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Low Compression HELP!


Guest Anonymous

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Guest Anonymous

A while ago I emailed the list about my woes of having to rebuild my engine for the second time in a year. Well, I ran it this time with a tiny single barrel solex and 1600 intake manifold. This helped break the engine in nicely. Unfortunately, the low compression in the rear two cylinders problem has come back... Less than 1000 miles after rebuild.

Front two cyclinders are pushing 120 psi Rear two are 70 psi. We found out today after getting my big 45mm sidedrafts on. Couldn't make it run worth a damn.

If anyone can shed any light on this problem I would dearly love to hear from you. At WOT it goes like a freight train. Pulls very good for such low compression. But only after the secondaries on the carb are open.

The potential is there, and I know I will love this car sooo much more when the compression problem is fixed.

Please help!

Lee Johnson

Kelowna BC

1972 2002

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Guest Anonymous

More details on your 'rebuild' many differing definitions of a 'rebuild'

What was done with the cylinders, new pistons? new rings? what kind of rings?

Were the valves replaced and properly seated? what kind of head gasket, what kind of head studs or bolts?

What camshaft are you running? What are the valve clearances set to? was the compression done hot or cold?

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Guest Anonymous

What was done with the cylinders

Cylinders were honed, but not bored.

new rings? what kind of rings?

No new pistons, rings are standard iron rings. Not the total seal rings.

Were the valves replaced and properly seated?

Valves were not replaced, and were properly seated. The headwork was done by a local head builder who has lot's of experience with BMW heads.

What kind of head gasket,

Headgasket is a standard gasket material with metal outer. Nothing fancy.

What kind of head studs or bolts?

Stock head bolts.

What camshaft are you running?

Camshaft is a Dr. Shrick 283

What are the valve clearances set to? was the compression done hot or cold?

Valve clearances are set to .007 and compression was done hot.

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Guest Anonymous

indicates they're out of spec. Sorry to be the bringer of bad news, but given the other costs and the PITA of a rebuild, the costs of new pistons and punching the block is pretty minor.

I wouldn't recommend the Ate' rings, metalurgy has substantially improved in the last 35 years. People I respect speak highly of Deves and my experience correlates with their recommendation.

It doesn't sound like you had your rod's reconditioned, either. . .

Live with the engine if you can, otherwise, do it over and do it right the second time.

Sorry,

John N

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Guest Anonymous

John,

Thank you for the words of advice. I have a few questions though...

What is involved in reconditioning the rods?

Is there anything else I should be aware of when doing another (3rd) rebuild? I want the car to run properly, with good compression in all four cylinders. Heck I would be almost happy if all four cylinders were at least even.

Thanks,

Lee Johnson

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Guest Anonymous

1) Punch the block for oversize pistons

2) If you have an E12 head, oversise 9.5:1 pistons go pn eBay for about $250-$280

3) Take the crank and rods to the same machine shop that did the block and have them check the bearing surfaces to make sure they're in spec

4) Have the rod's reconditioned at the same shop. They'll check each end for round and make sure all rods are the same length.

5) If you have the money, have the moving parts balanced.

6) When you put it back together, spend the extra $ for Deves rings.

7) A new oil pump is an excellent investment at this time too.

8) Ditto for a new timing chain

Cheers!

John N

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