block was bored and line bored/honed by a reputable shop that I won't yet name. I was running a used oil pump that was around the service limits so I got a new replacement. Measuring my main bore housings, I set the bore gauge to the top of the spec that king lists on their site. At the 6 and 12 position I get right at the max spec. At 2 and 8 I see -.0005, at 10 and 4 I see +.0005. Near the parting line I see up to +.003. I wasn't seriously concerned about this the first time around, but the low oil pressure has me wondering. Measuring with the bearings in, my initial clearances showed around .0018 to .0021 which I realize is on the higher side but I'm running slightly heavier oil for heat purposes. Before I buy another set of bearings I want some insight and opinions as to what others would do. Thanks for reading my story haha.

Re: Main bearing housing tolerances
 

Just wanted to add that this is my 6th engine build but the first I've ever had line bored and the first I've had oil pressure problems with. I also work as a machinist at a cylinder head reman shop so machining and tolerances aren't new to me. Thanks in advance for any advice.

Re: Main bearing housing tolerances
 

Bump for any suggestions... I realize that this is out of the realm of most people but I was really hoping to hear from some other experienced machinist/engine builders. The only people I have to ask around me build slow turning V8 's with heaps of clearance. Please someone save me with some intelligent enlightenment.

Re: Main bearing housing tolerances
 

Why was your block line bored ??? As for your measurements, the main bearing clearance is either a "Go/No Go" kind of thing with respect to the OE specs and aftermarket bearings like King, Clevite, ACL, etc... however, obviously the oil pressure will be lower with the clearance on the loose side of the specs and higher as the clearances get tighter. King bearings are a one-size-fits-all kind of product... so replacing them with more King bearings will likely result in the same measurements... maybe within a tenth or two. If you want to set up the main bearing oil clearances as you want, you will need to have the line bore set exactly at the middle of the factory specs and then you can utilize the OE bearings and select the color or colors you want.

Re: Main bearing housing tolerances
 

The block was line bored because it is sleeved and I'm using ARP main studs. I feel ya on the bearing clearances being a go/no go, but a go/no go gauge doesn't cover out of round. I guess my main concern is that all of my main bore housings are around .002- .003" big near the parting line which can't be fixed short of welding and re machining. Would you be concerned about this? I was also thinking of using Honda bearings to bring my tolerances in a bit more. Thanks for your reply!

Re: Main bearing housing tolerances
 

Are you saying that the main journals are larger at the parting line by 2 to 3 thousandths or 2 to 3 tenths ??? If it is thousandths... you cannot fix this. If it is only a couple three tenths... I would consider running top/bottom clearances a tenth or two tighter than your target value is to have an AVERAGE clearance value. This should keep the oil pressures up.

Re: Main bearing housing tolerances
 

I think you got that backwards didn't you?

3 thousandths of an inch is much smaller than 3 tenths of an inch is it not?

Re: Main bearing housing tolerances
 

Under normal circumstances TomCat, you would be correct. However, in machinist speak... the term "tenths" refers to one ten thousandths... or a "tenth" of a thousandth. So a thousandth would be .001" and a tenth would be .0001".

Re: Main bearing housing tolerances
 

Yeah, I know the difference between thousandths and tenths of thousandths. Sadly it's .002 to .0025 big, but only about 15 degrees above and below the parting line. What problems do you see that could arise from this?

Re: Main bearing housing tolerances
 

Awesome thanks for the enlightenment!

I will remember that for the future.

I've always assumed tenths meant tenths of an inch, would never have guessed it was machinist slang for tenths of a thousandth. :thumbup:

Re: Main bearing housing tolerances
 

I didn't mean to suggest that you needed an explanation... my clarification was to help TomCat better understand my answer about the terminology. This is good tech info for him by the way since he is always lurking around here and willing to help others... keeping him informed is good for all members here... both new and old.

As for your situation... that is a dramatic increase in size and my concerns would be that the oil film in that area of the bearing would lose the ability to maintain a consistent hydrostatic wedge. This in turn constitutes the loss of oil pressure across the main journals. Welding up the mains and re-cutting would be the only fix... and even then, if you have to move the top of the main journal more than .002" from the stock location, you run into oil pump longevity issues unless you remove the locating dowels which is a whole other rabbit hole you likely don't want to run deep into. I would say that unless you are emotionally attached to this block... scrap it and find another virgin main journal block to build.

Re: Main bearing housing tolerances
 

The road to Hell is littered with Honda blocks ruined by machinists :cry:

Re: Main bearing housing tolerances
 

True that B and B ;)