I see posts on using Mahle pistons from time to time. I am interested in the proper piston -to-wall clearance recommended/ used. It seems the manufacturer gives a bore size to use, but have not seen what the clearance ends up at. Can anyone report?
I used .020 over Mahle pistons in my rebuild. When I showed my machine shop what Mahle suggested for clearance he said that was too tight. Sorry I donât remember exactly what Mahle specified, I think it was something close to .00075". He felt safe with .0015-.002. and that is what he achieved. The finished bore size was 3.6455-3.646 I have yet to run the engine as the car is still apart.
I used Mahle pistons. Different pistons call for varied clearance. I fortunately had a 3.8 block which was in bore spec after sunnen hone with no bore. It was on higher end of spec, so I did piston research to find the largest nominal size. Then I went with the Mahleâs as they were the largest (due, no doubt, to the fact they had a small advised clearance.) I then decided to run the Mahle with a clearance more suited to other forged pistons - just as your engine builder advised. Now all guys say go with the specs of the piston manufacturer, but I doubt if any piston of any design wouldnât be OK with 0.002" piston to wall. While guys say âthe manufacturer knows, they are real smartâ, a street engine with very round bores and a good hone will be just fine at 0.002", careful assembly, round holes, no taper and youâll be dandy. Plenty of rebuilds with way worse if the above conditions are met.
Also plenty of shops which canât hold the specs they claim. Iâd worry more about your chains and all the other stuff that can go wrong - assuming you actually have the 0.0015" or 0.002" they try for.
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Responding to Billâs commentsâŚ
I agree with Larryâs overall message. As an engineer, it sort of rubs me the wrong way for someone to go out, and buy expensive pistons, and then ignore the recommended results of their R&DâŚsort of like all the guys who buy expensive ARP bolts, and then torque them according to their factory manual.
On the other hand, less than a thou of clearance is AWFULLY tight for a street engine. So, then you have to ask yourself what is the downside to going with a little more clearanceâŚmaybe a little more noise (particularly when cold), and maybe a slight loss in performance. Are you going to notice either of those?..probably not.
The other question you have to ask yourself is whether Mahle is going to come to the rescue if something goes wrong with their recommendations? The answer is âno wayâ. Your machinist also isnât going to help you at that point, as heâs (rightfully) going to say âI told you I thought that was too tightâ.
However, if something goes wrong with your machinistâs recommendations, heâll (hopefully) feel some obligation to help make it right, assuming heâs a stand-up guy.
If I was in.your shoes, Iâd be inclined to take the advice of your machinist.
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Piston clearance depends on how the piston is made - forged pistons expand more than hypereutectic, which expand more than cast. I would suspect that standard Mahle pistons would be cast, and Mahle would know best what clearance is good. Greater clearance, within limits, only means more piston noise.
Ask yourself whether Germany built its place as the worldâs leading exporter pro-rata of engineered goods, by deliberately shooting themselves in the foot through issuing suicidally risky user guidance with their products.
If two parties disagree, are you going to side with Bubba Greenhorn or good ole boy Flathead Fred against the advice of a global ISO 9001 corporation who can pick the best of the best from a technical and engineering educational system we can only dream of.
Using gut instinct based on yesterdayâs experience has its place, but using it to fill knowledge gaps about proprietory metallurgy and state-of-the-art piston manufacture is a poor substitute for unknown but vital unknowns.
There is a big conceptual difference between machining to avoid an angry customer with a seized motor and machining to produce efficiencies and performance criteria that were unheard of a few decades ago. I must have missed the Sinatra verse where he sang about boring blocks âmy wayâ. No wait - I remember it now -
âRegrets, Iâve had a fewâŚâ
LOL
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My Mahleâs should have 0.0005" per their spec sheets. Iâm a believer in significant digits from science training (the + or - as the engineers would say). Given this I could only measure my bores to + or - 0.0002". Therefore I have from 0.0012" to 0.0003" greater clearance than Mahle advises. Under 0.002" as a consequence. I know the math doesnât quite work out from what I have down here, but it is based on some assumptions - namely that the pistons were near perfect.
In any case I have tighter clearance, which Mahle advises, but not so tight that it would make âBubbaâ nervous fitting customer pistons. I still maintain most engine shops will have trouble holding 0.000x" , so Iâd be gun shy about them trying to hit that spec. Iâm happy with what I did.
As far as German wonderfulness, we had 3 BMWâs at one time and they (the Germans) are prone to some pretty wild foul ups. My k1300s dies at random and BMW says âWe fixed that in the previous model yearâ - Oh yeah? Our SSG auto stick (Italian electrics) needs to have so many ducks in a row in order to work that you need a talisman to assure reliability.
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Speaking of German cars. My day to day driver is a Mercedes ML320 SUV. I bought a Corvair new in 1964, owned a Rover 2000TC, a car R & T called the most unreliable car ever built (they were right), probably 15 Jags, TR3âs Austin Healey, etc etc, and let me say the Merc is, hands down, the most unreliable of them all. Finest German Engineering? They are incapable of building a diesel engine without cheating, and virtually all F1 cars are built in England. May I leave this with you all.
I have argued the same way and my gut instinct is the same but we had this discussion on one of the forums where a German guy was seeking the advice. From memory, when he wrote/spoke to Mahle they stuck to their spec and said they had developed the pistons using a 4.2 block. Seems tight to me but they didnât budge and of course it is much looser up at the ring end.
Larry
I also have a K1300s.
Best bike ever built!
Dennis 69OTS
I really like it. I donât have proper luggage, however. The die out at idle when you pull in the clutch is downright dangerous, however.
Iâm OK with what I did. I am close to that spec, by old fashioned standards. One and one half thousandth over their advice ainât gonna kill me.
I have not heard of that problem
Have you posted it on http://www.i-bmw.com/forumdisplay.php?f=195
It is a really good forum. Not anywhere near as good as this one!!!
Dennis
69 OTS
Supposed to be k1200s and âfixedâ. You canât believe what BMW said about it. The first fix was to put Textron gas treatment in. They said carbon build up on the back of the intake valves was causing it! They then just set up the idle with a computer fix. The reason for the gas treatment was the injectors were too close to the valve. The problem with any of these recalls or tech bulletins is many companies wonât say what they did wrong until a death suit hits them. Iâm not into paranoia, but when I worked for Chevy service they wouldnât cop to the flat cams or Vega blocks, so I have experience with avoiding expensive, after warranty claims.
What year?
I have 2012
I would suggest post the problem on the forum and ask advice
Dennis
Just wondering if any one fit Mahle pistons to less than .001" as recommended and have a report for us how it runs?
Doug
If Iâm reading the Jaguar service manual correctly, it calls for .0011 to .0017" clearance. Iâm going to use Mahle cast aluminum pistons that I just bought from XKS Unlimited. Think I should stay close to the .0017" to be safe?
wow. thatâs snug. I used to use .0035 in the Spit back in the day. Technology has improved I guess.
Frankly, that frightens me: back in the day with t-slotted pistons, Jag specced (IIRC) that clearance.
Dad, being an airplane wrench, followed that specâŚand the engine seized.
He took it apart, bought new pistons, had the bores cleaned up, and set them at 0.0025"⌠worked fine.
I donât have my notes for that spec handy, but⌠I do recall the Mahle spec was crazy tight. When this topic comes up many guys and experienced builders donât use the super tight Mahle spec. I, for one, did not either.
By no means should you just take my word, but my clearance ended up AROUND 0.0025"+. In other words I used what most engine shops would have done with random TRW slugs âback in the dayâ. My engine is quiet, doesnât smoke (rust, bust, implode, explode - runs real fine, lasts a long time). Just holding the MAhle clearance it seems would require a new, normaled Sunnen hone right there on the spot while you fitted them.
Many fancy pants cars have had BAD oil consumption in recent years from super thin, low tension rings and glassy bore finishes. They also have very fancy thermal expansion control struts and such inside the pistons. We donât have set ups like that, and with careful normal assembly we also wonât have seized engines using specs which donât apply to our situation.