DOHC on a SOHC engnie.

[two_68_510s]

Are you trying to justify a lathe?;)

[Mattndew76]

Are you trying to justify a lathe?;)

I wish I could get a lathe. But for now I can make these on the mill. Justify the purchase of a boring head maybe and a broach arbor....

This motor would prolly be ready for a test run if my commute to my man cave wasn't 50 miles one way lol. House shopping tonight...

[Mattndew76]

I was reading on the 1200 site about the A12 - B Head build thread and had seen where a person had posted that they went with larger pistons but offset the cylinder bores for 2 and 3 to match up perfectly with the combustion chambers on the head.


QUESTIONS:

How can you offset the cylinder bores for cylinders 2 and 3 with there being no play on the crank shaft to connecting rods?

Would the offset now be needed in the wrist pin connection within the piston?

[Byron510]

You'd definately have to change the crank if you changed the bore spacing, no small task!

Byron

[Mattndew76]

You'd definately have to change the crank if you changed the bore spacing, no small task!

Byron

Exactly.. I believe the guy who did this originally just went ahead and lined up the front and rear combustion chambers - cylinder and lived with the 4.445 mm offset chamber on cylinders 2 and 3. Since its a non interrupt head the 2.5 mm ledge must not have hurt anything.


I cant get any answers out of the Aussies. The one guy who is speaking for the OP on the site wants me to buy the kit they sell. I don't want to buy the kit. It looks marginal at best and I can design my own.

EDIT: If I bore over sized to a 78mm piston it takes the ledge away but still leaves the combustion alignment off by 4.445mm

[duke]

I would think that you could relatively easily reshape the combustion chamber with a die grinder and TIG welder. The spark plugs may not end up in the optimal position, but you wouldn't have any ledges to worry about.

[Mattndew76]

I would think that you could relatively easily reshape the combustion chamber with a die grinder and TIG welder. The spark plugs may not end up in the optimal position, but you wouldn't have any ledges to worry about.

So I looked a little bit more into the spacing. Me on my back looking up through the block at the heads alignment. I think I can leave everything as it but mill new valve relief in the top of the block. I can mill a 3.175mm head gasket and just force feed this thing air

[Mattndew76]

So i made a 3D rep of the head chambers to block cylinders. The inner 2 and 3 cylinders already have .100" deep valve relief cutouts. So I figured I might just go a little more with what I have to work with and mill my own relief in the block.

The cylinders 2 and 3 are .100" mismatched from combustion center and 1, 4 are .075" mismatched off center.

Red = Block

Gray = Gasket / head chambers

[Mattndew76]

Got 3 more jobs of Datsun parts and then I can thrown the block + Head into the CNC for some mod work.

Oredered:

*Main Bearings + Seals
*Rod Bearing
*Timing Cog Pulley
*Timing belt Kit ( Cog Idler, Tension Idler, Smooth Idler. )

Right now I am sourcing brass pipe thread plugs for the Engine Block and Head. I need to block those passages off.

Total Frankenstein.... With slightly off center combustion ha ha ha

[Mattndew76]

Crank shaft timing gear was waiting on my door step when I got home.

A12 Crank shaft drive post diameter is 1.125" Or 28.575mm

The I.D. of the Mazda crank shaft timing gear 1.065" or 27.051mm

Its made of some hardened steel.

I was curious if I could remake this part from 7075? I imagine with a .004" black hard anodize this would have plenty of wear resistance to the Rubber-Kevlar cog belt.

Key seat would have no play, so no torque wear would occur from that.

I am going to try this out.

[okayfine]

I am going to try this out.

[Mattndew76]

Im making this a non-interrupt valve-train. So if any fail in the timing system this will not damage the project. Just a "oops lets try this again."

Im excited to get this fired up. I need to fab up an engine stand for firing this up outside the car.

[mell00510]

Love this thread Matt!! Keep's me entertained on my saturday's at work, Keep up the awesomeness!

[James]

I love this whole thing - Can't wait to see it running

[510wizard]

Crank shaft timing gear was waiting on my door step when I got home.

A12 Crank shaft drive post diameter is 1.125" Or 28.575mm

The I.D. of the Mazda crank shaft timing gear 1.065" or 27.051mm

Its made of some hardened steel.

I was curious if I could remake this part from 7075? I imagine with a .004" black hard anodize this would have plenty of wear resistance to the Rubber-Kevlar cog belt.

Key seat would have no play, so no torque wear would occur from that.

I am going to try this out.

You might be able to cut the timing gear with a carbide bit or grind. How about turning the crank down .060 or a combo of the two? Seems to be a lot of work to make a new gear and have it anodized.