Boring out Cylinders- What Does It All Mean?

[Pittburgh-Stig]

Sorry if the title came off as a philosophical question,

I have seen people use the term "boring out cylinders" when referring to increasing piston size. Is it actually possible to bore out a stock 3.0 or 3.2 L cylinder, enlarge it to 98 mm diameter, and send the cylinder off to have them Nikasil coated?

Or do you always just purchase a new P&C kit for the desired size?

If boring is an option, is the trade off increased cylinder weakness in exchange for a moderate cost savings? Are there other trade-offs or downsides?

I've researched this topic on the forum but never came across a clear cut answer. Thanks for your responses

[Henry Schmidt]

Originally Posted by Pittburgh-Stig

Sorry if the title came off as a philosophical question,

I have seen people use the term "boring out cylinders" when referring to increasing piston size. Is it actually possible to bore out a stock 3.0 or 3.2 L cylinder, enlarge it to 98 mm diameter, and send the cylinder off to have them Nikasil coated?

Or do you always just purchase a new P&C kit for the desired size?

If boring is an option, is the trade off increased cylinder weakness in exchange for a moderate cost savings? Are there other trade-offs or downsides?

I've researched this topic on the forum but never came across a clear cut answer. Thanks for your responses

Generally cylinders can be "bored out". There is always a limit.
To directly answer your question: 3.0 cylinders (95mm) can be bored to 97mm.
3.2 cylinders (95mm) can be bored to 98mm. Both KS (Alusil) and Mahle (Nikasil) cylinders can be coated with Nikasil.
The only downside is quality control. New cylinders generally undergo closer scrutiny before being shipped out.
In theory, plating an Alusil cylinder with Nikasil produces a higher quality cylinder.

[Pittburgh-Stig]

Thank you Henry, I really appreciate you following up to my question.
With your experience, do you usually recommend one route or the other?

[trond]

also, if boring and plating; what is the correct surface roughness Ra ? I am getting conflicting information and it appears different people and companies are using very different specs. My EBS supplied came in at Ra16 and never stopped burning oil. i consider them junk. Others claim 4-5 to be more in line whit Mahle cylinders. Is there a concensus for Nikasil cylinders ?

[Henry Schmidt]

I was involved in a thread 15 years ago where we hammered out the parameters for honing Nikasil cylinders.
You can probably search out that thread.
If memory serves, 4-5 RA was the condition I measured on a used Mahle cylinder with 90K miles. Well broken in.
8-10 Ra was the measurement for a new Mahle.
I would agree that 16RA is too rough for a soft set of rings. They should however seat without issue. What will happen is excessive wear.
In today's world, where after market pistons find their why into replated cylinders, the real question should be: what RA best suit your specific ring package?

It is also important to keep in mind that there are different type of honing processes and each one will produce different RA number and still create a usable surface.

Unfortunately, RA alone doesn’t reveal much about the actual profile of the cylinder bore surface. A bore finish with tall peaks and deep valleys can have the same average roughness number as one with short peaks and shallow valleys. More measurements are needed to accurately analyze the surface. These include:

•RPK – the peak height;

•RVK – the depth of the valleys; and

•RK – the average core roughness depth based on the RPK and RVK measurements. A surface with a low RK value will have long life characteristics.

•RMAX is the highest peak-to-valley measurement taken from five samples.

•RZ is the mean highest peak-to-valley measurement taken from five samples.

When all these numbers are taken together, it creates a more complete picture of what the surface finish actually looks like

[Glenfield]

Thank you Henry. Your research around this a while back and the ensuing thread was a great resource. Not as polemical as the “who has the biggest spark” / Spark-gate thread, but not far off. Both were highly educational and I tip my hat to you for weighing in (and finding humor in it).

OP, I went with Millenium. The sales rep I spoke to said all their cylinders are ~15RA and that’s fine. That got my antennae up. Just be careful bc they do a lot of cylinders and most are not Porsche. I specified 5RA to Millenium and then had the finished cylinders sent to Charles Navarro at LN Engineering to have RA and all the other Rs measured. Sadly can’t rely on QC at millenium and others so it’s worth checking.

I posted my results on another thread to get folks opinions on the suitability of my RPK etc for Mahle rings. Ra has been thoroughly debated but the other Rs are second / third level derivative and so info was scant. You can check those out on the other thread.

[Aircooled4evr]

Hey Henry, do you know the thickness difference between original 100mm 964 cylinders, and aftermarket mahle 98mm, as well as "drilled" 95, for 97mm sets is?

Cheers, Mads

[Henry Schmidt]

Originally Posted by Aircooled4evr

Hey Henry, do you know the thickness difference between original 100mm 964 cylinders, and aftermarket mahle 98mm, as well as "drilled" 95, for 97mm sets is?

Cheers, Mads

To answer your question as simply as possible: all numbers are from memory so they're only close.
The standard spigot bore on a 3.0, 3.2, 3.3 case is 101.50mm
The standard spigot bore on a 3.6 case is 107.50mm
97mm from 101.5= 4.5mm /2 = 2.25mm wall thickness
98mm from 101.5= 3.5mm/2= 1.75mm wall thickness
100 form 107.5= 7.5mm/2 = 3.75 wall thickness.
We have 104mm base gaskets for 100mm on a 3.2 case.
100mm 104mm= 4mm/2 = 2mm
To install 3.6 100 cylinders on a 3.0/3.2 case they need to be cut to 104mm and the case bored to 104.5mm.
The 100mm 3.6 cylinder also needs the head stud holes slotted.

[Aircooled4evr]

Thanks Henry. Looking forward to buying a set from you! (You've got mail)
P.S. Sorry Pittburgh-Stig, for hijacking your thread!

[Pittburgh-Stig]

No worries @Aircooled4ver, This is what makes this forum great. I posted the question with a main concern over structural stability with a thinner wall cylinder and a secondary concern on the bonding process. I didn't even consider the variability in internal dimensions or roughness factors for the new Nikasil coating. Great information, thanks everyone, particularly Henry