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-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

bustert 19th December 2019 15:16

super duper +1 mr. hippy
don't you just hate it when the factory books hide info, vw was known for that one, cost me a cylinder head. having info is one thing but having correct info is, well, can't put a cost to it.
a man who can ? himself will rise to the occasion!

Hippysmack 9th September 2020 04:26

I am wondering just how much the air space in the oil tank plays into the overall pressure.
I need to put that on my list to test for crankcase pressure differences in oil levels.

edit: (got my years wrong)
Also, I need to compare a 1957-1976 Sportster primary pressure to that of a 1979 Super Glide / Electra Glide.
More on that coming.

Iron Mike 9th September 2020 04:58

My guess is that you won’t find much pressure in the 79 xl primary as it is vented to the atmosphere through the vent fitting on top of the trans.
As for the bt I would presume you would see pulses from the front chain oiling system controlled by the breather.
I would think they would be minute given the volume of space in the bt primary cavity.
Providing there is a stock oiling system.

Iron Mike 9th September 2020 05:02

I do vaguely recall doing a primary vac leak test, while bt is running. To verify primary was sealed. But if I recall it involved pinching vent lines during the test.
If I remember, used a bolt with center drilled out screwed into inspection or derby cover hole for vac port.

Iron Mike 9th September 2020 05:24

Hippy Always look forward to reading about your test results. Test remove all speculation without the bs.

Hippysmack 9th September 2020 06:22

Heh, I screwed up the years on that post but I fixed that.
Mike, your memory is pretty good.
I'm after 76< numbers at the moment but I can't actually do any testing since I don't have one.
But I'm bringing back some of the procedures I've found in the tech papers.
And some speculation.

Hippysmack 9th September 2020 07:16

Comparing A: 57-76 Sportster to B: 1979 FL/FX in regards to primary compartment press
If I misspoke some of this, please correct me.

Even in the two different engine designs, there seems to be similarities regarding primary pressure.

1: 57-76 Sportsters have a timing gear for crankcase pressure removal and a transfer valve in the primary wall.

2: When the Sportster transfer valve is done blowing excess oil into the crankcase, the only thing left for the transfer valve to transfer is primary air.
So the primary is subject to a 'slight variable vacuum' just as the crankcase is, only less due to sprocket windage.
The primary is also creating positive pressure while at the same time that pressure being changed between positive and negative.
( due to the opening and closing of the transfer valve)
So there is a pressure balance to be maintained in the primary. The clutch cable offers a small hole of atmosphere intake.
Some had a hole in the primary filler cap and some did not I believe.
So some had a closed primary (no primary vent) with the exception of the clutch cable.
It may have a slight mean positive charge.

3: Per Tech Tip #36 Click here to download the PDF

In October of 1993, a tech tip was issued for XL models regarding OIL CARRYOVER (oil usage) .
One of the running changes made was the installation procedure for the sprocket seal.
Beginning on April 1st, 1993, Harley-Davidson began to reverse the direction in which the sprocket shaft seal is installed.
The open side of the seal (exposing the spring) now faces towards the engine sprocket and primary/drive compartment.
This will promote better crankcase sealing, while reducing the potential for transfer of fluids and crankcase pressure.
The seal spacer inside diameter was also reduced to .995"-1.000" for a tighter fit on the sprocket shaft.
The spacer and seal part numbers did not change.
Consequently, a procedure was given to test the primary compartment for vacuum.
Check with crankcase gauge, part number: 96950-68 (manometer) or attach a clear hose to transmission vent line and drop in a glass of water.
If one inch or more of vacuum is present, check the sprocket shaft seal change information.

1: 79 FL/FX has a timing gear and NO transfer valve.
The primary gets oiled by positive pressure on downstroke to splash oil from the crankcase into a passage in the primary.
(front chain is lubed by oil blown into the chain case when the breather valve opens)

2: The FX primary is also creating positive pressure.
And at the same time, it's being pushed and pulled by crankcase pressure via the breather valve.
It has a dedicated primary vent to atmosphere so it has an open primary.
On upstroke, a certain amount of vacuum will be present until it equals out from the vent intake and whatever the primary is generating in reverse.
A slight variable vacuum to positive but it shouldn't last as long as the Sportster primary would due to the venting.
It may have a slight mean positve charge.

3: Per TSB M-745 Click here to download the PDF

On April 24, 1979, a service bulletin (M-745) was issued for FL/FX models regarding IMPROVING OIL MILEAGE.
(This bulletin is intended to aid in the diagnosis and correction of excessive oil consumption)
One of the tests the dealers were to perfom was a vacuum test on the crankcase.
Check primary chain case vacuum with gauge part No. 96950-68 (manometer).
To use the gauge, remove one of the screws from the primary chain inspection cover and install the gauge.
With the engine idling, the gauge should read 9 to 11 inches of water (includes primary vent air loss).
With the chain case vent line closed off (pinched), the gauge reading should be 25 inches of water or more.
If chain case vacuum is low, check for leaks.
Pressurize the chain case with compressed air and listen for leaks in the following locations:
All gasket surfaces, Oil seals and 0-rings, Around hose fittings.
Check at the Solenoid mounting surfaces, Starter drive mounting surfaces.
On the primary chain case cover, check the following:
Cover gasket surface, Front chain inspection cover, Clutch cover gasket, Starter drive mounting.
Then it goes into compression tests and etc.


What all this means is that the primary compartments can be tested with a slack tube for the presence of vacuum and how much.
Different engines will have different numbers by design.
But there should be a tipping point in any of them where say, before you go on a trip (long especially) ;

* You can test the primary for a possible sprocket seal leak.
The pressure may gradually change as seal quality changes.
* You can test the crankcase vacuum for problems that may occur.
This may help find possible problem areas especially when hound dogging a reoccurring leak.

Actually, any time is a good time to test it.
Then you have data for a baseline of what the pressure usually is.
I'm just thinking testing the pressure is a good thing to keep on the maintenance list.
I haven't seen much mention on the forum of testing CC pressure.
I've seen none on IHs.
One might think it's not necessary since nobody talks about it.
But I've read so many threads with oil leaks, gasket blew out, breathers puking, a Harley is just marking it's territory and endless others.
I'm currently working on IH oil leaks in the pedia and researching pushrod seals blowing out.
It seems like the theme is to replace the seals and ride.
But it also seems that if the pressure was tested more, they'd be less seals blowing out.

77> engines had more vacuum than 76< engines did per the MoCo due to the foofoo valve and other internal changes.
94 engines gained more vacuum due to the sprocket seal changes.
But there should still be a constant (or average) in 76< engines whether that be positive or negative.
I haven't considered chamber CCs but they apply as well respectively.

BTs were running 1200 and 1340cc in 79.

Hippysmack 12th September 2020 19:50


Originally Posted by Hippysmack (Post 5786097)
I changed the breathers today and found that a fiber washer on the front box was toast.
The middle broke out, got soft and bunched up around the hole and it now looked more like a smaller thicker washer.
The small piece feels much softer than the easily breakable fiber washer that I installed.
Maybe it bunched up into the hole. There was no oil around the bolt from the outside.
These have been re-used several times before, replaced them all today.

The umbrellas looked good to me.
The front was more soft than the rear but both were still flimsy.
It turns out I only replaced the front the last time I was in there, it had 'James' written on it and the other didn't.
But I replaced them both today anyway.

I got to thinking more about the fiber washers in this pic.
You can see 1 fiber washer (supposed to be a flat washer) with the center broken out from one of the bolts.
Note that the OD of the washer had gotten smaller than the other three and the part that broke out was curled up looking more like a small O-ring.
The entire washer was on it's way of imploding and getting sucked into the rocker box.
The fiber washers are crisp and brittle when installed. As oil and heat work on the center, so does crankcase pressure vacuum.
The engine these came from was tested making a pressure vacuum of 27-1/2“ of water column at hot idle due to one umbrella valve getting hard.
Notice that the other 3 are also showing signs of implosion in the center of the fiber washers.
The fiber will still seal until the whole OD is curled up or the thickness gets small enough to loosen the torque on the bolt.
This may be one example of how rocker box bolts get loose by themselves.
So, "How much vacuum is too much on a Sportster" may depend on gasket/seal material as well as the current condition of them.

There was no oil weeping anywhere. So it appears the fiber washers are the weakest seals in the system (unless other gaskets have been compromised otherwise).
If instead, you use the copper washers, that makes the next weakest joints somewhere else. Maybe the box gaskets or others.
So it may be possible to add more vacuum by beefing up the cover washers... until something else cuts loose anyway.

Interestingly, they don't use the fiber washers anymore.
Makes me think they raised vacuum with the rubbermount changes.
Maybe the smaller breather bolts and beefed up topbox washers were some amongst needed changes to accommodate or counter the affects.

I've seen questions in threads asking if they could or should use both the copper and the fiber washers.
If you install the copper against the cover, the fiber does nothing but add space between the copper washer and bolt head.
If you install the fiber against the cover, you haven't changed the fact that it will eventually implode as the one above did.

If you only use the copper washers, you have indeed impeded a future leak there.
But you may also set up a for future leak elsewhere instead.

The key is managing the pressure, not replacing gaskets.

Hippysmack 12th September 2020 20:25

Calculating Between PSI and Inches of Water
Here is the SP page on Using / Diagnosing with a Slack Tube (Manometer).

The standard conversion from PSI to Inches of Water: 1 PSI is equal to 28“ of water column (overall vertical movement).
When using a U-tube type manometer, as a visual aid, you could say 14 inches of rise in one leg = 1 PSI.
28“ of water divided by 2 = 14” of movement per leg per pound of pressure.

Example: To Convert PSI to Total Inches of Water:
4 psi = 112“ of water (4 x 28 = 112)

Likewise: To Convert Total Inches of Water to PSI:
112” of total water = 4 PSI (112 / 28)

In my first test with new umbrellas:

At idle, the mean vacuum pressure is -0.75 PSI (21“ of total water column / 28)
At 5000 RPM, the mean positive pressure is +0.0625 PSI (1.75” of total water column / 28)
The peak numbers don't seem as important as the swing space does (high to low to high to low etc).
The crankcase is pulsating between positive and negative influence due to the piston positions.
Higher swings (differential changes) will nudge the seals from there installed position faster than lower swings.

I brought this post in to keep it all together.

Originally Posted by Hippysmack (Post 5839483)
I found some info on CAT engines that limit to -1" to -2" of water column.
However, unlike a Sportster, the water pressure at all engine speeds seems to be fairly constant.

2016-2017 Encore, Cruze, Malibu and Volt found with seal leaks possibly starting with more than -5" of water.
Excessively negative range is below -16 inches of water for them.

TPEHAK 17th April 2021 04:12

Buell XB PCV Valve structure

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