7AG - a few questions

[eightsixboy]

Hi all,

I have been thinking about going a 7AGE for an "easy" torque increase with my smallport instead of going turbo, main reasons being cost and less cop bait.

I have read the threads on here about building a 7AG but there isn't any real data (so to speak) or info on exactly what C:R you get with stock 7A pistons, and a lot of the info is conflicting. Some say under 9:1, some say around 9.5:1 etc etc

From the threads on here I have gathered that when using stock smallport pistons you have to use a headgasket over 1.0mm or the C:R will be upwards of 12:1 or higher.

This has me thinking, why not just use a stock 7AFE bottom end with stock 7A headgasket (0.6mm?) and shave the GE head and/or deck the 7A block?

The only issues I can see with this might be, insufficient piston to valve clearance and that the pistons aren't designed for the head.

Thoughts?

[eightsixboy]

35590

After doing some calculations it looks like roughly 9:1 C:R with stock 7AFE pistons and headgasket, stock unshaved 4AGE head.

So assuming a head shave of around .5mm to .75mm you would end up with around 9.5:1, not ideal but better then a stock big port. would be higher still if you decked the block a little.

Would anyone know if there would be valve to piston clearance issues when using 7AFE pistons?

[Matt]

From my experience you will see around .5 of a compression increase on the piston that you would use in a 4a. So a smallport piston is around 10.5:1.

[Jimmee1990]

Use the smallport pistons, compression won't be unmanageably high. The valve cut outs match the head properly, as well as the crowns properly matching the chamber shape.

Even a standard MAP ECU has no issues running engines with 12:1 compression.

[Matt]

I've found allot of people will use the following when building a 7age

7a crank / block
After market 7a rods (stock can be used with new rod bolts etc)
7a Main / Conrod Bearings
4a Pistons ( 16v Big/Smallport, 20v Silver/Blacktop) - Matching the head used in most cases
4a Head (16v / 20v)
Cams to match head

Timing works out to be about half a tooth out due to block height differences, some opt for a re-cut key way, but adjustable cam gears are a much better option IMO as it gives you the ability to dial in after market cams properly.

A 7a headgasket is a great choice to use as it is much thinner than the 4a ones.

If the motor (Piston / Head combo / Cams) isn't interference as a 4a, then it won't be as 7a either.

[rthy]

will the 4age small port pistons fit with the press or will the floating pin need to be used?

7afe rod/pistons are 20mm bore - press fit pin and .5mm metal (?) head gasket
smallport 4age are 20mm full floating wrist pin 1.2mm head gasket (I think the 7 rib 4age also has 20mm pin?)