Ring gap & head stud confirmation for 2.7
 

Am in the process of reassembling the top end of a 2.7 and need a sanity check on ring gaps and head studs from the experts.

Mahle cylinders replated and new JE 10.5 pistons. Am thinking 0.18" gaps?

Engine has Raceware studs. Any info I got with these has long gone, but IIRC 24 ft/lbs rings a bell?

Confirmation or otherwise greatly appreciated...

Cheers,

Mark

We had a long discussion about head stud torques last week. Because you have a magnesium case, I think most of us agree that you should stick to the Porsche specs for the stud torques--24 Ft-lbs. Does the case have stud inserts?

Rings. Again, stick to Porsche specs. For the 3.0 SC and Turbo engines, the spec book calls for 0.15mm-0.30mm, or .006"-.012" for the 1 and 2 compression rings and the oil scraper rings. Yours should be similar.

You can ask JE what ring gaps should be for the piston/ring combination they sent you. Make sure they know it's nikasil cylinders in an aircooled, normally aspirated application.

Thanks for the replies. There is a sheet with formulae on it which is where my .018" came from. I just wanted to confirm that with others with recent experience.

I forgot to mention that this is built on a 901/14 case - i.e. sand cast aluminum. Does that make much of a difference for stud torque? Are they all the same regardless of the stud manufacturer?

.006-.012 ????

0.1mm = .004", right?

It seems pretty tight to me too, but I took that out of the 1978-1981 SC and Turbo spec book, p.41.

SC's have aluminum cylinders, which expand more than iron cylinders, so the ring gaps can be tighter. The 2.7 he wants to build should use aluminum cylinders too. That table is a "guideline only" and the context indicates it is for American iron block engines--completely different animal. Aluminum has about twice the thermal expansion of iron.

bumble: My opinion: Porsche specifies 24 ft-bs torque on the studs because that's more than enough to hold the heads on the cylinders. Keep in mind that is the same spec for the 3.3 turbo engines, which produce more cylinder pressure than the 3.0 or 3.2 or your 2.7. You could go higher, but there is no need to do so, It just puts more stress on the parts, including your very rare early aluminum case. Unless you are building a real race engine, stay at 24.

As stownsen914 recommended, check with the JE and/or the supplier for their recommended end gaps.

Thank you all so much for the replies.

The ring end gap table was what I used to get .018" for all rings given the engine use - weekend sports car/DE/canyon carver sort of thing. Not an all-out race engine but fairly hot all the same.

FYI:
Aly case, 97mm spigots & a LOT of mods to reduce pumping losses
Std crank cross drilled, knife edged etc etc...
Turbo oil pump
Turbo oil squirters
Std rods balanced and re-bushed using Raceware rod bolts
This was prepared for me by John Williamson (Otto - RIP) back in the day and did 10K miles before I 'retired' it

10.5 JEs
Resized and replated 90mm Mahle cylinders mooned to match the case. I was going to rebore to 92mm but was advised against it by someone (Henry probably).
Webcam 171e/149i cams
Refurbed rockers and idlers supplied by Henry
38/38mm ports with matched inlet manifolds
46mm Webers with 38mm venturis
Fabspeed SS headers
Magnalow 2 in 2 out
Twin plugged
etc

So, pretty much old school as could have been seen in the '70s.

24 ft/lbs it is for the studs then - thanks Peter. I couldn't remember whether there was anything particular about Raceware studs that might alter that - but it appears not.

Cheers

Sounds like a nice motor

You’re right, I was looking at the wrong numbers. So JE’s piston ring end gaps are way off. Out of the box they’re out of spec.

I’d do some more research on ring gaps before going with .018.

When I talked to CP about ring gaps for my 911 engine, they recommended larger than what was listed in the table due to being aircooled.

Why larger?

Dp, thanks for that reference. If Charlie Navarro says ignore JE and use the Porsche specs, that's about as authoritative as you can get.

Well, The Keeper of The List showed up and hijacked me to do 'more important things'. Normally this is a Bad Thing but in this case has prevented me from being a tad hasty. If the experts recommend Porsche's figures then that's what I'll follow. I'll measure the out of box gaps tonight and see how close they are to the recommended -15 - .30mm figures and go from there.

Thanks again for the input...

Measured the out of the box ring gaps. Very consistently they are:

Top - .010"
2nd - .020"
Oil - ..020"

Given what Porsche recommends, I think I''ll stick with these numbers. Saves a bit of filing...

Half the people say go with Porsche numbers and the other half say go with JP. Some say aluminum cylinders expand too much to go with JP numbers, others say mahle and JP pistons are different alloy and JP pistons will expand more than mahle and therefore you need bigger gaps.

I assume it's due to aircooled running hotter than the more typical water cooled applications covered in CP's published specs. I confirmed with a Porsche engine builder. This was an area I had no expertise, so I deferred to theirs.

The hotter an air cooled motor gets the more the aluminum cylinders expand. I guess it boils down to expansion rates between piston, rings and cylinders. I’m not arguing with you, I’ve got a set of these holding up a recent build and it’s sort of ridiculous that we can’t nail down numbers to go by. I guess maybe it’s worth the extra money to just stick with Mahle next time.

I also found it surprising that there wasn't more consensus on this.

As for heat, expansion, etc., the aftermarket piston manufacturers like JE and CP give different ring gap numbers for street/strip, circle track, boosted race use, etc. My application is race, so wound up using numbers closer to the "circle track" recommendations.

I also kept in mind a saying that a knowledgeable fellow shared here on the forum once:
Big ring gap = big catch can
Small ring gap = big trash can

I erred a little on the large side probably.

It will run just fine. What little difference it makes, you won't be able to feel.

Any problems with rings seating or oil consumption?

Assembly is about the start. I'm hoping to be able to be able to answer that question in the coming months.

I ended up going with .018 top and .020 2nd your cp’s might be different, rather have a little blow by than seize it up.