M20 Rebuilding Noob Questions

[XW70]

So Im going to rebuild the spare eta engine I have and mate it to the 731 and I electronics for some cheap performance (m in Australia so 731 is more common than the 885 and works compression wise). Im going to do bearings/seals/gaskets/rings and the like so I have a fresh engine, but Ive got a few noobie questions for those who have built before as its going to be primarily D.I.Y. and on a student budget:(

1) How will I know if the block needs to be decked/honed/machined. Obviously i can look at cross hatching and the lip around the edge of the bore but any other secrets? the bottom end is around 300,000km old. And could other parts like the crank need machining?

2) Should I replace pistons at this age? any tell tale signs to look for?

3) Are all rockers on M20 heads the same (200,731, 885)? can they be interchanged, cause if so then ill have lots to choose from and can pick the best of the bunch

4) How difficult is setting up tolerances and measuring them? Im aware of products like plastiguage but to what depth should I be doing this on a relatively modest m20

I have been reading up on engine building but just thought some m20 specific advice would be helpful, Cheers

[jlevie]

The odds of the block needing the be decked is pretty low. But a check with a precision straightedge will tell for sure. The head should get at least a minimal (0.12-0.13mm) surfacing cut.

At that mileage the chances are pretty good that the cylinders are past the wear limit, even if crosshatch marks are visible. They need to be checked with a bore gauge and if out of spec a rebore and oversize pistons will be needed. If within spec the cylinders must be crosshatch honed. If you skip that step the new rings will take a long time to seat, if they ever do. The pistons can be reused if the ring grooves aren't worn. The mileage is enough that the wrist pin bearings may need to be replaced (or replace the rods).

The crank journals need to be checked for wear and you need the dimensions of the journals to pick the bearings that will result in the right clearance (see the Bentley or TIS). If the crank is okay, have the journals polished.

The rockers are same on all of the heads. Check the cam for wear and if worn it can be ground back to shape or will need to be replaced. The valve guides in the head may need to be replaced also.

Do not have the block hot tanked. That will destroy the intermediate shaft bearings, which are a royal pain to replace (and hard to get). The measurements must be done with instruments that can read to 0.002mm. They are pretty expensive and if you don't have access to the instruments, have the shop that will do the machine work do the measuring. In my opinion, Plastigauge is good for a sanity check, but not accurate enough for determining bearing clearances.

While not strictly necessary for a street engine, balancing the rotating parts makes for a smoother engine that will last longer. I happen to have a race shop nearby that has the equipment and I'll have them balance the crank & flywheel. Then I balance the pistons and rods as all that requires is a scale, die grinder, and time.

If you rebuild the engine with care it deserves the result will be as good, or better, than a factory fresh engine.

[XW70]

Thankyou so much for that information jlevie, Ive got access to most of those measuring tools through friends so ill get onto that.