Rod Bearing Oil Clearance

[Mr_bob]

I have searched and read a reasonable amount, but I am curious as to the consensus regarding rod bearing oil clearance on the L6. I have encountered various different ranges from the different sources:

 

"rule of thumb" 0.001 per inch diameter

FSM materials (1972 = 0.0006 - 0.0022, 1982 = 0.0009 - 0.0026)

"how to modify" book 0.0023 - 0.0028

an engine builder to whom I have spoken said 0.0015 (regardless of diameter)

 

I was looking to target 0.002, and I currently have 0.0023 - 0.0027. I am concerned that I am potentially too loose. This is for an l28et rebuild with stock rods targeting ~300whp. So, for the L6, what is a recommended rod bearing oil clearance for such an application?

 

*Edited after checking the numbers in the various sources

Edited December 26, 2014 by Mr_bob

[TimZ]

I have searched and read a reasonable amount, but I am curious as to the consensus regarding rod bearing oil clearance on the L6. I have encountered various different ranges from the different sources:

 

"rule of thumb" 0.001 per inch diameter

FSM materials (1972 = 0.0006 - 0.0022, 1982 = 0.0009 - 0.0026)

"how to modify" book 0.0023 - 0.0028

an engine builder to whom I have spoken said 0.0015 (regardless of diameter)

 

I was looking to target 0.002, and I currently have 0.0023 - 0.0027. I am concerned that I am potentially too loose. This is for an l28et rebuild with stock rods targeting ~300whp. So, for the L6, what is a recommended rod bearing oil clearance for such an application?

 

*Edited after checking the numbers in the various sources

Look at this thread (especially the "off topic" bits)

http://forums.hybridz.org/topic/106082-power-drain-from-timing-chain-valuable-build-tips-herein/

[Mr_bob]

TimZ,

 

     Thanks for the suggestion. I did peruse the thread and it is a good read. PMC has some great tips but his section on clearances is from "how to modify" as well (just covering the mains); that definitely counts as a vote toward the larger clearances suggested by that book. I am definitely within that spec but that is on the high side of all the rest of the sources.

[Tony D]

There's an old saying "they run their best before they blow!"

 

Generally if oil clearances are under 0.003" you're good. Anything much closer and there is an issue getting proper oil flow through the journal and rod side clearance. You want oil flowing through there removing heat. That's why factory clearances were as low as 0.0015, and higher, but the performance build recommends higher clearances.

 

In high speed centrifugal compressors you can pick the bearings or pin ions run with close tolerances and mineral oil as they always have more varnish built up on them due to added heat from lack of flow. We use instrumented bearings in some machines, and you can see the difference in as little as 0.0005" difference in setup clearance and corresponding bearing temperatures (both oil throw off and actual bearing backshell temperatures.)

 

The flow from the oil pump has to be up to the task. If you start building excess pressure with that flow, the oil pump will start sucking horsepower as well. You need oil pressure/flow to get oil into that bearing area in sufficient quantity to maintain an oil wedge under rod reversal and peak cylinder pressure.

[Mr_bob]

Tony D,

 

     Thanks for the informative reply. I was tangentially aware of those other issues (side clearance, oil flow/pressure capacity) as well, but not well enough versed to incorporate it into my question; it did lead me to infer that the rod bearings are dealing with different issues than the mains. I was just concerned that the engines being built in "how to modify" are seeing much more severe service than mine (and thus those larger clearances aren't for me) and that I would be better off near the tighter side of the other sources. I definitely believe that 0.0005 can make a difference, and I don't want to be too tight like that either given what the oil needs to do in the rod bearings as you mentioned.

[Tony D]

Do you want to loose horsepower for the next 150-200,000 miles for those bearings to "loosen up" or do you want to give away that mileage for the horsepower immediately and live with an engine that may only have a bottom end capable of going 200-250,000 miles between bearing attention?

 

That's really the only question you need to ask yourself. Engine building is a series of decisions, made and weighed against payback and potential downsides.

Edited December 28, 2014 by Tony D

[Mr_bob]

Haha. Well, when you put it that way it seems pretty clear cut. Thanks for all the assistance.

[Tony D]

Just needed to get the right perspective on it!

 

Forged pistons suck 1qt every 1,000 miles right? So do worn stockers....

But if you got 186psi of compression, and drive 10,000 miles a year, you spend ten quarts of oil worth per year (say 40$ a year...)

 

I love guys that use "excessive oil consumption" to justify and overhaul ($3500 paid, how many miles you gotta drive to get that back?) then complain their new, tight engine is down on power and they just can't figure it out!

 

BMW did testing showing some of their engines were still gaining in HP at 30,000 miles. In other words, those tight clearances and rings wearing in/sealing results in progressively more power and takes a LOT longer than the 1,000 miles most people think!

 

Porsche, on the other hand just said "buck up"--they service with Synthetic from day one, and say "live with the oil consumption" (easy to do with a 21 qt dry sump admittedly...) on the philosophy the gave round bores, and the sealing is accomplished on the motoring Dyno to "sufficient" levels...so the engine may not be perfectly sealed, but will progressively seal better and better the more mileage it accrues.

 

When you really start "thinking" about automotive paradigms and work out the cost-benefit-analysis to YOU and not the OEM Aftermarket Parts Division you go "Ooooh those dirty...."

[Mr_bob]

Haha yeah those turbo porsches especially love their oil consumption; i knew of people who would just do periodic filter replacement and oil analysis as they were effectively adding a whole new sump full of oil just "topping up" between the suggested intervals.

 

I have been pondering and could ask the same question about the mains and ring gaps and all the other engine specs as well as I have been perusing all these sources. So, given the relative longevity of the L6 and lower mileage these might accrue as secondary/fun/weekend/whatever cars, is this a general endorsement of the "how to modify" specs even if I am not building a race motor? I understand it is ultimately my decision and depends on my application, but this is the collective experience I am trying to gather that I just don't see in reading all the numbers.