Engine Block - rebuild - standard size pistons

ae1969 · 10-05-2002, 01:27 PM

For a project I am doing...

I found a used block with about ~100,000 miles

The pistons were pulled out and the cylinder walls were in great shape. The machine shop says that it was still within spec?

So he recommends keeping the standard size pistons.

How commmon is it to have a high mileage engine and not need to re-bore and use oversize pistons?

What is the minimum I should have the shop do to this block before we begin reassembly?

Thanks.

drew1 · 10-05-2002, 02:34 PM

The silicon alum block wears pretty good. It takes special equipment to oversize the block, even carbide boring tools are eat up, have to use diamond, and special honing & kind of a lapping process to leave silicon crystals exposed. The block can be bored & sleeved like they used to do with Chevy Vegas.

drew1

ae1969 · 10-05-2002, 04:05 PM

thanks for the reply drew...

The measurements were all near spec.... what should I tell him to do to the surface?

oh the other question......I am replacing the rod bearings....but what about the mains...... I have seen lots of guys rebuilding blocks and not touching the mains?

Whats your opinion on this for these engines.

Thanks.

drew1 · 10-05-2002, 04:59 PM

To answer your question about surface finish, I'm ignorant. The Workshop Manual says lapping with silicum (I translate to silicon carbide lapping compound) and oil mixture. This is done with a hone, with felt inserts instad of stones. The Sunnen CK10 stops after 80 sec with a dull finish. I guess it means break the glaze so the rings will seat.

I would go ahead & put new mains in , too, while I had it down. On this board, some have said the rod bolts are stretch bolts & should be replaced. The 924 are & I'm waiting on a set of bolts. I don't know if 944 is or not but some replace rod bearing inserts with the motor in the car & how do you press in & out rod bolts?

If you can get Britwrench to reply, he would have better answers.

drew1

AFJuvat · 10-05-2002, 10:14 PM

Sounds like fun! If you take your time you can stretch out the expense and have a lot of fun with a rebuild. By the time you are done, you will know these engines up/down/left/right

If you are within spec and have no warpage (measured at a minimum of three places, or better yet a cylinder bore gauge), you can get away with not having them bored and re-coated. with a well maintained engine, it isn't uncommon to have a cylinders in great shape after many miles.

If you have it all torn apart, go ahead and do the mains too - it adds little to the cost and will give you piece of mind. Do have the crank journals checked for out-of-round or taper - again, on a well maintained engine, this should be within spec.

Replace all con-rod bushings, nuts, (the con-rod bolts are part of the con rod and cannot be replaced seperately - you do have to replace the nuts.

Take the opportunity to have the block completely cleaned - the water jacket does tend to collect crud/scale from years of anti-freeze immersion.

Clean all oil galleries with a plastic rod and a bronze or stiff plastic rifle bore brush.

Do the balance shafts too.

Have fun with it!

AFJuvat

drew1 · 10-06-2002, 06:16 AM

Alex,

You're welcome for thanking me. Maybe I kept the post up enough for AFJ to see it.

drew1

ae1969 · 10-06-2002, 01:02 PM

Thanks AFJ and drew...........

I am tearing down my clutch as we speak so I am working on this block as I take a breather from laying under the car.....

This should keep me busy for the next month or two!! (have some time to kill, wife is back in school and studying at night....so I get kicked out to the garage .... poor me

)

Thanks will keep u upto date.... and will be begging for help as I go.

Mike B · 10-06-2002, 04:08 PM

Hi Alex...When I stripped my block down it was in good shape except for a ring ridge at the top of the cylinder...Natural wear...

Anyways hoped I could simply insert the standard pistons and save myself $2200 Cdn...I could have have the cylinders lapped my the machine shop but they advised me to go with the oversize pistons due to the the ridge...The concern was the new rings would run into the ridge at high rpms and POSSIBLY CAUSE castrophic damage...YIKES...I went with the advice from the machine shop and a master Posrche Tech ( Marc at www.aplineautowerks.com ) ...It was expensive but for me it was the way to go...After 150 miles it humms like a angry kitten....

Alex, you may get away with lapping (the final stage of the cylinder procedure listed in the manual) and have no problems...A knowledgeable machine shop should know...Best of Luck brother!

ae1969 · 10-06-2002, 06:44 PM

Hi Mike,

No ring ridges for me! ...thank the almighty for that one....!!! The the measurements they took were dead on.........the guy kept asking me? How many miles on this block????

It looks like I will be able to slip them right in...! I'll take a little lapping over boring any day!

Hey mike, what is the procedure to break in the engine anyways???? As far as I know all new porsches are already broken in? So how are you going abouts breakin yours in?

Maybe in the summer ....I will get to ride your kitty!