Exhaust Valve Pocket Size

[schleven]

I just received my new 100mm Mahle Motorsport 10.5:1 pistons from EBS and am puzzled by the size of the exhaust valve pocket. It measures 40mm across. Standard exhaust valve sizes for 3.0L+ is 41.5 and I would imagine valve pockets need to be wider than that. Here's a picture of my new pistons (right, w/ 40mm pocket) next to a set of older Mahle Motorsport 100mm pistions (left, w/ 43mm pocket). Notice the difference in exhaust valve pocket size. Am I missing something here?

[Neil Harvey]

Good catch.

Be careful and do not just look at the pocket diameter. The angle and the location should also be checked.

Should question the supplier what pocket diameter, valve angle and pocket location they spec'ed out.

[Trakrat]

unless the picture is playing tricks on me... the piston on the left looks bigger as a whole compared to the one on the right.

Just give EBS a call and ask them. I've called and asked questions about their pistons without ever buying anything from them.

[racing97]

Well you can see how the Piston on the left has been reworked it has milling marks in the pocket and lathe marks on the deck. Mahle makes a lot of Pistons and they have far superior material processing than just about any other manufacturer but they do not cater to specific needs of cars not currently in the production pipeline for Porsche Builders one of the important aspects is compression ratio Mahle computes them at zero deck, thats why they are always producing lower comp ratios when you actually measure them. CP for example computes ratios with deep pockets and deck clearance up to -040 as the Mahle piston on the left also shows, so if you want to drop them in Mahle gives EBS what they think they need and yes they are the best but if you want to get the most out of them you will be machining a bit,

[Peter M]

I had to further relieve the reliefs in my Mahle Motorsport 98mm pistons to gain adequate clearance with my 993SS cams. Only way to tell for sure is to assemble and measure with plasticine and then you know for sure.

[schleven]

Thanks all for the comments. Just puzzling why they'd spec a 40mm pocket on a 100mm piston (it's not like someone is going to run these on a <3.0L block with, let's say, 37mm exhaust valves). Going to continue to look into what my options are at this point.

[cgarr]

Is that 40mm measurement at the full diameter of the circle? The pocket looks to be less than a half circle?


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[schleven]

The 40mm is measured at the widest area of the pocket. It does appear that the pocket is less than a half circle. See photos comparing old Mahle motorsport 100mm (no, no machine work has been done to this piston) and my new 100mm motorsport piston.

Old


New

[Tippy]

You can set a head directly on the piston with the exhaust valve installed, and see if you get a stroke difference. That'd be the quickest check.

[MBruns]

Here's what I like to do and it to me the most real world, as I set up one side to check v to p clearance with light springs in head # 1, I leave the valves out of #3, set cam timing to look at #1 then put #3 at TDC and use a transfer punch the same diameter as the valve or an old valve stem, machined to a point and bump a light mark in the piston and that will be your center of the pocket as that head see's it. I have seen pistons that the pocket will be off center in relation so just opening the current pocket may not be enough, you can sweep the pocket with the set diameter of the valves to see if it's close or set it up in the mill with a piston vice at the proper angle and put the valve in the collett and move it into the pocket, I use a pin gauge of 1 mm as a min. to check the entire pocket.
Hope this helps, Mike Bruns