Boxster piston ring end gaps?
 

Hi All,

I`m wondering if i should buy new rings for the pistons. Not sure if this has ever been released by Porsche, does anyone know the end gap tolerance values for the 2.5L engine?

Thanks!

Generic answer.

Is there pressure buildup from the engine case breather, if not, rings are probably fine.
If the car is for high RPM, a slightly larger gap is ok as very little gas/air gets through the gap a such a short time.
Hasting Rings says 0.014 " per inch of bore.

I'm sure there will be after market rings for sale. Most ring widths today are roughly the same for similar size pistons. I haven't stripped a 986 motor but I presume they are nikasiled, so get compatible rings. Ie not for cast iron.

I have a 2.7 motor in the shed just waiting for some spare time to give it a full overhaul.

Hi,

Thanks for the answer. Unfortunately I know nothing about the pressure buildup bc I`ve never seen this engine run. I looked at a Haynes manual for my SAAB 900, based on that values the second ring is out of spec (SAAB uses Mahle pistons and rings as well, the bores are larger though, so I don`t know how accurate that is for a Porsche). Based on what you are saying it really depends on the bore size so it still might be good, I`ll check that out. My worry is mostly about the side rails for the oil ring, if I place them into the bore I can see through the ring and the bore.
Interestingly, you can buy aftermarket rings for all the other engines but not for the 2.5L. For my car the only option I found is to buy the originals, that cost around 2K so it`s out of the equation. I did some research and found that certain BMW rings are the same in size, I`m not sure those are good for Nicasil though..

Are you sure they don't say 0.004" per inch of bore? I think 0.014" would be the total end gap

Sorry you are correct, from the web Example: 4" (bore) x .0035 = .0 14 minimum gap. I work in mm so I didn't click that .014 was wrong figure.

I understand the 2.5 and 2.7 are basically the same motor. The 2.7 has a 6 mm longer stroke and the pistons are 3mm lower to fit.

My guess is the rings are the same if you can get 2.7s.

I'm sure there are aftermarket ones available for a few $100. Rings aren't rocket science.

I`m not sure about the bore, the ring sets for the 2.7 are listed as 86.5 mm standard size. In my 2.5 I measured roughly 85 mm. But here for instance they claim the two engines are the same bore-wise:
https://www.auto-data.net/en/porsche-boxster-986-2.7-boxter-24v-228hp-6710
I`ll check what the manual says about this.

There is no nikasil in any 986 engine.

Thanks for elaborating on that!

Yes, the bore size both for the 2.7 and the 2.5 should be 85.5 mm, so rings listed for the 2.7L should be ok for the 2.5L, unless I`m missing something else.

You should measure the cylinder bore's for wear, if they are worn beyond spec you should probably reinstall the original conforming rings in their original positions.

Well, I don`t know the specs and I don`t have precise tools to measure the wear. The bores show no sign of wear by feel and the ring gaps do not change much when I slide the ring all the way down the bore, and there`s no difference if I place the ring to the top of the bore (the area that the ring normally does not reach) vs. if I slide it down. So I assume the wear is not substantial. I think the compression rings could be reused, my worry is about the oil rings, the side rails seem deteriorated, I can see through between the cylinder wall and the side rail when I place them into the bore.

Ohhh.... i'd probably back the bus up abit, if I were you. At least lay on the brake some...
If you're trying to do this even partially "right", you shouldn't be letting $100 worth of snap gauges and a caliper stop you. Take your time, do it right. You'll be glad you did.

Sent from my SM-G930T using Tapatalk

I do have a caliper and snap gauges, but those are useless to measure cylinder wear. For that you need a bore gauge, and you are right, they are actually much cheaper than I remembered, so I might go ahead and invest into buying one or just take the cases to a machine shop and get them measured. Still, I don`t know the tolerance values for the bore, not even sure if they were released by Porsche.

This would do it, for sure no machine shop would measure the wear and ovality for me for this much:
https://www.amazon.com/Blesiya-50-160mm-Cylinder-Internal-Measuring/dp/B07K59JPYH/ref=sr_1_8?s=industrial&ie=UTF8&qid=1547515233&sr= 1-8&keywords=bore+gauge+metric

I still don`t know if the 2.5L and 2.7L piston rings are interchangable. Based on the dimensions (bore size and thickness) they should be identical. However, the sellers say the 2.7L rings won`t fit my 2.5L engine based on the catalog. Am I still missing something here? I already posted some info about the ring end gap measurements in my show&tell gallery and asked the same question there: http://986forum.com/forums/show-tell-gallery/73917-blue-boxster-resurrection-project.html
Perhaps more people read this thread and there`s somebody out there who has seen both of these engines apart and know the answer? Thanks!

Dial the dealer?

Post up on Rennlist?

The ring gap will seldom change down the bore. The ring wears and opens up the gap. If there is wear in the bore, you will feel a lip just above where the ring stops at the top. No step, bore ok.

Thanks Otto, the dealer is gonna tell me what the catalog says. I can try Rennlist, but mostly members there and here overlap, don`t they?

Yeah, I checked that, I think the bores are fine, there`s no lip. It`s possible though that there`s some ovality, and the wear may be different along the bore due to the different piston speeds at different heights. So I`ll measure this just in case. BYprodriver also had a fair point when suggested that if the wear is substantial I may have a better seal with the original rings in the same position than with new ones. But most likely it`ll be fine with a fresh ring set once I find one. I think I`ll just go ahead and order a set for the 2.7 and send it back if it doesn`t fit, but I doubt that.

Check for 996 103 033 02

Mahle piston ring kit Mahle 504 40 N0
EAN 4009026929575

Contains:
Mahle 63504, Rechteckring, Nitriert, Heigth 1,2 mm
Mahle 63745, Nasenminutenring, Heigth 1,5 mm
Mahle 65959, Ölabstreifring (mehrteilig), Nitriert, Height 2,0 mm

https://catalog.mahle-aftermarket.com/eu/product.xhtml?eid=1195707#divider

Fits:
M 96.20, 85,50 Ø, 2.480 ccm
M 96.25, 85,50 Ø, 2.687 ccm
M 96.22, 85,50 Ø, 2.687 ccm
M 97.20, 85,50 Ø, 2.687 ccm
M 96.23, 85,50 Ø, 2.687 ccm

You'll find infos about correct gap in the workshop manual.

There are also piston ring sets for overbore pistons available:
Mahle 504 40 N1
Mahle 504 40 N2

Regards from Germany,
Markus

Thanks Markus,

This is helpful, seems the same ring set works for both 2.5 and 2.7 cars, as I thought. Unfortunately the Mahle sets are out of stock everywhere I looked at. Now I think I`m gonna go with this:

https://www.ebay.co.uk/itm/183341980498?ul_noapp=true

It`s an SPD product, which I`ve never heard about...

Workshop manual: does such thing really exist??

In the price range of the eBay ones i would go with OEM: https://www.********************************************** **************************************.com/oem-parts/porsche-piston-rings-99610303302/?c=bT0x

Edit: uhm - seems to be a competitor of pelicalparts.

Shure, an official pdf workshop manual in english language for the 2.5 exits.

Yes, but it contains nothing about the engine internals. Porsche never intended techs in the field to work on these, so they never published anything.

Yes, I have the Bentley workshop guide, which is useless for engine rebuild, Wayne`s book, and a parts list with diagrams I found on the web. That`s all I have.

Markus, I`d go with that too if it was in the same price range, but that`s for one cylinder..

Markus, I`d go with that too if it was in the same price range, but that`s for one cylinder..

Rings can rotate during their life so there are no original ring positions. Always give a light hone before installing new or old rings to help bed them in. .

Cant find the link on running in a motor but if you are using old pistons, a few squirts on the accelerator with a back of is all that is required.

Have you ever see race cars do 10000 miles as slow speed to run them in ?

New pistons, new hone, 2 - 3 heat cycles and pedal to the floor.

Quite and old video of mine.

https://www.youtube.com/watch?v=PsEFKYULNDc

Cheers Wallace

Thanks Wallace,

Yeah, I meant the same rings to go back to the same cylinders.
What do you use for honing? Just a honing tool in a drill or you take it to a machine shop?

12500 rpm is not bad for a 4T single :cheers:. What can the CB125 twin do, if the singe can be spinning like this?

One of these will do, https://www.toolfactory.co.nz/products/ampro-3-jaw-engine-cylinder-hone-50-175mm-75mm?variant=39640318349&gclid=EAIaIQobChMIhN7U-pyL4AIVCSQrCh33Mg57EAYYAiABEgK7IPD_BwE

Quick hone (20-40 sec) with some cutting oil is normally fine. Just roughen up to help seal the rings.

Just think of it this way, take a sharp knife and draw it across and oil stone. Home many strokes does it need to sharpen, 6 or 12. Same with and engine, the stone tears the liner microscopically, a few slides of the piston rings beds them in .

Seen some aero engines documents where they 'lap' the rings in by hand with a piston lapping jig.

Have 2 CB125 twins and one is bored to 150 cc. I need to do my magic on the motor but that will be good for 14000+ .

Will do. Honing will also help keep oil film on cylinder wall.
I also like to play with vintage Japanese bikes, I have a TX750 Yamaha and a CB750 K1.