Head Gasket and Bore Size

[Steve]

Hi,

Just wondering if anyone can tell me what the bore of an RB25DET is - exactly. I was under the impression it is 86mm, but after investigating headgaskets, it appears the Japanese companies make them in a range of sizes, some 88mm, some 87mm, some 87.5mm but none 86mm bore size.

I am planning on decompressing slightly, so I can wind the boost up with stock internals (Sydneykid, I know what you would be thinking!). I have decided to give this a shot, see how long the engine lasts and then drop in a 3L when/if I blow it up:D

Cheers

Steve

[sidewaymambo]

steve..exacly what I have in mind..hahaha...

But I decide let it run as stock compression and see what happen..

[Steve]

Its nice to know I'm not alone.

Have just checked Meggala's site, states the bore IS 86mm, so why dont any of the Japanese gaskets have 86mm bore size.

Do they have some sort of 'squish' factor reckoned in?

Very confusing

[sidewaymambo]

at least I don't think decompress the engine = install forgie, so reason for 87, 87.5,88 instead 86..

http://www.tomei-p.co.jp/_2003web-catalogu...B25_gasket.html

another question is most forgie I can find are 87 and 88...and I just can't find 86

http://www.tomei-p.co.jp/_2003web-catalogu...B25_piston.html

[Steve]

Thanks for the first link. I think it may explain the magical gasket/bore indiscrepancy.

It gives different compression ratios when different size gaskets are used on an 86mm bore, eg the 1.5mm 87mm bore gasket gives a 8.2:1 CR when used with 86mm pistons, but the 1.5mm 88mm bore gasket gives a 8.3:1 CR. Looks like the 1.2mm is the go, gives an 8.6:1 CR - just above RB26 CR, and its the cheapest of the lot.

It is interesting that they have a gasket that gives an RB25DE a 8.9:1 CR - might interest those contemplating a DE to DET conversion!

Cant make any more sense than you of the second link, but at 2400 a set, I think they are a little pricey? Certainly out of my league.

[Sydneykid]

Hi guys, the different diameter of bore in head gaskets is to allow for oversize pistons. The thickness of the headgasket is what alters the compression ratio. The thicker the headgasket the more it lowers the compression ratio.

You should always use the right diameter headgasket to suite the bore diameter. I certainly would not want combustion occuring between the head and the block, which is what happens when you use too big a diameter head gasket.

As for using a thicker head gasket to lower the compression ratio, well just don't get me started on that old world shortcut. If you truely have to lower the compression ratio, then do it properly and enlarge the combustion chamber volume.

Hope that clarifies.

[Steve]

Thanks Sydneykid, yeah I knew head gaskets would hit a raw nerve:D The only problem seems to be that the aftermarket gaskets dont come in 86mm bore, very confusing.

Could you tell me how much it would cost to get the combustion chamber resized? Also what sort of effect would this have on the flow of the head? I was under the impression that it can be dangerour to start messing with its (CC) design? I take it this would involve stripping the head down first?

The reason I am thinking head gasket is as a quick fix, they only cost $350 odd, and another $300 to lift the head. I do know people who have had good results from this sort of thing, and the engines have run well for years.

Cheers

Steve

[Sydneykid]

Hi Steve, try Advan in Sydney (ask for Peter) for 86 mm headgaskets. I would rather do the combustion chambers, good excuse to do the ports and give the head a polish at the same time.

What we do is gradually remove the squish zones at the outside of the valves, reduces the compression ratio and improves the flow at the same time. It needs to be done using a flow bench and with constant reference to combustion chamber sizing.

Regardless, I still am not convinced that you need to reduce the compression ratio. We have run 9 to 1 on engines with standard internals up to 260 rwkw, no problems, daily driver, more than 18 months and 30,000 kilometres, 4 or 5 drag meetings a year and 3 to 4 trips to the circuit a year. Not just one fluke of a car either.

My suggestion, don't take the head off, use the saved $650 on getting the tuning right.

Hope that clarifies my position.

[adam 32]

from my personal experience there isnt much to be gained as far as machining of the head goes (according to a headshop here in perth) so as sydneykid says invest the $$ in a better cause