99 pistons in 88 barrels - advice please

I talked with John Gregory about this before, the man who built the Hogslayer Norton drag-bike for T.C. Christenson to ride. He also had a very fast Model 88 roadracer in the 1960s. He said that he turned the later domed Model 99 pistons down to a model 88 oversize to make light high compression pistons for his race bike.

Also I have a .040 over 88 cylinder here I would like to use someday. Standard Model 99 pistons would be right about .080 oversize for a Model 88. On my cylinder I measured the cylinder wall thickness at the front where the fins are shaped to allow the nut to go up onto the front cylinder head studs and I found that they are currently .108" thick there, which means that bored to .080 I might have as little as .068" of wall left at that point. I am sure that it would be easy to find recommendations for cylinder wall thickness and see how it compares.

For touring I would think you could get away with a lot of things like this. For some reason back in the day Norton only offered up to .040 over in the parts books, and I know the racing boys like to find them as thick as they can get them.

I have thought about asking a good piston engineering company if they would take a few sets of old 99 pistons I have and turn and profile them to a 88 oversize, it is on my list.

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.

Some of the race boys, in the late 60s and 70s, used to do a similar thing plus adding a shorter stroked crank to give themselves a squarer, higher revingengine.

There is usually plenty of meat on the 88 cylinder liners to allow for overboring. This is not the case on the 99 barrel linerwhich really goes very thin after plus 40 thou. The external liner diameter is around 72mm on both 88 and 99 barrels so they possibly used similar liners to start with. Just the length altered to cater for the shorter 88 barrel.

Checkout the attached photo of 88 & 99 barrels together. Both are on plus 30 thou. I'll let you decide which is the 88 barrel.

As both the early 88 and 99 engines used the same cylinder head, there should be no issue with valve pocket clearancewith the respective pistons.

Turning down 99 pistons to 88 dimensions can be risky. You may alter the thermal characteristics of the walls and hence promote distortion and weak spots. I once tried this out on a set of Powermax pistons in my 750 track day engine. I had them turned down from +20 thou to near standard and they partially seized 3 times despite a lengthy and careful 1000 mile running-in. But I have met others who have had better success. I guess the type of piston and who does the trimming are key factors in this option.

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The 88 and 99 cylinders might look the same on top, but I have a set of alternator Model 99 cylinders that have more material on the bottom where the sleeves go down into the crankcases, and just like 650cc cylinders they have the heavier reliefs where the base bolts need clearance to screw on. I do not have a generator Model 99 cylinder to see what it looks like.

The late Model 88 cylinders for alternator bikes, even the one on the 1962 88ss I have, has no improvements over early generator cylinders in thickness anywhere that I can see.

So we need someone to compare the bottom of generator 88 and 99 cylinders so we can see if the 600cc bikes always had reinforcements, or if they started off with weaker cylinders than the later Model 99 had.

Of course the 650cc cylinder had the re-shaped fins on it's front side, but other than that it does not appear any stronger than the alternator 99 cylinder I have. Bracebridge Street kept separate part#s for the 99 and 650, later after AMC shut down the Norton works, the 1964 parts book lists the same cylinder part# for both 650cc and Model 99.

So in the case of the alternator cylinders anyway, I can say that a 500cc cylinder would be weaker than a 600cc cylinder when bored out to 68mm.

When pistons are "turned" down, they can of course be chucked onto a lathe and turned down as a true cylinder, but that is not how good pistons are turned down or manufactured.

Factory supplied pistons were for many years ground to what was called a "cam" shape, sort of an oval, so that when they are at operating temperature they would distort to a shape that was nearer round, or at least a shape that was not prone to seize.

In Irving's famous book Tuning for Speed it talks about this, and even how there were designated standard shapes pistons were ground to designated by particular names, I think letters were used.

This sort of equipment would of course not be in home workshops, or even motorcycle shops and dealerships. Some old piston manufacturer, or ex-employee of the same, may still be grinding pistons on such a machine and may do custom work. Maybe a manufacturer on the continent such as Mahle etc. might have the service available.

In the early 1980's there was a nice article in one of the major motorcycle magazines about someone upgrading their Triumph engine in which new Hepolite pistons were being fit. There was a diagram and instructions to file a couple of areas of the Hepolites so that they could be fit into the bore more tightly without seizing, thus running more quietly and lasting a bit longer.

A top-notch and very experienced mechanic/engineer probably could do a very good job turning down and shaping a piston at his home with a small lathe, a micrometer/caliper and some good hand files, and even though most of us are not that person, it is fun for us to try such things on a whim.....

With 99 pistons you have to be careful the bottom of the skirt does not contact the flywheel. 650 pistons are fine in this instance, having shorter skirts. The cylinder wall thickness is rather minimal when you do this but I can report that it does work OK.

Previously bruce_lindsay wrote:

Hello folks

I am rebuilding 2 wideline dominators - one an 88 & one a 99.

I have amassed a few sets of barrels and original pistons and am keen to utilise what I have rather than buy more components - I'm especially keen to avoid fitting the new repro pistons which are all that seems to be available just now.

I have a few sets of 88 barrels at larger oversize bores e.g. +.030 and above and I also have some good std hepolite 99 pistons.

My question is - can a set of flat-top 99 600 pistons be successfully used in a 500 engine?

I am aware the distance from the centre of the gudgeon pin to the piston top is the same and gather that providing valve size& piston recesses have been checked first (I am using standard first-generation alloy heads) there should not be a top-end issue.

My main areas of concern are liner thicknesses when taken out to 68mm and also I have had it mentioned in the passing that piston skirt/flywheel clearance might be an issue.

Any advice will be appreciated, particularly first-hand experience from anyone who may have done this before.

Oh - if anyone has a good pair of +.060 500cc pistons available (not the new repro type)please let me know.

bruce_lindsay

I realise that the post I'm responding to here is quite old (2012) so you may very well have resolved your problem regarding cylinder barrels. I am presently scouring the world for a decent 88 cylinder barrel and having no luck. You mention that you have "a few sets of 88 barrels in the larger oversizes. I'm not much bothered about the bore size as that issue can be addressed. What can't however is the stroke. I have 99 barrels but no 88 barrels. You might just be the man to help me out. Victor Garman

Previously victor_garman wrote:

bruce_lindsay

I realise that the post I'm responding to here is quite old (2012) so you may very well have resolved your problem regarding cylinder barrels. I am presently scouring the world for a decent 88 cylinder barrel and having no luck. You mention that you have "a few sets of 88 barrels in the larger oversizes. I'm not much bothered about the bore size as that issue can be addressed. What can't however is the stroke. I have 99 barrels but no 88 barrels. You might just be the man to help me out. Victor Garman

Victor

I know of several 88 barrels available in Canada. If you are interested send me a private message.

Bruce

Many years ago I bought a 'box of bits' Dominator and successfully built and ran this engine as an 88. It is a 1956 Dominator crankcase with, I believe, matching Barrels and Head. I have had it bored to 600 as the LH cylinder wiped slightly due to oil issues. After a noisy test run I removed the Head and found witness marks suggesting the pistons are contacting the Head.

Has anyone else encountered this issue and how was it overcome?

Any advice on the problem or how to create greater clearance between Head and pistons would be welcome, e.g., machine top or edges of piston or how best to machine Head. Thicker cylinder base/Head gaskets? Any advice would be welcome.

You can buy a cylinder compression plate that will raise the cylinder and head.

Bought from RGM , I fitted one to the Atlas  to  make it a bit easier to kick over, in theory it could make a small change to the rocker geometry. In the event it has compensated for a bit of valve seat recession and actually improved the action. No problems.