piston replacement for a 79

[mendozeal]

hey there ya'al
I popped the right side piston on my 79.. looks like the crank sprocket is worn and the chain skipped a link, boom.. middle of an 1800mile trip too..

a year later, I have the cylinders removed, and the piston size is the 81,96 e=9,2.
that piston is expensive.. at like 371$ from BMW, and must be ordered from Germany. the 8,2 is much cheaper, as is the 81,97 from the 1980's and on..

(I got a great looking head with valves on eBay!)

I found a used set of cylinders and pistons from an 85, for the same price of a single new piston.. will this fit without any further alteration?

any advice at all on how to source a cheaper piston and ringset would be sooo appreciated, even aftermarket sources and approiate sizes would be super helpful

and if anybody has some laying around, please pm me!

infinite graditude for this forum!

jeffrfsparrow@gmail.com

[Barry]

81.96 is the standard piston size and it is a grade A
81.97 is also a standard size and grade B

Pistons and cylinders were matched by grading them A, B or C

9.2 is the compression ratio.

Motobins have pistons from £75 but I don't know if they have them graded. You would have to talk to them.

If you measure the bore you might be OK with the 81.97 bearing in mind it's only 0.0004" bigger. There is no doubt it would fit in the cylinder it's just a case of measuring the running clearance which should be between .0012" and .002"  from new and a max. of .003" on worn parts.

With it being the smallest size there is probably not much call for the A grade piston as a replacement in worn bores.


Pistons also have a weight grade and should be stamped  + or -  so it would be good idea to have the replacement the same weight if possible.

[montmil]

Jeff, that looks nasty.

Thinking you're going to need more than a new piston and head.

There's bound to be a buncha metal in the crankcase, perhaps the oil pump, filter and passages and some other areas -so the pan needs to come off for a complete inspection and clean out. Even though the oil pickup in the sump has a perforated "screen" on it, it will pass bits large enough to make a mess later on.

Have you checked for bent pushrod/s? You might take the time to confirm the piston rod has not been tweaked in the impact.

There's really quite a bit of work to accomplish before it's time to refit the jug/s. As such, you could continue to watch eBay for a pair of useable jugs n' slugs.

Oh, and sounds like you'll need a full timing chain kit and sprockets, too.

Sorry for your loss. Best thing would be to take your time and do it proper. Lots of Airhead sources out there for parts with better than retail costs. Good luck.

[Luca]

Sound advice from Monte.

If you were looking at those conrod bolts and scratching your head, they take a 10mm triple square socket.  Avoid the urge to buy a cheap one.

[Justin B.]

The Nikasil jugs can be made to fit with some modifications, but can't remember exactly what is entailed.  I may have a matched set of 1979/1980 "slugs and jugs" up in the attic if you can't find anything else.

[Luca]

Sparrow, was wondering if you had a rough/loping idle or could hear the timing chain slapping around before the whole thing went tits up?

If not I think you should take a close look at the lubrication system.  The valve timing gear is the last place that oil gets delivered, and it is through a pressure relief valve.  If you had done an oil and filter change before your trip, you might look to see if the filter got crushed or if the $2000 o-ring was nicked, not compressed enough or compressed too much, etc.  Timing chains usually give you plenty of warning, but I have a feeling a lubrication failure could burn out a set of valve timing gear in 1,000 or so miles.

Pretty sure the chain tensioner on 79 and up is hydraulic as well.

[mendozeal]

thank you all for the solid replies!


can't comment on the idle, was at freeway speed for well over an hour before she went. I had been pushing her way too hard trying to make it to the east coast over night. But now that I think of it, she did sound a little "off" the last time I fueled her up...
 where should I start checking out the lubrication system? and what is the $2000 o ring?

I was planning on pulling and cleaning the pan, and doing new filters before the rebuild anyway. anywhere in particular I should go way out of my way for thorough cleaning?

the pushrods and the piston rods still seem remarkably true.. But how might I preform a more precise inspection?

when I pulled the timing cover, the little metal disc that held the spring for the chain tensioner just fell out. the circlip was MIA.. my best theory is that no tension on the chain allowed it to skip a link on the crank sprocket, and boom..
unless I'm missing something I don't see a hydraulic tensioner.

the chain and can sprocket still seem to be in great shape, is there a reason I should replace them anyway?

my current plan is to measure the running clearance and go for two new 81.97 slugs. is this crazy? and cand I still use standard rings for that piston?


thanks again everyone!

[Barry]

my current plan is to measure the running clearance and go for two new 81.97 slugs. is this crazy? and cand I still use standard rings for that piston?

Piston rings are not graded like the pistons and bores so the standard rings will fit any grade of standard size piston. If you think about it piston ring size as measured by the end gap is determined by the cylinder bore not the piston so provided they are undamaged and within spec. I don't see a problem. Specs are from the BMW workshop manual applicable to the early model R65. If you see any other published figures that differ slightly they are probably for the 81 - on models with the nikasil bores.

[Lucky_Lou]

and what is the $2000 o ring?
thanks again everyone!

Its the "O" ring on the oil filter cartridge..... danger Will Robinson if not fitted correctly.
Lou

[Luca]

The $2000 o-ring is the o ring that goes between the oil filter and the oil filter cover plate (bolts into the throttle side of the engine)  It is accompanied by a metal shim, sometimes two.  Often the cover plate gasket is removed and the joint is left as metal to metal.

The main idea is to compress the (usually white) o-ring enough to seal the filter to the cover without overdoing it.  If that o-ring fails, the oil pump ends up sending oil right back into the sump.  My little theory would be that if there was some sort of partial failure, the timing gear would not get enough oil because it is lubricated by excess oil passed through a pressure relief and spit out of a little straw.

I'm not sure how you would check the condition of the oil pump, but I was thinking more along the lines of making sure that your bypasses and oil passageways aren't stuck or clogged.

Sounds like you have a spring tensioner.  Mea culpa.  Your theory sounds good.  If the cam sprocket seems ok you should be able to re-use it, but you really want to check every square inch of it very closely, as well as the chain, for damage.  Cam sprockets last the longest of the three components.  Crank sprockets are the shortest lived of the three.

There is one good reason to replace the crank sprocket and chain while you are in there.  They are cheaper than another top end.

[montmil]

A worn timing chain usually announces itself with a rattling/slapping noise as you roll off the throttle while still in gear.
   Under acceleration the chain is pulled tight by engine torque and doesn't make a lot of noise.

The timing chain tensioner is a spring within a tube within a casting that applies pressure to the chain thereby taking up slack. The springs can and do lose tension over years of service. I know this! On the other side of the chain and sprockets is a plastic shod chain guide that is bolted parallel to the chain when it is under tension. It's a guide, not a tensioner.

All these rotational bits are lubricated from a small tube that simply spits oil in the chain's general direction. Not too high tech, that. The problems begin when there's not enough oil pressure to get the oil to spit... as happens when the revs are kept low. Same issue with the battery charging -low revs equal minimal to no charging nor oil to the timing chain and sprockets.

Keep your revs up, lads. Your engine and battery will thank you.