3.0 heads... Easy big valve convo for a 2.5?

There's a lot of if's and but's in my mind at the moment but I think it might be do-able.

It would be a hell of a peaky engine. If you were going to do it you'd have to consider using the 3.0 intake pipes and runners and GTA cams as well. [] [LOL]

So, valve sizes according to an ebay shop are:

2.5 inlet:

Head Dia: 33.50mm - Stem Dia: 7.00mm - Length 102.00mm

2.5 Exhaust:

Head Dia: 29.00mm - Stem Dia: 7.00mm - Length 102.20mm




3.0 Inlet:

Head Dia: 35.60mm - Stem Dia: 7.00mm - Length 102.00mm

3.0 Exhaust:

Head Dia: 31.10mm - Stem Dia: 7.00mm - Length 102.10mm


The heads are listed as different part numbers which raises some questions regarding compatability.

As the 3.0 is a larger bore, is the combustion chamber too big?

Will this affect the compression ratio?

As the 2.5 is a smaller bore, is there enough room to accomdated the larger valves?


The last question I think I can answer myself as I have a pair of 3.0 heads and I think I have a spare 2.5 piston and liner. Problem is I don't have a 2.5 head to compare it with.

Discussed it over on 156.net. We think it'll lower the compression ratio... Bummer.

it will only lower CR if you fit the 3.0 heads to a 2.5.... the dynamic CR depends on the cam duration and is only applicable at low rpm...

yeah I know. That was the whole point of starting the thread.

see if the valves can be fit to a 2.5 head.... and get the head ported, 3.0 valve seats fitted etc...