68mm pistons in 88 barrels

I've seen many mentions of this combination, and with my piston need for building 500 motors, as well as the mix-and-match coming up more frequently, I thought I'd do some measuring and encourage others to add to the brain trust.

My (admittedly lax) measuring protocol is measuring directly above the two or three abbreviated fins on the cylinder, where they seem to neck down a bit, front to back. I'm using some outside dividers, and using a digital calipers to measure the distance. Not terribly accurate, but we're dealing with the world of horseshoes and handgrenades, where the outside dimension has little bearing on the concentricity of the bore within. The seam on the outside of the castings on the cylinders has been ground down about the same amount, but bears mentioning that's it's in about the same spot I'm measuring. I added 650 cylinders as a control for what a std 68mm piston would need for their own (up to +.060") reboring.

650SS spigotless-- L 3.109", R 3.115"
650SS spigotless-- L 3.103", R 3.106"

88 dynamo-- L 3.065", R 3.042"
88 dynamo-- L 3.005", R 3.023"
88 dynamo-- L 3.010", R 3.025"

88 alternator spigoted (not the SS fin shape)-- L 2.905", R 2.912"

My brief survey says it might be worthwhile to explore boring an 88 dynamo cylinder to 68mm (+.080") depending on a check for inside/outside concentricity. The alternator barrels I measured seem much smaller and maybe not worth the risk of knocking through the wall.

This obviously would add more capacity than is legal for racing a 500cc bike.

Same dimensions are worth noting for lining 88 cylinders in addition.

Please add to these dimensions and/or first hand experience!

Seen this?

kernel65 said:

Seen this?

99 pistons in 88 barrels - advice please | Norton Owners Club

www.nortonownersclub.org

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Definitely, but there didn't seem to be a resolution as to first-hand, "yes, it worked", or, "no, it didn't work".

I'm really considering it in order to build a mongrel 88 and not take good, usable parts out of circulation. Would be great if it worked.

One reply did say:

“When 88 pistons were hard to get a few years ago, this overboring of an 88 barrel to plus 80 thou and fitting standard size 99 was a neat trickof getting round the shortage problem. You ended up with a capacity of around 625cc but theexternally the engine looks like a standard 88.”

I want a mongrel 88!

Mark Savage said:

I think that will be 525 not 625.

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Same bore + shorter stroke = bigger capacity? I see a future in sales for that person!

Oops, was just quoting the paragraph in its entirety for completeness, didn't mean to add confusion.

kernel65 said:

Oops, was just quoting the paragraph in its entirety for completeness, didn't mean to add confusion.

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No worries! That quote perfectly illustrates the quality of information out there on the subject.

Seems to be a common mod at one point in time. Haven’t seen DynoDave around for a while, wonder what his take would be?

I recently acquired a Dominator 88 dynamo engine with another bike and those 500cc barrels had been opened up to accept std size (68mm) pistons from a 600/650. I'll add some measurements soon.

My current conundrum is that the pistons that came with the bike are trashed and I found a nice set off +.010" 600/650 pistons on EBay for a decent price. Debating whether it would be safe to open those up even more......

Fritzer_108 said:

I recently acquired a Dominator 88 dynamo engine with another bike and those 500cc barrels had been opened up to accept std size (68mm) pistons from a 600/650. I'll add some measurements soon.

My current conundrum is that the pistons that came with the bike are trashed and I found a nice set off +.010" 600/650 pistons on EBay for a decent price. Debating whether it would be safe to open those up even more......

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That's interesting! If you have any way to measure the wall thickness, that would say a lot.

There are 66mm liners available from LA Sleeve as well as Hepolite if +.090" (!) is not feasible.

Original std 650ss pistons are not that unusual to find with a bit of patience.

Other than 850 Commandos, Norton twins probably do not have through-bolts holding the barrels down. If you overbore you might get a bad failure if the cylinder wall thickness is too thin below the fins.

Ha, I completely forgot about this thread. I took some measurements of my 500cc barrels that have been bored for std. 68mm pistons. Here are pictures of the caliper...the captions will say which measurement it is: https://flic.kr/s/aHBqjAtsyZ . The "OD" pictures refer to the barrel spigots.

I noticed that the left cylinder was bored crooked and the bottom wall thickness is very thin compared to the right side. Let me know if there are more useful pictures or measurements I can provide. Maybe I'll get around to it in a few months .

How about hardly boring some 99 barrels to fit some 88 liners and 88 pistons , the point being to retain metal in the barrel and preserve strength ? sounds a bit odd but might be a way of saving some 99 barrels .

norton bob said:

How about hardly boring some 99 barrels to fit some 88 liners and 88 pistons , the point being to retain metal in the barrel and preserve strength ? sounds a bit odd but might be a way of saving some 99 barrels .

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The barrel height discrepancy between the 88 and 99/650 complicates that, but I have since found some good 88 barrels, so I've backed away from the edge. The sticking point is now good, light pistons without resorting to parts worth more than the value of the entire bike otherwise...

Your post is a good reminder to measure my new barrels and add to the above list for a daring individual.

Sorry ,was thinking about a 99 motor . My 99 has 650 SS pistons and the 650 has Nomad 99 pistons .

When there is a barrel height problem, it is sometimes possible to raise the barrel a bit with an alumium gasket-shaped packer. I usually try to get the edge of the piston crown level with the top of the barrel at TDC. The people who do rebores can usually supply sleeves. The don't usually come from motorcycle parts dealers. I would not over-bore to fit larger pistons - smaller ones might be lighter, and help the motor to spin-up better.