Cylinder head / piston

Production K Models

Cylinder head / piston

Postby Monti » Fri Jun 05, 2020 4:28 am

I am new this forum but have some experience with other HD models (mainly Panheads).

I have taken the heads off a KH engine. There was a 1/12 " thick copper cylinder head gasket between the cylinders and the heads. Thanks to this super forum I know that KH engines do not have a cylinder head gasket and that the piston comes out of the cylinder at TDC.
To make sure that I put everything correctly together, I would like to know how much the piston actually comes out of the cylinder at TDC and how deep the recess in the head is above the cylinder bore. It would be really helpful if somebody could provide this information.

Thanks a lot in advance!

Kind regards

Monti
Last edited by Monti on Sat Jun 06, 2020 4:18 am, edited 1 time in total.
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Re: Cylinder head / piston

Postby thefrenchowl » Fri Jun 05, 2020 7:02 am

Hi Monti,

Welcome...

From memory as I have not a std dismantled engine to check...

Pistons stick up about 40 thou and the recess is about 120 thou, giving you 80 thou squish band. With 15 thou gaskets under cylinder bases.

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KHK
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You can play with base gaskets to give you more or less squish.

My 1954 KHK has an S&S forged steel 4" 5/8 crank and the pistons stick about 60 thou for 60 thou squish with thin alloy plates on the bases.

One could reduce the squish further to 40 thou like on the KR racers and I was thinking 20 thou would still be OK but have not tried it yet.

Patrick
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Re: Cylinder head / piston

Postby panic » Fri Jun 05, 2020 8:52 am

Is that copper gasket 1/2"? Decimal measurement please.
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Re: Cylinder head / piston

Postby thefrenchowl » Fri Jun 05, 2020 9:08 am

1/12, Jeff,

so about 80 thou, 2mm, that's a lot of wasted copper...

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Re: Cylinder head / piston

Postby LDB » Fri Jun 05, 2020 11:48 am

Concerning quench distance, I'm not sure that one could get by with .020 if there's any parallel with automobile engines. With them, .035 is the minimum practical to run without engine damage with engines having tight bearings and pistons and modest RPM. Unfortunately, the operating quench height varies in an engine as RPM and temperatures change and a motorcycle engine would be no different in that respect.
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Re: Cylinder head / piston

Postby Ferrous_Head » Fri Jun 05, 2020 6:40 pm

Limits on piston to head height will vary from engine to engine.
Sportsters and K's have a very stiff crank and they don't flex much. Try revving a 750 Norton to 8 grand and watch as the fire goes out.
(Pistons will hit the spark plug tips and close them up. I have seen this happen.)
Rods strech. Inertia is your enemy this time. The faster an engine revs the more stretch you will get.
The problem with setting a qench (squish) band is they are ineffective above 40 thou.
With something like a K model I would think you could go as low as 20 thou with no issues. If your above 40 there's no real point in bothering.
Sportsters in fact (stock ones) do not even have a qench area. And that's one engine that could REALLY benefit.
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Re: Cylinder head / piston

Postby Monti » Sat Jun 06, 2020 5:04 am

Thanks a lot for all your replies, especially Patrick for giving me his values.
On my engine the pistons comes out about 63 thou (1.6 mm) of the cylinder at TDC, but the recess in the head is only 102 thou (2.6 mm). The reason for that is that the sealing surface must have been machines after the heads were welded in the past. I think I will rise the cylinders with a little bit thicker base gaskets so that the pistons will be less exposed to the heat on the recess towards the vales and go for a piston to head clearance as recommended in the engine specifications (towards the 78 thou/1.98 mm for a more reliable and easy staring engine). Any thoughts on this from your side?
Is it possible to seal the gap between cylinders and heads just with aluminium paint or is it better to use a thin copper gasket?
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Re: Cylinder head / piston

Postby thefrenchowl » Sat Jun 06, 2020 5:56 am

Heat resistant aluminium paint only, thin/sparse full coat on both surfaces, let it go sticky then assemble.

But 1st, do hone both cylinders and heads on a dead flat surface with either valve seat cutter thin paste, sulphur paste, tooth paste, lapping paste or other fine abrasive.

lappinga.jpg
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Since there no copper to compress and give you an idea of torque, I fit copper washers between std steel washer and head. This gives me a bit of "give" like with a normal gasket.

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No torque value given, don't go mad.

Personnally, 1mm/40 thou squish seems good to me, the tighter the squish the better the burn...

Cheers from Patrick
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Re: Cylinder head / piston

Postby Monti » Sun Jun 07, 2020 3:01 am

Patrick
Thanks a lot for all the feedback and the informative pictures.
The hint with the copper washer is a good idea. I will make sure that the surface of the cylinder and head will be flat before I use the heat resistance aluminium paint as recommended. So based on your experiences I should go for a 1mm/40 thou squish. Do you know if this will make it more difficult to start the engine? I ask because the owner of the KH has a knee problem which makes kick starting more difficult for him.
Thanks again for all your help.

Monti
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Re: Cylinder head / piston

Postby thefrenchowl » Sun Jun 07, 2020 4:39 am

Hi Monti,

If it's not your bike, I'd go for 60 thou squish with thicker base gasket... You don't want an uninvolved third party testing my gut feelings!!!

These engines are low compression, so easier to start than most other Harleys.
I don't think the squish band has an Impact on the starting procedure.
Just make sure the engine is properly flooded (2 kicks with choke fully on and throttle fully opened) before switching the ignition on.
All engines have their own quirks. Mine on AP did start 3rd kick on the button with full advance, 1/4 choke and nearly no throttle. If it kicks back, it's too much throttle.
Now that it's supercharged, it's another story!!!

I've never like the big Linkert carb, I found it to be an antiquated pig to tune with my old KK and actually never ran one on my KHK. Always favoured early Sportsters DC and even Tillies/Zenith/early Keihin. Basically anything with a butterfly rather than a slide

Bye for now,

Patrick
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