Small / Large push rod tubes

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Mjolinor
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Small / Large push rod tubes

Postby Mjolinor » Wed May 08, 2019 6:47 pm

Can someone educate me about the push rod tube diameters. I bought a pile of bits off ebay and there is one cylinder and piston. The cylinder has small pushrod tubes which I thought were older but the cylinder looks and feels like Nikasil and has no wear, hone marks clearly visible, no lip. It has no cutout in the cylinder base for an 'O' ring and came with two cylinder base plates for lowering compression.

This is what I bought:
https://www.ebay.co.uk/itm/BMW-R75-1976 ... 2749.l2649

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Rob Frankhamr
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Re: Small / Large push rod tubes

Postby Rob Frankhamr » Thu May 09, 2019 9:50 pm

Research suggets that the 16mm tubes were fitted up to 09/75 with the 18mm tubes being used thereafter. This combined with the fact that it has no 'O' ring groove suggests that the cylider dates from before 09/75 which, of course, means that it won't be Nikasil; since Nickasil cylinders were not used prior to 09/80. I would suggest that it has recently been rebored if there is no wear lip and the hone marks are unworn. You can normally see the cast iron insert quite clearly if you look at the head - cylinder mating face.

Rob
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Mjolinor
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Re: Small / Large push rod tubes

Postby Mjolinor » Thu May 09, 2019 10:24 pm

Cheers. I haven't measured it yet. It has a piston, maybe the piston is undersized for the bore. I will measure it.

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windmill john
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Re: Small / Large push rod tubes

Postby windmill john » Fri May 10, 2019 6:59 am

You got all that for £25!
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Bumble, my 700 Transalp.
Gupta, my 350 Royal Enfield.
Sold my Airheads, what an idiot, but Honda is also great.
Too many bikes have come and gone, trying to be sensible now!

Mjolinor
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Re: Small / Large push rod tubes

Postby Mjolinor » Fri May 10, 2019 10:27 am

You got all that for £25!
And a whole lot more actually. Carb kits, points, gaskets, inner tube. Seems it was the last of the airhead stuff that he wanted shut of. He had three "new" BMWs in the garage. (new = made after 1990, not airhead. The ones where you can't see the bike for the plastic)

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windmill john
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Re: Small / Large push rod tubes

Postby windmill john » Fri May 10, 2019 2:14 pm

I know, I posted because I saw the eBay picture, side panel as well for example.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Bumble, my 700 Transalp.
Gupta, my 350 Royal Enfield.
Sold my Airheads, what an idiot, but Honda is also great.
Too many bikes have come and gone, trying to be sensible now!

ts
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Re: Small / Large push rod tubes

Postby ts » Fri May 10, 2019 7:45 pm

Apart from how the pushrod tubes fit in their corresponding bores near the top of the cylinder, both the narrow and wide tubes, when fitted with their matching rubber grommets, go into identical size holes in the crankcase. More important is the diameter of the cylinder base spigot; 97 mm for /5 engines, and 99 mm from approximately 1976 on (these match the change from 16 to 18 mm pushrod tubes). I know that someone have claimed they managed to fit 97 mm cylinders in a 99 mm engine block - but I don't know what they did to avoid the barrel from "walking" in the engine case bore. It is possible to turn down 99 mm nicasil cylinders to 97 mm, so they can be used in a /5 (I did that myself with a pair of R80 barrels I used for a while!). There are even people who sell home made (?) R80 nicasil upgrade kits for /5 bikes on German eBay . . .

Plenty of options for those who like to play :-)
ts
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'73 R75/5, '78 R80/7, '83 R80RT

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Re: Small / Large push rod tubes

Postby Mjolinor » Fri May 10, 2019 8:42 pm

I already have an R65 block that has been bored out and a pair of R80 Nikasil that will fit and with the short stroke crank and conrods off the R65 it gives a 700 cc engine. I also have an R75 block spare with its crank and conrods. But at the moment it is altogether too many decisions to make so it's looking like staying a pile of bits at the moment. :)

What I could really do with is another R75 small tube barrel in good order.

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Rob Frankhamr
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Re: Small / Large push rod tubes

Postby Rob Frankhamr » Sat May 11, 2019 10:07 am

I already have an R65 block that has been bored out and a pair of R80 Nikasil that will fit and with the short stroke crank and conrods off the R65 it gives a 700 cc engine.

Not sure how far you've looked into this but, in order for this to work, you need to shorten the cylinders to achieve a workable compression ratio. I would say that the amount would be half the difference between the width of a 'long stroke' engine (746mm) and a 'short stroke' engine (688mm) (i.e.33mm) but that would assume that the crown to gudgeon pin distance was the same for both piston sizes. I would also want to check the depth of the cylinder head casting and you would have to calculate and take into account the compression ratio you're aiming for... quite a complicated calculation. I have also read that there is an issue with the piston skirt fouling the crankwebs.

It can be done... it has been done but it ain't just a matter of bolting up R80 cylinders to a R65 block.

Rob
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Re: Small / Large push rod tubes

Postby Mjolinor » Sat May 11, 2019 10:20 am

I already have an R65 block that has been bored out and a pair of R80 Nikasil that will fit and with the short stroke crank and conrods off the R65 it gives a 700 cc engine.

Not sure how far you've looked into this but, in order for this to work, you need to shorten the cylinders to achieve a workable compression ratio. I would say that the amount would be half the difference between the width of a 'long stroke' engine (746mm) and a 'short stroke' engine (688mm) (i.e.33mm) but that would assume that the crown to gudgeon pin distance was the same for both piston sizes. I would also want to check the depth of the cylinder head casting and you would have to calculate and take into account the compression ratio you're aiming for... quite a complicated calculation. I have also read that there is an issue with the piston skirt fouling the crankwebs.

It can be done... it has been done but it ain't just a matter of bolting up R80 cylinders to a R65 block.

Rob
Which is exactly why it is all sitting in a pile right now. :)

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Re: Small / Large push rod tubes

Postby Mjolinor » Sat May 11, 2019 2:13 pm

Much measuring and examining later. This R75 barrel is cast (not Nikasil) and it has done zero miles in my opinion. What it does have is the mother of all scores where a circlip has come out so it's kafurkled until I rebore / sleeve it. I reckon it has not run like that and is possibly one from a failed PDI check when the bike was new, that's why it still has very prominent honing marks visible.

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Re: Small / Large push rod tubes

Postby Mjolinor » Fri May 17, 2019 7:22 pm

I found out a couple of things today. The Nikasil coating is 2 thou thick and that water can get between the Nikasil and the aluminium. This causes aluminium oxide to form and makes the Nikasil blister. I have an R100 barrel that this has happened to.

I mention this because to me it seems like a potential time bomb for any out of use barrels that are stored somewhere damp. I suspect the problem is porous aluminium on the barrel skirt.

This problem could make good cylinders a nightmare to find and clearly non-Nikasil becomes a more desirable option.

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Rob Frankhamr
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Re: Small / Large push rod tubes

Postby Rob Frankhamr » Fri May 17, 2019 10:27 pm

I found out a couple of things today. The Nikasil coating is 2 thou thick and that water can get between the Nikasil and the aluminium. This causes aluminium oxide to form and makes the Nikasil blister. I have an R100 barrel that this has happened to.

I mention this because to me it seems like a potential time bomb for any out of use barrels that are stored somewhere damp. I suspect the problem is porous aluminium on the barrel skirt.

This problem could make good cylinders a nightmare to find and clearly non-Nikasil becomes a more desirable option.


This is a known issue with early nikasil coated cylinders (not just on BMWs, car or bike) It only affected a small number of cylinders... I think one can safely say that any cylinder that has survived this long is pretty much safe.

Rob
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Re: Small / Large push rod tubes

Postby Mjolinor » Fri May 17, 2019 10:32 pm

It must be an early one as it has small pushrod tubes which I thought went when Nikasil came in. So it must be as early as they get.


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