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3.9 head 

 

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 Post subject: 3.9 head
Posted: Mon Aug 03, 2009 8:36 pm 
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I neet to change the head on my ed 3.9 but good 3.9 heads are hard to find
I believe a 4l head on a 3.9 lowers the compression ratio
can I machine a 4.0 head enough to keep the same ratio or even raise it
I am runing on gas so increaseing the cr would be good
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 Post subject: Re: 3.9 head
Posted: Mon Aug 03, 2009 10:58 pm 
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If you plane 25 thou off a 4.0L head and fit it to a 3.9L block, the compression ratio will be 9.0:1. The combustion chamber will be reduced from 64cc to 60cc.

I had this done to a reco'd EF head & fitted to my old 3.9. The calculations were done using computer program (by a cylinder head reconditioner).

 

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 Post subject: Re: 3.9 head
Posted: Tue Aug 04, 2009 12:47 am 
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I have just done this to my 3.9.. but they only shaved 10 thou off, and ported it. I am running the AU metal gasket. Head's on, but it's not finished, have to finish off timing case crap. Is there a way to work out my CR for this combo?

 

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 Post subject: Re: 3.9 head
Posted: Tue Aug 04, 2009 12:04 pm 
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Without working out the exact ccs in the chambers, it won't be accurate. More of a guesstimate. The computer program also took into account the bore diameter, piston clearance, deck to piston clearance, etc. With the AU gasket and 10 thou off the head, you might be around 9:1 (on a 3.9 block).

 

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 Post subject: Re: 3.9 head
Posted: Tue Aug 04, 2009 7:26 pm 
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Thanks for the info looks like that will solve my problem
Another questoin on au head gaskets
1 As I understand I cant use an au vrs on a 3.9 because the timing cover top seal is bifferent
2 I was told if I buy a 3.9 vrs I will get an au type head gasket anyway as the gaskets are interchagable, is this true,
3 or do I have to use an head gasket with 3.9 timing cover top seal
Thanks
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 Post subject: Re: 3.9 head
Posted: Wed Aug 05, 2009 10:43 pm 
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the head gasket doesn't seal the timing cover on ford ohc 6's.... the rocker/timing cover has a different seal but the covers are also changable given you have the matching seal. the other difference maybe that the later models come with the rubber seal and the older ones the cork gasket for the timing cover top seal.have noticed this as i've bought a few... heh

 

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 Post subject: Re: 3.9 head
Posted: Thu Aug 06, 2009 12:02 am 
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Not to hijack, but will EA 3.9 rocker gear work in ED 4.0 head? Reason I have to ask is the ED HLA's are smaller, 11mm, whilst the EA HLA's are the larger 14mm.. and since I was running a Wade 1636 in my EA head I had doubled up on shims.. therefore I don't have enough shims for the ED rocker gear.

To add to it, I may have lost some shims since 8/12 rockers got smashed in my head, and I haven't got round to counting them all up.. If I'm lucky enough to have the full 24 x 14mm shims, I could run my spare 3.9 rocker gear that I grabbed the extra shims from to run the cam in the first place. :shock: If not, I have to buy / find 12 x 11mm shims.

 

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 Post subject: Re: 3.9 head
Posted: Thu Aug 06, 2009 1:54 pm 
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Yes, the 3.9L rocker gear will fit and work on the 4.0L head OK (pre-AU). I used to have 3.9 rocker gear on an EF head without any dramas. I thought the 11mm HLAs came out with the EF/EL engines...unless they started using them earlier (like the ED engines that came out with EF crankshafts).

 

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 Post subject: Re: 3.9 head
Posted: Fri Aug 07, 2009 12:44 am 
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Good stuff, I salvaged my busted rockers, and counted 24 of the 14mm shims. Phillip Head services are now acid bathing my spare EA rockers, so when I finally get this donk back together, I can run my cam again :D

 

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