Custom pistons M117 V8 - PeachParts Mercedes-Benz Forum

Tony H · #1 03-07-2014, 12:22 PM

I am considering having custom pistons made for my iron block 5.6L project as I am having second thoughts about using the 560 pistons. Has anyone specific experience with custom pistons? (especially the M117 V8) the net is loaded with vendors that can provide custom pistons-would like to hear about a specific vendor. These would essentially be stock replacement cast pistons-don't need forged as this is a NA stock street motor with maybe 9:8.1 CR. Don't need coatings as this is an iron block.
Questions-are custom piston rings manufactured for each situation or do they go with specific bores so they can use their off the shelf rings. I would assume they would make the pins to the original diameter so they fit the rods.
I'm thinking about bumping the bore to 97mm if I go with custom pistons.

Frank Reiner · #2 03-07-2014, 03:17 PM

Tony:

I have been impressed with the direction you've taken on this project; I thought that the use of the OEM 5.6 pistons was an excellent choice for the application. If combined with a head with a smaller chamber (I'll have to go back and reread your threads on the combination) and the cams you chose (56/57?) the compression pressure should be close to what can be reliably tolerated with the available street gasolines.

That said, on the subject of custom (i.e., non-OEM) pistons, cast pistons will be what you can find in the bore and pin height combination that will be usable with the minimum of re-machining. Casting molds and cores are too expensive for one-off work. They are, however, less expensive per piece when the production run is in the thousands. For low volume work forgings are rather less expensive. Common OD and ID dies can be used to produce blanks that will satisfy a range of finished diameters and pin locations. High quality pistons in low run quantities are available from CP (Carillo/Pankl), Diamond, and JE at the top of my list. For an outright race engine CP is my first choice; for your use I would try JE, as they also manufacture a line under the SRP label, some of which are made using the 4032 high silicon alloy, which would be a better choice for street use due to its lower expansion rate, as that in turn allows a closer fit. The preferred alloy for race use is 2616, which is stronger than 4032, but which must be fitted with additional clearance.

Back to cast pistons; if my memory serves me right, the Ford 312 has piston dimensions (bore and pin height) that are close enough to your requirements to be a starting point. I also think that I mentioned this in a post to one of your threads from the evolution of this project.

Tony H · #3 03-07-2014, 05:53 PM

Thanks for your interest Frank. I have been in touch with Steve Nelson @ JE pistons and he says no problem-whatever I need they can do. I have not discussed specifics with Steve yet but searching past posts it looks like about $1400 for 8 custom forged pistons (they only do forged)with rings/pins from JE. I am now going to use 02/03 hydraulic camshafts from a Euro 500SL. Some initial work with the 560 heads shows they will work on the 4.5 block-the down side is the large combustion chamber. If I have custom pistons made I can reduce the size of the dish to get my CR to my goal-about 9.75:1.
Frank: can you give me specifics about the Ford 312 pistons-diameter, compression height, pin dimeter, maybe a link to a picture so I can see what the crown looks like.
I had considered using another piston that might be close enough to modify.

Tony H · #4 03-07-2014, 06:13 PM

Did some quick research-they are a posibility. The diameter is in the right range. The 312 pin dia is a little smaller so the piston could be machined for the 560 pins. the compression height is 1.76" for the 312 vs 1.714" for the 560 so the piston would stick out of the bore a bit but depending on crown thickness could be machined down. Some issues I can see is if there is enough crown thckness for valve pockets. Another issue is they are flat top pistons and will result in 12.00:1 CR. I looked on Ebay and they are very inexpensive. A new set is only a few hundred$. There is a used 312 piston on Ebay for $32. Might get it and take a look. You might be on to something. If it works out I will buy you a beer if you are ever in Nevada City

Tony H · #5 03-13-2014, 03:32 PM

Just in case anyone has interest or is following this project: Ford 312 pistons are not good candidates for this application because: They are flat top (need a dish) and there is very little aftermarket support except for cheap cast pistons(my machinist said don't use them)
What I have found is Buick 231 V6 pistons are 96.5" bore(with oversizes available to fit my needs), have a dish very similar to the 560 piston and have a compression height of 1.800" which is slightly more than the 1.714" of the 560 piston so I can mill to the exact squish clearance I want. The Pin is slighty smaller than the 560 pin so I can either have the Buick pistons made with the 560 pin diameter or have cutom rod pin bushings made to fit the Buick pin. A forged set would probably be in the $600 range. This whole exercise is to save the cost of custom pistons which would be in the $1500+ range.

Frank Reiner · #6 03-13-2014, 04:57 PM

Tony:

Good eye on the Buick piston; I missed that one.
Are the rings set low enough to take .090 off the crown and still have a minimum margin of ~.200?
Unless the pin boss is unusually thick, a .084 increase in pin diameter may be a bit much, I'd think.

Tony H · #7 03-14-2014, 03:10 AM

Yea I have some research to do. Apparently .100 is no big deal to take off a piston top. The piston is dished so I don't need to worry about crown thickness. The main thing I'm concerned with is skirt clearance to the crankshaft and having the piston machined for circlips since the Buick pistons are press fit to the rod. I ordered a cheapo piston from Ebay to play with and see where I am. It should fit in the bore since it was bored to the 96.5 pistons.
After doing some research I have found swapping pistons for odd configurations is very common. It will probably be several weeks before I get my block back from the machine shop as it is a one man operation and he is in demand.

Stretch · #8 03-14-2014, 04:12 AM

Why do you think dished pistons in particular are necessary?

I thought a dish or alternatively a bulge was primarily used for compression ratio configurations - with the right conrod length and piston dimensions I can envisage a myriad of flat top, dished, convex topped and customised "dented" designs. I guess secondary design considerations would be swirl / burn characteristics that might be improved with one piston design or another - are you looking at this kind of consideration?

Tony H · #9 03-14-2014, 03:11 PM

Hey Stretch,
The 560 heads have 50cc combustion chambers and with no dish the CR would be in the range of 13.0:1. The dish also serves as the valve pockets. The squish area is the surrounding flat area of the piston. I am going to set up for .040" squish. Flat top pistons may not have enough crown thickness to machine valve pockets or a dish to reduce the volume. I need a 20cc dish to get me in the 10.0:1 max range I am going for.
Most older American V8's have large combustion chambers so there is no need for a piston dish-usually the other way around(dome)

Stretch · #10 03-15-2014, 07:17 AM

Well at 13:1 you could run LPG quite nicely! But yeah otherwise I see your point...

dirtyharry · #11 03-19-2014, 03:48 PM

My M117 is running 10.4 or 5 to 1, stock iron 4.5L with 3.5l heads, seems to be holding up just fine.

Tire shredding power is great.

Tony H · #12 03-21-2014, 12:31 AM

Looks like I'm looking at custom pistons-the Buick ones don't have enough to take off the .135" or so it would take to make them work and I cannot find anything else with aftermarket support close enough. I may be able to have semi-customs made by using the Buick ones as the basis and having the pin hole moved to get the correct deck height. Apparently the Buick "Grand National" V6 has quite a following and lots of companys make forged pistons for them.
Harry-are you sure about 10.5:1 CR? The 3.5 volume is only 6cc less than the 4.5 heads.(39cc vs 45cc)That's worth about 1/2 point.

dirtyharry · #13 03-23-2014, 12:09 PM

I didn't get a chance to CC my 3.5l heads but I read through the forum the volume is less then that, like in the 35cc range, also the crush in the head gasket to factor in as well. I crunched the #'s through software, and it was above 10.3:1. the only difference is there is less squish area in the 3.5l head, but there is a lot less. either way I can break traction while rolling in 2nd gear in my W111 coupe, and I'm running 9in wide tires. its pumping out the power.

Tony H · #14 03-23-2014, 06:31 PM

DH-Sounds like a decent increase in power and torque. Did you have a before and after comparison?
After considerable discussion with my machinist I have decided to scrap the custom piston idea and use the 560 pistons. He feels confident there will not be a piston compatibility issue. I'm having the block parallel decked .020" to raise the CR a bit since I plan on using the larger combustion chamber 560 heads. By time I machine the piston top (about.035") I should have around 9.4-9.5:1 CR.

dirtyharry · #15 03-24-2014, 01:58 AM

I didn't get a chance to do before and after. The m117 came from my w108, which I crushed. My w111 coupe retains the stock 6cyl 4.10 diff, with a g76\27a behind the modified m117. The thing about the head swap, is that the 117 has 20 something mm more stroke, so with the reduced Chamber volume even though the piston doesn't come all the way up, it's a significant bump. The rod ratio of the 4.5 is like 1.6:1, very good for revving. It probably could be safely taken to 7k without valve float. Also the 52\53 cams are the hottest production cams, the 46\47 have more lift, but less duration. That's why the 3.5l has valve reliefs cut in the pistons.