KH Piston Clearance

[Geodoc]

I put 81mm Wiseco's in my '73 Ducati 750 GT at .003" by the package info if I recall correctly, though that's with iron sleeved aluminum barrels and OHV. I'd imagine the Enfield Racing guy would have a note in the package about that, but will be sure to double check before it goes out for boring.

As an aside, do all K series (inc. KR's) run the same piston except for the KH / KHK with the skirts relieved for flywheel clearance?

Stock for street is 1 to 1.5 clearance. Cast type. Forged definitely grow more. Might give Wiseco a ring see what their spec is for this application. Yes they will stick at 1.5 clearance...They probably have different clearances for different applications. Being street stock you want less clearance than if you're running fuel and drag racing. The two ends of the scale.

[EKHKHK56]

Right. For cast. Forged have more mass and therefore grow more, depending on combustion chamber temperatures for how much. When I raced Sprint Cars the forged ROSS pistons were the same for street or top fuel. Only difference was recommended clearances for different applications. I don't recall exactly but it would be something like .002" mild street to .010" top fuel.

[EKHKHK56]

Sorry, just missed your post George. Bore size, lubrication, block type, cooling method, actual piston material....all play in. I believe all K Factory pistons are the same stock. I don't think they had to modify skirt. It's -32F out or I would go look at my piston collection. When I stroke these to 5" very little is removed from stock skirt. After everything is computed I think your 3 number should be ok with easy breakin. Get good Racing oil with Zinc and some straight heavy weight. I still have old 60 70 weight I use. Maybe 20-50 Racing Oil for breakin. Then I use straight 60 70 in my Ks. The K motor is more like a transmission with all those gears than an say an OHV that needs thinner oil to go upstairs to lubricate. Erik

[EKHKHK56]

When assembling valves dip the stems in STP profusely. Poor things never oil unless you flip your bike

[wz507]

Just bought a set of pistons w/ rings, wrist pins & clips + Manley valves for a KHK from:

http://enfieldracing.com/

His pistons are Wisco that he had made to suit. I haven't got them in my hand yet, but Wiseco usually comes with installation instructions for clearance & ring end gaps. Being forged rather then cast, I'd imagine that the piston clearance would be larger than with the stock ones.

Here is a picture of the K Model Wiseco Piston sold by Enfield. There is a factory slip in the ring box with the ubiquitous ring fitting instructions but no information on piston fit (of course Wiseco has no idea what someone is doing with a custom piston, so they're not likely to make any fitting suggestions other than no matter how you fit it there is no warranty). Enfield recommends 0.004 - 0.0045” piston fit.

LR K Model Wiseco Piston.jpg (106.33 KiB) Viewed 10931 times

[Ferrous_Head]

If your using stock pistons go with what the manual says. But there is a huge difference between what most cast piston and forged pistons run. This will (as I understand it) relate to the silicon content.) But in any case I would ask Wiseco for their recommendation.

[wz507]

If your using stock pistons go with what the manual says. But there is a huge difference between what most cast piston and forged pistons run. This will (as I understand it) relate to the silicon content.) But in any case I would ask Wiseco for their recommendation.

I think the ultimate piston fit for the subject forged piston relates to several critical variables, e.g., the aluminum alloy (in this case 2618), the piston style (cam ground, barrel shape, or modern CNC hybrid design), the cylinder material, cooling, and end use (putting around or riding it like you stole it).

I looked back through some old notes I made and see that Wiseco recommended fitting at 0.0025”, Enfield recommended 0.0035 – 0.004” and the race engine builder who recommended 0.004-0.0045” (I misquoted this earlier when I said this was Enfield’s recommendation). I would never think of running this forged piston at the Wiseco recommendation of 0.0025”, because Wiseco lacks experience with this particular engine design, it’s intended end-use and it’s operating temp. I guess the only way to really know the lower limit of fit is to start snugging them in and determine at what point things start to tighten up (but that doesn't sound like much fun to me).

For what it’s worth, the Wiseco part# for the ring pack is encoded to provide the nominal ring size. In my particular case of 0.060” OS pistons the ring pack part # is 2815XC, so the nominal ring dia is 2.815”, and the piston, at it’s largest dia is ~ 0.002 -0.003” smaller than this, so all in all completely consistent with the Wiseco recommended fit.

[Ferrous_Head]

While no manufacturer is going to know exactly how the intended target engine will be used I would suspect they would all be familiar enough with an engine if the make pistons for said engine. I would expect they would have built a "safety margin' into the recommendations. It's always possible they will make a mistake or specify too close tolerances for a piston that might see serious high speed work. But these aren't new engines and I would expect that Wiseco feels it's making pistons not just as replacements but for engine builders who intend to use the bikes in competition.
I'm not trying to defend Wiseco - I don't use their pistons. But I have to assume that anyone who makes pistons for a particular engine would have some knowledge about how it should be fitted. I doubt they just guess.
Different engine builders will all have their preferences for clearances. When your building an engine for someone else it's always a compromise as you try to get maximum performance with minimum risk. No one wants to build an engine for someone that locks up at the end of the main straight. But you do want that engine to pull harder than the competitions engines. Compromise there.

If you went with Wiseco's recommendations and then the engine seized you would have a comeback on them. My best guess is they would ask for full details and provide you with some form of relief. New pistons at a minimum.

I run nearly everything in my engines with "loose" tolerances. They don't get much in the way of a "break in" and with an engine that heats up like a frying pan it just makes sense to me. And I can afford to drop a small percentage of power if it means the engine lasts for the entire meeting. My engines never see 1000 miles before a rebuild so I don't care about getting maximum life from them. Just enough to get over the finish line.

[EKHKHK56]

I would venture Wiseco knows everything they need to. I ran their pistons with no issues in my 1976 Elsinore that ran a completly tweaked Al Baker 1974 cyl with boost port modifications. Good enough for the 76 title when the screaming yellow zonkers were on top of the tech game. Race clearance is set loose for immediate competition after building. Extra oil, less friction, particularly at high RPMs. Normally temperature in a K is not a problem unless you let it sit and idle.

[wz507]

If you went with Wiseco's recommendations and then the engine seized you would have a comeback on them. My best guess is they would ask for full details and provide you with some form of relief. New pistons at a minimum.

Probably not, as this is the warranty included in each piston box - quite typical for a race part in my experience.

LR Warranty Disclaimer.jpg (186.7 KiB) Viewed 10919 times