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The AMU engine which is the nearest in Bentley to the APX as far as I know has the following spec:

Compression rings 0.15 to 0.40 (wear limit 0.80) mm
Oil scraper ring 0.25 to 0.50 (wear limit 1.00) mm
 

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Discussion Starter · #4 ·
Thanks.

Just to be sure, the 2 top rings shall both have a gap of 0.15-0.40mm.

The lowest ring shall have a gap of 0.25-0.50mm. Here I ahve 2 rings and some u-shaped thing that shall be inbetween the 2 rings.

What does "wear limit" means?
 

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Yes that's the standard oil scraper construction.
The section should look like this when pressing against the cylinder wall: |U|

The "wear limit" is a bit odd - that's what it says. I've always seen spec max as the wear limit. It's almost like saying a new engine should be between min and max and an old engine being reassembled can have up to the "wear limit" i.e. to programme in and ensure some useful life. If you were taking it apart though wouldn't you get a new set of rings if the gap was anywhere near the "wear limit" in that case? Perhaps it's to encourage you to make that judgement call?

It says to measure it ~15mm from the bottom of the bore - using an un-ringed piston to keep them square. Maximum wear occurs near the top of the bore so you can also measure them here to assess bore wear.

Piston ring to groove gap (i.e. how much the piston can move before dragging the ring) is:

Compression = 0.02 - 0.07 mm (wear limit 0.12)
Oil scraper ring 0.02 - 0.06 mm (wear limit 0.12) - this one is always inaccurate as the things are so springey!
 

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Discussion Starter · #7 ·
Just tried the rings. Gap for top one for the different cylinders are between 0.2-0.25, second one is 0.4-0.5 and third ones something like 0.4-0.5.

Started to look at how the rings shall be directed and I'm not sure I understand this.
Everywhere else I have been looking it says that the ring gap shall be 120deg apart from each other. If I look at the instruction http://www.jepistons.com/PDFs/TechCorne ... rc4032.pdf and at the pic in the lower rigth corner at page 2, it says that the gap for the top ring shall be to the right and the gap for the second ring shall be to the left.

Then for the 2 oil rings and the u-shaped thing, shall the gap for the 2 rings be at 7 o'clock and 11 o'clock?
Shall the opening for the u-shaped thing be according to pic9?

Maybe my English is too poor or my technical skills isn't enough but for me this instarction is not very clear.

Anyone that knows this?
 

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The main thing to check regarding the position of the piston ring end gaps when on the piston in the bore is that the end gaps are in different positions ( Not all the same it helps stop the compressed gasses leaking past the rings when on compression stroke) :)
 

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Discussion Starter · #9 ·
127ultima, when you did your engine, did you used the old bolts for the crank and the piston rod or did you buy new bolts?
If you used the old ones, what torque did you use?

According to ELSA, the bolts shall be replaced but I see no reason for that if you just use a correct torque for the old ones.
 

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Discussion Starter · #10 ·
Shall all four of the #26 be there for an APX engine?

As far as I remember there were only the bottom ones in mine (engine was dismantled 6 months ago). The kit that I bought has all four, ETKA says four but there are only the two bottom ones in ELSA.
 

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Offset all ring gaps by 120 degrees from top to bottom (including oil scraper rings).

Always use new con rod bolts and nuts - 30Nm +1/4 turn (oil threads and surfaces) - they are stretch bolts and will weaken every time they are re-used.

Two thrust washers shown on APX on bearing cap No. 3 with locking tab.

Mains bolts 65 Nm + 1/4 turn - always replace stretch bolts.
 

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Piston rings rotate freely, gap positions are random after about 1min run.
Setting position is hifi feel good thing, but it only takes seconds to do. So :p
 

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Discussion Starter · #14 ·
John-H said:
Offset all ring gaps by 120 degrees from top to bottom (including oil scraper rings).

Always use new con rod bolts and nuts - 30Nm +1/4 turn (oil threads and surfaces) - they are stretch bolts and will weaken every time they are re-used.

Two thrust washers shown on APX on bearing cap No. 3 with locking tab.

Mains bolts 65 Nm + 1/4 turn - always replace stretch bolts.
In all manuals, it says that stretch bolts shall be replaced but is that really necessary? Why can't you use the old ones if you just use a proper torque?
In my case, I used the old main bolts but found out that the quality for two of them wasn't that good so I had to buy new ones anyway. I bought all 10 since they weren't that expansive but still, 8 of them could have been used.

For the con rod bolts, these are rather expensive and same here, why do I have to replace these if I just use correct torque for the old ones?
 

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There are generally two methods of tightening fasteners to a specific preload (bolt tension). The first is by using a torque wrench which measures the torque or turning force on the bolt and presumes that at a certain torque there will be a certain bolt preload. This method, although generally good for most fasteners is prone to error caused by variations in friction in the threads. A dry corroded bolt will bind and result in low preload at a certain torque, whereas the same torque on an oiled or greased bolt can result in many times the bolt preload and even cause the bolt to snap!

In certain critical designs such as big end bolts, cylinder head bolts etc. it's important to have enough preload to stop the fastened items separating under load which will cause movement and the bolts to unwind. In a critical design the bolts are usually required to operate near the limits of their capabilities, so obtaining an accurate preload is critical.

To solve this problem bolt manufacturers often use "stretch bolts". These are usually recognised by their tightening specification of an initial torque setting, which takes up play and produces an initial preload within the elastic limits of the bolt material plus an angle tightening spec - often +90 degrees. The angle tightening specification takes the bolt out of its elastic zone into its plastic deformation zone where it becomes permanently stretched. The idea is that the plastic zone is determined by bolt diameter and material properties which are tightly controlled in manufacture. Tightening the bolt just into the plastic zone therefore gives a very accurate bolt preload - much more accurate than the torque method alone.

The problem with re-using these stretch bolts is that each time they are used the bolt diameter necks down and gets thinner resulting in less preload, e.g. 1st use 100%, 2nd use 85%, 3rd use 60% to 4th use 40% then snap! That's just an example but illustrates why they should not be re-used. The consequesces of failure in an engine are clearly not worth the risk.
 

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Stock Oem rods are not worth reuse either.
After market upgrades are not that expensive.

[smiley=devil.gif]
 
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