535cc Dommie

Quite a few owners had to do this over-bore when 500cc pistons and rings were in short supply. I am not so sure that things have changed for the better!!!

You can happily bore your cylinder block to plus 80" which will be then around 68mm. The standard 99/650 bore size. There will be plenty thickness of metal left on the liners. Plus you can use 99 or 650 pistons.

Now there are 2 drawbacks to doing this mod.

1) You will probably have to rebalance the crankshaft because of the bigger pistons that are now about to swoop up and down the bore. If you don't do this the vibes will be very bad.

2) It might be a good idea to inform your insurance company about this conversion. Just to cover yourself. As a rule most just note the mod without upping the premium. Some ask for an admin fee.

I have done this too. Best to use 650 pistons - the 99 ones have a longer skirt which could touch the flywheel on an 88 crank. I didn't rebalace the crank - vibes were still the usual modest 88 ones. Would the insurers worry about what is after all just a rebore? I informed insurers that I had reduced a 175cc Bantam to 125ccand they demamded a more than doubled premium - because it was now modified. I changed insurers.

Some insurers will use any excuse to get out of making a payout. Some will consider any part changes, away from standard, as a modification...which in the small print often states "you must notify of such". This was an issue raised on the NOC website many years ago when owners were bolting on Mikuni & SU single carb conversions. Even converting to a disc brake or double disc set-up were also technically modifications even though in most people's eyes a very sensible move with regard towards improved safety.

Boring oversize may well be considered as a performance enhancing modification. In which case I would recommend as least contacting the insurer to check out their rules.

A footnote to this...........a few years ago a local chap, I knew, managed to lose control of his newly restored Triumph Bonnie and wrap it (& himself) around a lamp post. When he got out of hospital the Police prosecuted him for careless driving plus a whole host of minor law infringements due to changes he had made to a standard bike. Things like adding dropped bars, racing shocks, no chain guard etc. There were quite a few altogether. He admitted guilty to all and was consequently clobbered with a huge £600 fine plus 3 months loss of licence.

The charge and fine, that really stood out as absurd, was for 'fitting the rear chain spring link the wrong way round.' His insurance company claimed no knowledge of any of the mods, he had made, and refused to pay out. No bike, no money and then no job.

Best not wrap your bike round a lamp post then! It does seem the police were determined to find fault somewhere - something that does happen when they are somewhat aggrieved. But back to the over-bored 88. Going up from +.060 to + .080 is just a normal rebore, not whatI would consider an attempt to gain extra performance. I suppose a particularly vindictive insurance company could strip the engine and quibble.Interesting dilemma. But where would it stop? There are very few factory spec bikes being ridden (the emphasis on being ridden, rather than trailered to shows) out there. I have 12v electrics, heated handlebar grips and a non-standard r/c head on my Domi - 3 strikes (well, modifications) and you are out? That's what some insurers specify.

Does the factory owner's manual or repair manual state anything about a maximum re-bore tp +.080?

If it does, I would think that this is enough to show that the re-bores to this size are normal for wear.

I recently renewed my insurance with FJ and was asked if I had done any performance increasing modifications and I said not. Down to one carb on a Commando is hardly that. But it does make me think about trying a larger main jet in my Poor Man's Manx!

As a part of agreed value, I have to send photos of all sides of insured bikes and so it would be no use them saying they didn't know about the RGM front disc conversion!

On another point with regards to Insurance: (Yes I know slightly off topic) you may have noticed a Speed camera van where years ago there was none. With this in mind and phone calls coming in about my coming car insurance (Not FJ) I asked the question: What if in six Months time I got 3 points on my licence? The agent said that I would have to declare it at the next renewal but he said he would ask his manager (pause) NO, I would need to declare it straight away and also pay a mid term adjustment! Strange, because you enter into a 12 month contract that was correct at the time the policy was taken out.

It's a bit like RPI rate of inflation for putting up prices but CPI for putting up your wages and pensions. In other words, you lose and THEY win.

Just like Ethanol, this is another real dogs dinner the Norton rider (+ others) have little or no say on.

Previously David Cooper wrote:

Thanks for all the posts - happy and sad! Computers are making insurance increasingly impossible as they micromanage everything we do and stamp conformity everywhere. My bike has electronic ignition, concentric (not monobloc) carbs, stainless steel spokes and most fasteners, a luggage rack and it might have a standard (instead of correct SS) camshaft (I can't be sure). It has 'modern' tyres since the Avon GP has vanished. Sometimes it has had only one carb, and sometimes a siamesed exhaust. And the rear numberplate is probably not the correct type, and PO removed the front plate entirely. Worst of all, I suspect half its originally horses have fled the stable. I wonder if my insurance is valid?

Hi i think the best way would be to note down all that has been modified and that to the best of your knowledge all is correct then have an engineers report done on the bike showing them exactly what has been modified or changed ect then its up to the person doing the examination to get it sorted for the insurance. then peace of mind.

when i rang my insurance i told them i had my es2 in cafe racer trim from standard the person just asked has is it been done with period parts i said yes and that was it. as they say they record all calls for training ect so it should be all logged down there end what was said.

Neil has now got me really worried as I have just upped the main jet in my newly fitted Premier Concentric 32mm carb. It came with a standard 220 jet which I tried but found that it seemed to dull the top end performance. I had forgotten to tell my insurance company about this possibly important change. I did mention, to them, 5 years ago that the bike was purchased with a single SU carb.

Michael......all of the early service manuals recommend rebores to no more than +40" . This is certainly good advice on the model 99, early 650 and Atlas engines with a top barrel spigot. If you go to +60" then the spigots get thin and susceptible to breaking at either end. The 500cc engine starts with a much thicker liner which could probably be taken out to +100".

Returning to the issue of a +80" rebore of a 500cc barrel and then fitting standard size 600/650 pistons. The problem of re-balancing will probably centre around the type of replacement pistons used. Checking out the like of GPM pistons as supplied by AN reveal a serious increase in weight. Too much to be ignored. This is one reason I stick to NOS hepolite or good used pistons in my rebuilds.

Several years ago my Poor Man's Manx received a new std size Australian Piston (Excellent quality) following a re-sleeve

I took it as a straight swap so didn't weigh against the genuine +60. (Piston good but bore knackered) It was a bit lumpy over the old one, especially at higher revs. The compromise was a 22 tooth engine sprocket (std 20T) and a 20 tooth G/box sprocket. (std 19T)

Yes it pulls these gears fine, acceleration less good but 70 mph manageable at low revs, without loosing fillings and reasonably smooth at 50 mph. Rather than another engine rebuild you might want to try raising the gearing and finding 100 mpg at 50-60 mpg is on with a low revingengine. (Will be slightly different on a twin)

Don't try this fix on a Model 50, it won't pull it. You may just get away with a 20TG/box sprocket but that's about it.

I wonder if stress from your big end failure caused the damage to your Australian piston, David? Can't have been pistonfatigue?

David, can you tell us the make of the offending piston? I have seen several similar examples, all original Hepolites. What can happen is that the hole appears directly above the small end of the con rod and as the engine continues to run, albeit not very well, flame torches through the hole in the piston onto the the rod which lets go. In a way, you were in fact lucky!

The spigot on your barrel looks quite chunky ........ suggesting the possibility of an over-bore without the need for expensive liners. From memory, the standard 88 barrel can be rebored up to +80" easily ........... which is standard 99 piston size.

Greetings from South Australia, Hope this is not considered 'off-topic' but JP are a helpful precision engineering & piston supplier. Theymake very good motorcycle pistons & custom 'specials' from UK sourced 'blanks!' However, these are renowned for being heavier than stock. I am just restoring a 1964 Triumph Tiger 90, (OK I know) but the original pistons at +0.060" (over maximum Triumph manual recommendations of + 0.040") were cactus. Fortunately, I had a second cylinder which was bored to +0.030" and JP provided suitable unobtanium 9:1 pistons (specified to 0.030"), which is a non-Triumph size but the correct 'specification' compression. On weighing, the blank-I.E no rings circlips or pin, old 0.060" piston came in at 154 grams. The new JP at +0.030, also blank, was a massive194 grams (40 grams more each!) heavier. As the flywheel would have originally been factory balanced for stock pistons of an unknown weight, this is now a balance dilemma. Rods have been 'tuned & polished'to be +/- 2 grimes (as are the pistons straight from the factory)but the original weight unknown piston stock and thus balance weight vs. 'obese' JP may cause an issue. I spoke with an engine tuner and he seemed little fussed....... JP offer up-to 0.080" oversize in most M/C pistons if your cylinder will take the strain but the slugs are of course heavier again.(in my case ~ 50 grams each heavier, as down at the factory we weighed a 0.080" suitable blank).JP are super to deal with and know their stuff, stillprivately owned and have been making pistons for25 years!

Comments of a post card please?

Regards Steve (Ex-pat, but now also from the home of JP pistons Adelaide) and no, before anyone starts, I am just a customer!

I'm off topic now David, but Coopers still are the best commercial beers brewed over here but I've moved over to the James Squire ales. Still not as good as Theakstons, Greene King India Pale or London Pride though!

Since some Dominators are now over 60 years old (mine is 62), they may have been fitted with a variety of pistons over the years.

I am unlikely to go down this route, but, does anyone know the weight, of an original 88 item? If so, is there enough metal, on a 600 piston, to machine it down to the original weight? This would presumably solve any balancing problems. I can?t believe any insurance company would bother to strip down, a crashed bike, to check bore/piston sizes.

Just a quick but certainly "Off-Piste" topic for Ian, what about the wonderful Henley breweries Brakspear's? Only going since 1779!

OK enough!

Let me know how you go with the pistons. I certainly have an original but worn Dommie piston or two here (ex-650SS) and will weigh at the weekend if that assists.

Rgds Steve (Hic!)

Attachment is from S Robinson Tuning Book. It suggests 88 pistons were around 260g. Replacement hepolite Pistons for 99 in the 1970/80s used to be about 280g. The original GPM 99 pistons could have been used to anchor a ship. Especially those on +60".

I fitted 650 pistons to my 99 and removed a lot of metal from inside the skirts and knife edged the bottoms to reduce oil pumping power losses. What with the shorter than std skirts I would not be surprised if they weighed less than the orriginals, I have had no issues. With regard to new replacement barrels for 650,99 and 88, many months ago I raised the issue with a mate who makes similar stuff. I understand that he is "on the case" and that there is light at the end of the tunnel.