KH Piston Clearance

Production K Models

Re: KH Piston Clearance

Postby MIKE100M » Sun Mar 18, 2018 6:57 pm

EKHKHK56 wrote:I would still do some heat cycles before running it up on the dyno. I just let idle, blip throttle a little till warm/hot than let completely cool. A few of those at least will ease break in. Dyno is good with load, never want to rev an unloaded engine. As we know, that's when they try to fly apart. Might watch ex temp, use heat gun, big cooling fan. If exhaust port is around 750 she's at operating temp. Over 900 or so too hot.

In my opinion its best to not idle fresh rings for longer than it takes to get oil circulating. To get a good ring seal and seating requires the rings to immediately be loaded and pushed against the cylinder walls. Especially important with the narrow low tension rings. Start it on the dyno and bring revs up under a little load. When up to operating temp do some pulls. No lugging and no over revving. The first few minutes are the most important in getting a good ring seal.
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Re: KH Piston Clearance

Postby EKHKHK56 » Wed Mar 21, 2018 2:31 am

The rings are best seated with on throttle loading, and off, reverse loading. A few heat cycles won't effect that, and with only .001" to .0015" wall clearance I feel better about letting the parts get to know each other gradually before doing full throttle runs. Idle with blips, motor is warm within couple minutes. Cool down...a few times. I use the newer Hasting Rings with 3 piece oil ring and modern Compression rings. I don't have a dyno, your method sounds fine also. Loading - unloading the engine is key. Cast rings are pretty forgiving and come back to life naturally to a degree, by loading, unloading after cyl repair unlike ones with Chromium or high Carbon steel construction.
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Re: KH Piston Clearance

Postby wz507 » Fri Sep 28, 2018 5:50 pm

GuS wrote:wz507.
The Enfield pistons are beauties..
I would really appreciate if you took your time to measure the piston hot/cold. Front to rear top and bottom of skirt. Sideways also, but since the sides are shaved off, that would only make sense to measure on just below the oil ring.

The +0.060” OS Wiseco pistons were measured on the fore and aft sides just below the oil ring groove and at the bottom of the skirt and found to have a taper of 0.003”. Piston and cylinder were equilibrated in an oven for 3 hr after which a Fluke meter indicated a temperature of ~ 250F. The piston dia increased by 0.0065” at both the top and bottom, and the cylinder grew by ~ 0.0031”. The coefficient of thermal expansion (CTE) for 2618 Al and grey cast iron differ by ~ 2:1, so the measured values are reasonable, with both materials expanding in ~ a 2:1 ratio. If the temperature were increased further, the differential in size should continue occurring at ~ 2:1 ratio, and of course with increasing temperature the piston/bore clearance decreases since the piston is growing at twice the rate of the cylinder.

Whereas the above measurements were made at steady state the issue in the real world is that our motors don’t run at steady state, i.e., the piston and cylinder are not equilibrated at the same temperature. When you hold the throttle wide open the piston heats up much more quickly than the bore which in turn decreases clearance and is a theme common to many seizure events.

In my opinion the above values suggest strongly that the piston manufacturer recommended clearance of 0.0025” is not viable (of course I was never going to follow it anyway).
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Re: KH Piston Clearance

Postby sportyparts » Fri Sep 28, 2018 6:36 pm

I have a couple sets of K pistons at +.010 over and one set at +.060 new for $100. and a bunch of N.O.S. K ring sets for $30. a set. 714 879-0500
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Re: KH Piston Clearance

Postby Noot » Wed Oct 03, 2018 12:03 pm

K and KH pistons (cast) I give +.002
Ironhead XL +.0035-.004
If your bore size on K/KH/XL is nearing the .060/.070 over, I give 'em another 1/2 thou....
Never had a problem. Break it in nice. ;)
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Re: KH Piston Clearance

Postby Coolbreeze » Tue Oct 30, 2018 5:42 pm

For my KHK, I am told that cast pistons should have 0.004" clearance (0.002" on each side). Bruce at Enfield Racing tells me his forged pistons should have 0.005" clearance.
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Re: KH Piston Clearance

Postby EKHKHK56 » Tue Oct 30, 2018 6:13 pm

I run the cast pistons at .0015" to .002" clearance fresh. They don't grow that much, with ez break in.
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Re: KH Piston Clearance

Postby EKHKHK56 » Thu Nov 01, 2018 2:58 am

From my Service Bulletins. K piston .001" to .0015" side clearance. I would go .002" on the long stroke KH & KHK just because. with Cast Factory Type Pistons.
K Service Bulletin April 1952
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