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A new normal with the M8?

Started by rich1, February 06, 2021, 05:24:36 AM

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rich1

Months ago I posted some photos of my pistons that seemed to have a lot of carbon on the tops. I was told by some of the members that it was normal for an M8. Since then I have seen a lot of other posts on other sites with pistons that look exactly like mine. So it does indeed appear it's an M8 thing. My compression and leak down test numbers are really good. And the crank case is vented to atmosphere. Interestingly the bikes vented to atmosphere from day one are just as bad as those that aren't. I notice that when running in hot weather that I get a lot more oil out of the crank case vents than I ever did with any TC engine. But with such good compression and leak down numbers it appears my engine is sealing well. Engine is a stage 1 114 running oil level at half way mark. Just trying to understand why the excessive oil from the crank vents compared to the TC. Just normal for an M8? Nothing you can do? Thanks

Don D

Every stock M8 cylinder I have had here for boring has been very far from straight and true, I mean thousandths not tenths off. received the same information from another builder last night. When you rebore and hone them in plates and have good ring seal from day one there do not burn oil. This is oil getting into the combustion by the rings. You can have a decent leakdown test and that does not assure you there is proper oil control. Carry over out the vents is not an influence because they are vented out but a sign, that when hot, the ring seal may not be as good as you think.

rich1

Thanks for the info. Did the pistons and cylinders have a better fit on the TC's? If so that's a real step back in quality control

OldMike

February 06, 2021, 06:43:56 AM #3 Last Edit: February 06, 2021, 06:49:38 AM by OldMike
 I posted a picture of my 2020 Low Rider S pistons @ 3,500 mile teardown. I installed external vents when the bike was new, have the new 8 lobe oil pump and keep my oil level 1/2 way on the dip stick. Only had a bit of water vapor coming out of the external vent hose. The air filter and air cleaner area are all oil free. Behind the throttle plate though is a thin oily or oil like film that looks like it migrated back to the intake valves but not coming from the valve guides.
New 131" build will have Harley 131 cylinders and CP pistons and new stock Harley 131 extreme heads. Since the external vent was just water vapor, I'm going back to stock venting with a Harley air cleaner.
I have a cheap bore scope to see how pistons look going forward. What you see in the picture I think is normal for M8's.


Don D

Take those 4.310 cylinders and throw them in plates and measure them before putting it together.

OldMike

Quote from: HD Street Performance on February 06, 2021, 06:47:53 AM
Take those 4.310 cylinders and throw them in plates and measure them before putting it together.
I'm just a private, at home assembler, no plates or bore gauge. I'm trusting the machine shop for a good fit so I can drop them in.
My original plan was to get 128" cylinders bored and fitted to 130" but I got curious about the new Harley 131 cylinders after seeing a mechanic make note about the a better fitting piston in these cylinders compared to a stock Harley 114" fitment. Never know, this bike might see new higher compression cylinders/pistons in the future.

Don D

4.310 cylinders are Harley Steel liners, right? I would assume nothing and insist they map the bores, based on what I have seen. If the pistons are standard then a cleanup to get them straight and true may put the designed clearance in question.

OldMike

Quote from: HD Street Performance on February 06, 2021, 07:14:23 AM
4.310 cylinders are Harley Steel liners, right? I would assume nothing and insist they map the bores, based on what I have seen. If the pistons are standard then a cleanup to get them straight and true may put the designed clearance in question.
Yes, these are the steel liner cylinders, have you seen these to be out of round?

Don D

Yes, and as I said a few thousandths to clean them up and now a piston that was supposed to fit at .0025 is at .0045. ring end gaps grow too and often require a .005 OS ring set.

OldMike

Quote from: HD Street Performance on February 06, 2021, 07:28:29 AM
Yes, and as I said a few thousandths to clean them up and now a piston that was supposed to fit at .0025 is at .0045. ring end gaps grow too and often require a .005 OS ring set.
Understood, thank you.

kd

Don, are the Harley piston rings more narrow compared to the KB or CP offerings?  If so, do you think the .0025 should be squeezed down a bit and have you been fitting them for coating to tighten up the tolerance to minimize rocking and stabilize the ring control?
KD

Don D

To be clear this is "rework" and can be avoided going in. They get line2line coated. Also some of the STD KB pistons are .001 OS to allow for a new cylinder to be honed. KB doesn't have a lot of shelf M8 pistons yet though. With M8s they all use thin rings .042 is common on oil scraper and compression, I like a 2.5mm oil ring pack but the Mahle 2.0 seem to be working well. I have completed several 150-160+ hp builds

The better solution is to buy S&S cylinders, season them in plates, and hone then in plates to proper size, my spec +.0002 - 0 @ 65°F. I have a hone recipe that breaks in fast and seals well. I have also sleeved the stock cylinders. This type of work is expensive and takes a certain customer that appreciates the work and will pay for that. Many have the Amazon / Walmart mentality and to them this is all not necessary. Cylinders and pistons are just like buying a toaster, right?

Ohio HD

Quote from: kd on February 06, 2021, 08:23:31 AM
Don, are the Harley piston rings more narrow compared to the KB or CP offerings?  If so, do you think the .0025 should be squeezed down a bit and have you been fitting them for coating to tighten up the tolerance to minimize rocking and stabilize the ring control?

from CP's new catalog, the M8 ring packs are as below.

CPN Piston Ring Packs for M8
Gas Nitride Steel 1.00mm Top Ring
Taper Hook Groove Napier 1.20mm 2nd Ring
2.80mm Flex Vent Oil Ring

I've used a lot of different piston and ring brands in my time. The CP offerings have performed the best in my experience. Pistons are repeatable, always the same dimensions and strong. Ring design seat well and hold up under hard hot engine use.




Rokinrider

Because I like the way my 107 stage 2 runs. I thought about getting a set of take offs on EBay, getting some CP pistons with a real oil control ring and sending them to someone for a good hone/piston ring fit and replacing the stock cylinders. I keep hearing go bigger, but at 65 this girl runs great.
Mclintock! swell party were the whiskey?

To The Max

Quote from: Rokinrider on April 24, 2021, 09:49:18 AM
Because I like the way my 107 stage 2 runs. I thought about getting a set of take offs on EBay, getting some CP pistons with a real oil control ring and sending them to someone for a good hone/piston ring fit and replacing the stock cylinders. I keep hearing go bigger, but at 65 this girl runs great.
I had a 107 stage 2 and it was nice. i then went to 117 at 10.5 to 1 1t was  fantastic very nice to ride and very smooth, and no real lose of fuel ecomomy at cruise. then went to 124 at 11.0 to 1 and the power was much better, but at around 3000 rpm it now develops a slight harmomic vibration and the economy isn't so good if i cruise about 70 mph. i would go the 117. hope this helps. Max

Don D

Piston gross weight is often times not considered. It should be! Aftermarket weights vary and this affects balance. Time marches on and now I am using gas ported top rings on some builds.  Again not a part for the box store consumer.

kd

Don, I have wondered about the gas porting on the street use Harley engines.  With a proper quench zone (some are not full circumference) that surface is "usually" swept clean and there should be very little concern about carbon contamination. Are you experimenting with positioning and size or do you have a formula you are working from? 
KD

Don D

Having them put in custom pistons and using the total seal rings. Hard to quantify the benefits.

misfitJason

I honestly think the buildup is from the gas nowadays
2006 Dyna, Kraftech Evo Softail