Boring R60/7 Cylinders

[IronHorseCustoms]

I have a project bike I'm working on. It is a '78 R60/7. Bike sat for a long time on the sidestand and cylinder number 2 became corroded. My engine shop said they had to bore it out to 75mm to make it smooth again. So that puts it beyond second boring spec of 74.970mm.

I know many have successfully bored their airheads up a size. From 500 to 600 and 750 to 840.

I have been advised that because the liners are slightly hourglassed from spigot to head I need to be careful. Empirically, it sure looks like I could go ahead and bore to 82mm for 750 pistons.

Does anyone know if this is doable? I would hate to get the liner too thin.

Thanks in advance.

Cheers,

RD

[khittner1]

Well, since 60 folks have looked at this question, and no one has answered, I'll have a go at it, if only in the "if we don't know, we'll just make something up" spirit that Duane always cites as a benefit to forum participation.

I really don't know anything about R60/7s or bore jobs, but it was always my understanding that the basic jugs of the several /6 and /7 variants (600cc, 750cc, 900cc, and 1000cc) were essentially identical, and that the only difference was the size of the hole bored into them. I don't know what that means in terms of the wall thickness of the iron liners in the pre-Nikasil bikes---were the liners all the same OD, and varied only in their ID/wall thickness, or did the iron liners have different ODs to correspond with the displacement variations? Either way, it sure seems like boring 7mm worth of iron out of your current bore sleeves is a lot. Can't you have the problematic jug re-sleeved, bored, and honed back to original 600cc spec?

[Kurt in S.A.]

1978 R60/7? I didn't think that bike existed. I thought they stopped the 600cc at the end of 1976 (maybe it was 1977), 1977 was the R75/7, then the R80/7 came out in 1978. Maybe the R60 was only for the US market??

The various manuals, such as Haynes, gives the sizes up to 3rd over. You really will have trouble going past that I think. I'd probably feel uncomfortable going beyond 1st over. I faced that with my '78 R100/7...the cylinder was out of specs and my research found that 1st over on the 1000cc was problematic and that I might be facing problems in a small number of miles. I only wanted to do the job once, so I opted for replacement Siebenrock cylinders.

Those are my thoughts...

Kurt in S.A.

[Bamboo812]

1978 R60/7? I didn't think that bike existed. I thought they stopped the 600cc at the end of 1976 (maybe it was 1977), 1977 was the R75/7, then the R80/7 came out in 1978. Maybe the R60 was only for the US market?

Phil Hawksley's website lists the R60/7. Interesting that it didn't come with the drum brakes retained by the R60/6:

[IronHorseCustoms]

Well, some replies are better than no replies...lol

It was my understanding that the cylinders were all the same, just sleeved with different liners for the various bores. I have also been advised that the liners are hourglass shaped, thicker at the spigot and the head than in the middle.

I just cant find any definitive information on whether or not I can go for an 82mm bore. Sure, if the 100 used the same cylinders, there would not be much left to bore. But heck, the liners look so darn THICK! So if I was to base it on the head and spigot, yeah, I could go to 82mm. It's just that darn hourglass shape I'm worried about.

I guess I could always be a guinea pig and just go for it. I just don't want to waste the $280 for boring both cylinders.

So I guess this post is really a bump, hoping that someone could chime in.

Oh, and year, that pic is the bike, but ours is red... soon to be gray and bobbed

Cheers,

RD

[Kurt in S.A.]

RE: the cylinders and sleeves... I just measured the liners on my R100/7 cylinders...I did my best to measure right at the top where the edge of the sleeve is exposed. Not a very accurate number but it's about 6mm. So, if the cylinders are the same for the R60/7 but with thicker liners, what does yours measure? The bore of the 1000cc is a little over 20mm larger than the 600cc (94 vs 73.5). If the same cylinder was used, that would mean that your liner would be around 16mm thick. Seems like a lot of weight to carry around...not to mention the heat retention.

Kurt in S.A.

[Major Softie]

RE: the cylinders and sleeves... I just measured the liners on my R100/7 cylinders...I did my best to measure right at the top where the edge of the sleeve is exposed. Not a very accurate number but it's about 6mm. So, if the cylinders are the same for the R60/7 but with thicker liners, what does yours measure? The bore of the 1000cc is a little over 20mm larger than the 600cc (94 vs 73.5). If the same cylinder was used, that would mean that your liner would be around 16mm thick. Seems like a lot of weight to carry around...not to mention the heat retention.

Kurt in S.A.

Yes, I can't believe it. Keep in mind that this would also include the R50, with its 67 mm bore: 27mm smaller than an R100. It would be crazy to put all that steel into a barrel liner. Plus, what would be the point of having cast-in displacement markings on the barrel castings if they are actually all the same? Much more sensible to just make one barrel and stamp the the displacement on them the way they do the A B C.

[bbelk]

How can a motorcycle that classy be boring. I think its interesting as Hell.

[khittner1]

If the iron liners among the displacements were all the same OD, then presumably one could do a whole lot more rebores on a 600, than on a 900 or 1000cc machine, since there would be so much more liner thickness to work with. But I've never seen more than a 3rd-over bore spec, so the liners must have comparable wall thicknesses across the displacements, and varying ODs, even if the surrounding aluminum jugs are all the same outside dimensions. And I would think that the "hour-glass" shaping of the bore is the result of normal bore wear at the top and bottom of the stroke; the bore starts out as a cylinder, and becomes hour-glass shaped during its service. Some allowance in the iron liner's thickness probably has to remain after a rebore, in order to accommodate that normal future wear pattern, hence, your machinist's remark. So I'm back to 7mm seems like a lot of liner material to remove from a 600cc original to get to a durable 750cc result. I think you'll need new-to-you 750cc jugs to get to that result for very long. But this is just thought experiment; someone with actual machinist knowledge/experience may enlighten us, and that'd be good.

[Major Softie]

But this is just thought experiment; someone with actual machinist knowledge/experience may enlighten us, and that'd be good.

Yeah, actual liner thickness knowledge would be nice too, even though our speculation seems very logically based.