advice one bore job

[71datsun240z]

how much should i bore over my cylinders for my triple dellortos ill be getting soon they are 40 mm carbs i can get the borejob done for free but i dunno how much over i should go

[2ManyZs]

The amount you bore the block is dependant on how out of round the cylinder bores are to begin with. Have someone check them with a micrometer and only take it as far as is needed to give you a good true bore. You may only need to go to 20 oversize. Don't go to 40 over just to get the extra little bit, if in the future something goes wrong and you need to bore it again it will not be possible. The more you take it out now will only hurt you in the future. Once the bores are at a certain size they can't be taken out any more due to the amount of material in the block that has already been taken out.

The bore size is not as critical as the type of head, pistons, cam and related items when you decide to run the Dell Orto's, Webers, Mikuni's and the like. However bore size will make it necessary to adjust all the other components to compensate for the change in chamber volume, and compression ratio. Find out how much needs to be taken out to true the block and then match the rest of the components to work with that.

[Dave Sommers]

As usual, the advice of 2many is dead on.

The horse power of the z comes from the head and not the extra little bit you get from boring larger. Only do it if you have to.

[Ivan]

just adding on to the other posts, get it checked but it will be around 2mm (42mm)

[shock96]

I would not go over 2mm on the overbore unless you have an F54 280ZX block and then 3mm is possible, but I would still stick with the 2mm.

You still have to get pistons for that as well. I am not sure that they make any in 88mm. You might have to have them made. 2mm is 0.80 over if I am not mistaken.

If you are going to have pistons made anyway, go 2mm. What the hell! There are a ton of used engines and blocks out there if you scrap yours.

Mike

PS: You also increase static compression slightly with an over bore.