rebuild a 420
 

howdy guys..I have a 1987 420 with 210k miles..it is smoking at idle as described is my earlier posts..so I decided to pull heads and have them rebuilt..question now is .. is it terribly hard to put rings/brgs/seals in these motors ?.... is there parts that should be replaced if i go that far that might be overlooked?..anyone that has pulled one of these motors down want to reply would be appreciated ..thanks

Vavle job time. Unless someone roasted the engine, the bottom end looks like new. You have the aluminum block with siliconized cylinder bores, so unless it was grossly overheated, they will be nearly unworn at this milage (they wear at 1/5 the rate of high nickel cast iron, and this design usually goes 500,000 in the iron version without ring wear....).

You may in fact only need valve stem seals, not even a valve job.

Peter

I like you Peter
 

As far as I can tell..the original owner has never had any major problems with overheating ... the motor runs smooth and of course no bad signs of abuse..Ijust replaced the timing chain, guides and tensioner and under the vlv covers were very clean with just a light brownish tint ....I wonder if the valve seals could be an in car thing.....thanks for the info .

Valve stem seals get old and hard and fail to seal, usually last 100,000 miles or a little more. Can result in using a quart of oil in 1000 miles easily. Replacement doesn't require removal of the head, so I would do them before jumping into anything else. If you have compression leakage past the valves on a leakdown test, then valves need work, but not otherwise.

Be aware that on the earlier aluminum blocks, the threads may pull out in the block when you take the heads off, and it requires a rather expensive jig to install TimeSerts or Helicoils prior to re-assembly.

Meaning, of course, that if you don't have to remove the heads, leave them on!

Peter

I thought i read somewhere that you need a special spring compresser..is that correct?.is there anything i need to do while i am doing the seals?..... thanks for the info..most helpful to say the least

Replace the plastic fittings on the oil tube. They will be VERY brittle anyway, and will most likely get broken while you are changing the seals. This will oil starve the camshaft and ruin it and the rockers once you start driving it!

Cheap insurance.

Start it up with the valve covers off and verify that you get oil dripping off the tube onto all the rockers after you get the new seals on.

Peter