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khale 02-24-2013 04:19 PM

R75 piston ring gap, new gaskets, etc
Hello all,
I just had my heads rebuilt (valves replaced, new guides, new exhaust seats, new springs). I'm looking for a complete walk through of a proper top end check and put back together.
I'm pretty new to a top end check, so speak to me like I'm 5. I'm pretty sure I don't need a rebuild, as the piston and cylinder wall looks good. I can still see good cross-hatching on the cylinder wall. See pics below.
What measurement do my piston ring gaps need to be? Do I need to remove them and place them in the cylinder to check the gap? If I have to remove the rings, I have read you need to be very careful. What's the best step for ring removal and refitting? The Haynes said to use metal shims, could I use 3 feeler gauges to use to slide on and off the piston rings?
When the rings are put back on, do the gaps needs to positioned in a certain way?
When putting the new gaskets and push rods seals on, is there anything I need to know? I have universal sealant, should this be applied to the cylinder base gasket? I noticed there was a sealant around the pushrod seals that almost looked liked teflon tape, but I don't think it is. Any idea of what this is and if it is necessary?
What should I use to clean and resurface the piston head?
Should any sealant be applied to the cylinder head and valve cover gaskets?
I'm also aware that a high temp copper anti-seize should be applied to the exhaust threads before re-attaching the headers/exhaust nuts. Any recs on this anti-seize?
Thanks for any help you guys can offer.
pushrod seal gasket?

disston 02-24-2013 04:50 PM

What year R75 might this be?

It is customary to use some system to prevent the rod from contacting the edge of the block bore where the cylinder has to fit. Use some rubber bands in and X on the cylinder rods to hold the rod off the surface of the bore. A small dent here, to small to see even, can cause a failure to seal later.

This is a time when you need a manual. The subject is generally too large. After three or four of us have posted pointers you are likely to miss something important with out a manual. Do you have a Clymer's or a Hayne's?

Over the years advice on installing these parts has changed. It used to be Hylomar was the most popular and it was also put on the push rod tube seals. I think now, clean bores, and only engine oil are advised. That's how I did it last time.

I don't know what is recommended most for the base gaskets, if your engine is old enough for base gaskets, but I see Yamabond 3 bond, I think it's called, recommended. Same stuff maybe on the cylinder bases if they don't have shims/gaskets.

Careful with the placement of the head gasket. It may look like it fits either way. It has a correct way.

There's more I'm sure.

khale 02-24-2013 05:01 PM

Thanks Disston. I have a 1973. I have a Hayne's I'm reading through as well.

disston 02-24-2013 05:17 PM

Alright. I really don't like to talk with people that have no manual. Don't know why but maybe because I don't want the responsibility, or something.

There is a tool made specifically for removing and replacing piston rings. You can find this tool in a hundred places on line and over the counter at larger Sears stores. There are several variations but the most common one is this;

I believe the tool is better than the method with out the tool using two or three feeler gauges. Even with the tool you have to be careful. BMW rings have a top and bottom. So if you take these rings off to measure them don't flip them over. Take the rings off in order, from top to bottom and replace in order from bottom to top.

Allowable ring gaps are listed in the Haynes manual. You tell us what it says.

Rings are inserted into cylinder bore and squared with the piston (you have to remove the piston to do this) and measured with feeler gauges. Measure rings one at a time. If the rings don't have a lot of miles on them you could just measure the top ring and run with it.

How many miles on these rings? Are they original?

disston 02-24-2013 05:33 PM

This is the tool I bought just a few months ago for the next time I do piston rings. It's made by OTC. It's a kit that covers a wide range of ring sizes and does removal and replacement. Also does ring compression for getting the pistons back in the cylinder. This kit costs around $100;

Some riders have tricks for getting the pistons back in the cylinders. I prefer to use a ring compressor. And I prefer to have the pistons on the rods with the wrist pins held in place by their circlips when I put the cylinders on. The other way involves to much contortion to fit the piston to the cylinder and then have to finagle the wrist pins and circlips on with the piston hanging half way out the bottom of the cylinder....etc.

A third way involves a champher at the base of the cylinder that can be used to compress the rings. Many like to use this. I think it's hard to get the rings properly spaced doing it this way. But I have done this method.

I prefer a ring compressor.

The rings should be spaced at 120* intervals. If you will read Snowbum's advice on this he has a really intricate system of spacing the rings. Takes everything into effect I think even the phases of the moon. :lol3

Kt-88 02-24-2013 05:45 PM

As for the gap placement, I was always told to put the oil ring (the thickest one, closest to the case) with the gap facing up because boxer engine oil collects at the bottom of the cylinder.

khale 02-24-2013 05:45 PM

I have a ring compressor, so no worries there.
Here's what I have for piston rings for /5:
Thickness - 1.790-1.810mm
End gap - installed - 0.30-0.45mm
Side clearance - 0.060 - 0.070mm
Thickness - 2.030 - 2.050mm
End gap - installed - 0.30-0.45mm
Side clearance - 0.050 - 0.060mm
Oil Scraper:
Thickness - 4.010 - 4.030mm
End gap - installed - 0.25-0.40mm
Side clearance - 0.030 - 0.040mm

khale 02-24-2013 05:55 PM

I'm currently reading through snowbum's advice on this topic:

Anything else I should know?

khale 02-24-2013 06:10 PM

"AVOID allowing the piston rod to fall against the engine cylinder opening....this will create nicks....and the nicks WILL prevent the cylinder from properly sealing. If you nick the case, dress that nick out!"
I do have a tiny nick in the cylinder opening. What would I use to dress the nick out?

Bill Harris 02-24-2013 06:36 PM


Originally Posted by Disston
This is the tool I bought just a few months ago for the next time I do piston rings. It's made by OTC.

KD Tools make a similar band-type of ring compressor. I've been using this type on VWs and BMWs since the '70's:

The pliers and individual bands are available for NAPA. I don't recall the NAPA numbers, but they can look them up.


khale 02-24-2013 07:21 PM


Originally Posted by disston (Post 20802071)
How many miles on these rings? Are they original?

I have 56k miles on my engine. This is the first top end work to my knowledge. These are the original rings, piston, cylinder, etc.

supershaft 02-24-2013 08:02 PM

I like BMW ring compressors but the PN's are confusing to look up.

khale 02-24-2013 08:09 PM

I wish there was just a more concise article with step by step pictures. Anyone know of any? Don't get me wrong, Snowbum's article/info is great info, just a little too 'early' internet for me, too wordy as well.

Plaka 02-25-2013 12:02 AM


Originally Posted by khale (Post 20803132)
I have 56k miles on my engine. This is the first top end work to my knowledge. These are the original rings, piston, cylinder, etc.

that is about 1/2 to 1/3 of the mileage you should be getting out of those parts.

notice the brown streaking on the pistons below the rings? This is from blow by. (I just pulled the pistons on my RS BTW at about 100k and there is no streaking).
Anyway it could be worn rings or it could be gummed up and sticking rings. The rings work by pressure getting behind them and forcing them outwards against the cylinder wall. They have to float freely in the ring grooves. Wouldn't hurt to check the gaps and if they are in spec then just clean everything up and put them back. Put them back exactly the way they came out. assume the blow by was due to sticking.

hate to tell you this now but you don't need ring compressors. You put the pistons in the jugs on the bench and just use your fingers to compress one ring at a time while you press the piston into the jug with your beer gut. (no gut? get busy!) Leave the piston pin clear at the base of the cylinder and connect it to the rod (WHICH IS SITTING ON A CLEAN RAG ALREADY!) as you put it in place.

You clean ring grooves with a piece of broken ring. if you don't have one most automotive machine shops can give you one out of the trash can. It is an operation requiring a whole lot of care. often a better option is to take the pistons to an automotive machine shop and just let them hot tank them. Shouldn't cost much.

Dress out dings in the cylinder bore with emery cloth dampened with oil.

base gaskets come in both higher and lower compression. Pick yer poison.

A book very worth having is called "How to keep your volkswagen alive..."

The air cooled flat four VW engine is very similar to the flat twin BMW engine. (the clutches are within a few mm of each other). you will learn a vast amount about everything you are doing and a lot more besides, like setting the valves and doing the timing. It is very readable, fun, and assumes you know nothing. Lays down a solid mechanical understanding and all the basic techniques---like dealing with rings and heads for the type of engine you are working on..

don't clean up your pistons at all (unless the rings grooves are cruddy, see above). No need. The left one is going to get carboned up again real quick anyway. If it was really bad you would clean them. your's ain't really bad. You can observe your piston crowns (and valves) anytime through the spark plug hole, just use a pencil flashlight. this is my favorite tool for the job:

it is also good for operations like looking up your nose which you may want to do sometime (usually in the event of some kind of damage).

You can clean up the cylinders and pistons with plain water---with the engine running. You wind it up and mist some water into the intake tract. you want enough to show up in the exhaust. Steam cleans things very nicely. Don't overdo it, you want minimal cooling lest you screw up a valve. Then change the oil. This operation should be reserved for situation where carbon deposits are creating a problem---like raising the compression ratio causing detonation with your preferred fuel or pre-ignition caused by glowing bits of carbon. You are nowhere near these issues (especially with nice clean heads). I found even going into my 75/5 at over 100k (converting it to a 90/5) I had all sorts of carbon (lots more than you have) and no problems.

I use a nickel based antisieze on the exhaust header nuts--and get generous with it.. Also a smear around the inside of the ports. these are areas I also keep real clean along with the threads on the heads. brass wire brush and some scrubbing. For freshly done heads the threads are clean but when you do your first head bolt retorque and oil change remove the nuts and add a little more antisieze. The oils in the stuff soak into the castings leaving the threads a bit dry. I remove the nuts and clean the old antisieze of the head threads and replace it every oil change. Takes a couple minutes and I am insuring I never have stripped threads due to seizing (expensive). use the high temp antisieze on the header clamping rings also.

if your rings turn out to be worn invest in 4 new conrod bolts and some plastigauge and check the conrod big end bearing clearance. instructions come with the plastigauge. Worn out rings at 56k speaks of poor maintenance (not enough oil and/or filter changes maybe). if the bearings are scored up and/or worn out of spec replace them. You can dress minor crap out of the crankshaft journals with crocus cloth. In any event while you are there remove the lifters one at a time with a magnet and and check the faces for pitting. Replace them with some assembly lubricant. If any pitting replace them with a good one (available used maybe. I had one with extremely minor pitting out of the four and fortunately 2 spare good ones from another engine). Eyeball the cam (and pray) while you are at it. Doesn't hurt to start running some moly in your oil also. (but never in the gearbox)

Also suspect your rocker assemblies. if these are bad your valve adjustments won't last very long. just put it together and if you have a problem deal with it later. You don't have to tear the whole thing down again and you want to update to the needle bearing assemblies anyway.

Always replace the pushrod tube seals. You need to own a pushrod tube seal drift in any event. (you give 'em a smack when, not if, they leak). It's a BMW special tool but like the exhaust nut wrench, not expensive. Theoretically you can make one but hardly worth it unless you are extremely poor and the real deal is nicely made (and hardened). The third tool you need is one big ass pair of channel lock pliers. Cheap ones are fine. You use these to compress the rocker arm support blocks when you torque the head bolts and also at retorque/valve setting time. They need to open up with jaws near parallel to the width of your rocker assemblies---end to end. Measure and go shopping. You may not find them this big in hecho en China. Then just break down for real channelLock brand. It is worth having the best clicker type torque wrench you can afford. Beam types will do you but there are too many places where they really suck (oil pan bolts).

Pull the oil pan, check for sludge. Found clean it up and plan on some serious oil flushing. Run some Seafoam in some cheap oil, ride 50 miles gently, change it, repeat. Do it 3-4 times then an oil filter change and some decent oil. Hopefully you will get the oil passages cleaned up.

Last time I did my /5 I used hylomar on the base gaskets. They "weeped" eventually anyway. I decided, screw it and just cleaned the bases when I washed the bike.

When you put the air cleaner assembly together, stick two fingers in the obvious hole to guide the long bolt into place.

pommie john 02-25-2013 12:35 AM

If it were my engine and I'd just had a total head job done, I wouldn't bother checking the old rings, I'd replace them. That way everything in the top end is new. The bores need the glaze taken off them if you're fitting new rings. ( It is an iron bore in that model isn't it?)

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