Cylinder head and inlet manifold

[Bandit1200]

I'm about to replace my heads with 20 thou skimmed reconditioned units.
I've read the BLMC manual with it's recommended sequence of tightening 1 head, torquing the inlet manifold to both heads and then torquing the 2nd head
Then there is the view expressed by some on the SOC technical section that the heads should be torqued and the manifold made to fit.
Both seem logical but I have a question, If the V is 90 degrees and 20 thou has been shaved off the heads, should a like amount be removed from the inlet manifold/head interfaces to compensate.


HELP!!

Steve

[Stagdad]

Probably Steve,When I fitted a new inlet manifold it was unskimmed and I had 15 thou taken off to match the skimmed heads,I came up with this figure by loose fitting the manifold with only the bolts and not the gaskets measureing the gaps under the inlet then measuring the gaskets and minus a few thou for compression then taking one from the other came up with 15 thou to come off the inlet.however my heads were still on so there may be a bit more play if you have everything loose to start with and you could possibly get away with it but my guess is it will need matching with the heads.


Cheers Steve

[dasadrew]

.... or use the thick head gaskets?

[V Mad]

Then there is the view expressed by some on the SOC technical section that the heads should be torqued and the manifold made to fit.
Both seem logical
Steve

I dont know who said that and why but that does not seem logical to me at all. When the heads are fitted, there is a small amount of clearance between head and fixings (studs and bolts); therefore if the heads are not fitted parallel when tightened down, they can not mate properly to the inlet manifold except by luck! IMO the manual is the correct procedure to follow.

By the way, do you know whether or not your old heads and/or the inlet manifold were already skimmed?

Skimming of the inlet manifold may be necessary if both heads are skimmed and refitted with same thickness gasket, but 15 thou is not a lot and some relieving (opening up) of the fixing holes may be sufficient to enable bolts to be fitted. If standard head gaskets were fitted then using thicker gaskets will avoid any problems.

[Ian Durrant]

I dont know who said that and why but that does not seem logical to me at all. When the heads are fitted, there is a small amount of clearance between head and fixings (studs and bolts); therefore if the heads are not fitted parallel when tightened down, they can not mate properly to the inlet manifold except by luck! IMO the manual is the correct procedure to follow.

By the way, do you know whether or not your old heads and/or the inlet manifold were already skimmed?

Skimming of the inlet manifold may be necessary if both heads are skimmed and refitted with same thickness gasket, but 15 thou is not a lot and some relieving (opening up) of the fixing holes may be sufficient to enable bolts to be fitted. If standard head gaskets were fitted then using thicker gaskets will avoid any problems.

I really disagree that opening up of the bolts holes is the right way to go. That will simply result in a mismatch of the manifold to inlet port. Tuners spend time matching these to improve power over the factory tolerances and so simply correcting a head skim by opening up the bolt holes is not the best plan if you care about power (and maybe even fuel economy).

[V Mad]

I really disagree that opening up of the bolts holes is the right way to go. That will simply result in a mismatch of the manifold to inlet port. Tuners spend time matching these to improve power over the factory tolerances and so simply correcting a head skim by opening up the bolt holes is not the best plan if you care about power (and maybe even fuel economy).

Ian, I was just trying to be helpful/pragmatic. Production cars often have ports that are irregular in shape and the port sizes differ considerably due to casting tolerances. Ports that may be misaligned by 15 thou are, IMO unlikely to affect perfomrance noticeably. If you are looking for precisely matched ports you can spend a lot of money or time for little gain.

[GDPR]

More importantly Ian, the opening of the bolt holes will misalign an already very marginal cooling aperture, and reduce coolant flow to a significant degree, or leave insufficient gasket area aligned and clamped that could could cause leaks under pressure....

[elvimto]

I have the skimmed heads,thick head gaskets and i made thinner manifold gaskets after I had a water leak and did not want to loosen the heads back up.When the heads are torqued the intake manifold is really tight I almost did manage to get all the bolts started but it was a struggle so I gave up on that and made gaskets at work which were I think about .020 thinner used Hylomar to seal no problems since.

cheers Stuart