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Allan G
Allan G
West Yorkshire
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Lightning head skimmed
#797548 02/04/20 10:19 pm
Joined: Oct 2017
Posts: 561
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Nick H Offline OP
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A head I'm considering using has been skimmed on the head gasket surface.
Quite a lot I would say leaving barely a step. The valves look good though.
[Linked Image from i.imgur.com]
I think it's an early small intake head which I read many prefer.
Could someone confirm that?
Intakes measure @1.1" and are parallel.
[Linked Image from i.imgur.com]

Will this head be useable with the skimming?
Boost my compression some anyway but I'm not sure I want that.


1966 BSA Lightning
(2) 1967 Triumph "Choppa"s
1974 Indian ME125
1960 Harley Servi-Car
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Re: Lightning head skimmed
Nick H #797553 02/04/20 10:46 pm
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If you're worried about the compression use 2 or 3 barrel base gaskets.
Each will give you about 5-8 thou. Or have a thicker head gasket laser cut
there are plenty of firms around that can do that.
That head will be fine, some didn't have much of a step on them anyway.
It looks like the ports have been opened a bit so you may loose some of
the small port advantage but if you're not racing it, why worry?

Re: Lightning head skimmed
Nick H #797554 02/04/20 10:54 pm
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For normal use the head will be fine, plane as little as possible but also check the barrel for flatness as these can go out of shape too. Then make sure your head studs thread all the way in by hand without washers, thus proving that when your tightening your bolts down your not just screwing against the bottom of the blind hole.

All the 68-466 heads did not come from the factory with a balance pipe, this was fitted with the larger port heads. However that port looks played with. There should be a taper for the first half inch don the small port head, also the port doesn’t look central within the mating surface. They should start at 1 1/8” and taper within that first inch to 1 1/16”. Then be parallel from there. It also looks like 40mm or bigger valves have been fitted, they should be 38mm. Some prefer the bigger valves but on a small port head I find you loose something at bottom end and gain a tiny bit at top.


In a nut shell. You could probably put some flat top/low compression pistons with it and it be fine. But personally I’d look for another head.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Lightning head skimmed
Nick H #797555 02/04/20 10:56 pm
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It also looks like exhaust spigots have been fitted. Which will mean custom pipes are needed.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Lightning head skimmed
Nick H #797556 02/04/20 10:58 pm
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A65 heads can take quite a bit of planning......ive seen plenty which still have enough valve clearance left when they have been planed so much that the step is gone and the machining actually extends right out onto the bottom of the lowest fin ..........if you are not wanting to increase compression use a thicker head and or cylinder base gasket.

try to get your machinist to agree to do a MINIMUM clean up (good luck) if you are relay confident in his ability tell him you want a light "witness mark" left, that should stop him just hoeing into it ........i would use the one you have after planning just going by the pic with a min clean up done.


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
Re: Lightning head skimmed
Nick H #797563 02/05/20 12:17 am
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You will need to drill the rearmost bolt holes to 3/8 clearance if you use a later barrel too.
As for head bolts/studs, use the 71/72 type they have greater threaded length into the barrel,
not essential but a nicer job.

Re: Lightning head skimmed
Nick H #797594 02/05/20 2:33 pm
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Nick H Offline OP
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Thanks. I think there is a slight taper at the first half inch or so. My measuring skills are not great.
The port does look off center.
Are people suggesting I have this head skimmed even more and if so, why?
Alan - good eye on the exhaust port stub! Kind of a chunky weld there.

I've got another head coming for consideration.

Last edited by Nick H; 02/05/20 2:33 pm.

1966 BSA Lightning
(2) 1967 Triumph "Choppa"s
1974 Indian ME125
1960 Harley Servi-Car
Re: Lightning head skimmed
Nick H #797611 02/05/20 5:50 pm
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i would use that head, it looks fairly fresh, check the mating surfaces with a good straight edge , then with engineers blue on the barrel joint, that will show if its true or not.
With provisos, all threads are OK? Valve seats not too pocketed? Guides in decent nick?
CR is easy to adjust with gaskets, check the combustion chamber volume with some oil in a calibrated burette on trial assembly , do the math , then decide if its too high or not .


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod

Moderated by  Allan G, Jon W. Whitley 

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