Dominator 500 twin

Very Strange.......HC pistons just about poke above the top of the 88 barrel. The tops of standard compression pistons are about level.

A possible cause of your problem is the wrong crankshaft being fitted. Has a99 shaft made its way into your engine? They do look similar. The difference in stroke would be easy to check. Just turn your crankshaft and then measure and half the distance measured between TDC and BDC. If it comes to around 82mm then it the wrong shaft. The distance should be 72.6mm.

Checkout the attachment. It shows the central flywheel rib that the 99 crankshaft has got. The 88 crankshaft has a plain round outer surface.

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photos-020-jpg

88 stroke is 72.6mm, 99 stroke is 82mm. So a bit of arithmetic on fingers (or back of envelope) indicates that sticking an 88 barrel and pistons on a 99 crank would have the pistons protruding 4.7mm too high. It was not unknown for people to put a 99 top end on an 88 - gave a nice low-compression easy-going engine. Just right for modern fuels!

Previously alan_wooldridge wrote:

Can anybody help me with the correct numbers for left and right pistons and the length of them please, as the pistons I have at TDC protrude 3.5mm above barrells.

Are they like this?

My 88Ss photo in 2005.

No measurement to hand but looks like 3mm or so.

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dommie-88-23-may-05-jpg

Previously David Cooper wrote:

Previously alan_wooldridge wrote:

Can anybody help me with the correct numbers for left and right pistons and the length of them please, as the pistons I have at TDC protrude 3.5mm above barrells.

Are they like this?

My 88Ss photo in 2005.

No measurement to hand but looks like 3mm or so.

hello what numbers are on the top of those pistons They look like High Compression piston to me the Original pistons were stamped BHB . LH 22504 and RH 22505 7.8 to 1 compression ratio

All very confusing.

Originals were slotted type: the left one is numbered

LH 16338 STD MADE IN ENGLAND

The ones (Australian) in the picture are new May 2005.

One of these subsequently developed a hairline crack from the gudgeon pin up one wall to the crown. Found only by accident when I changed big end bearings. Scary... no idea why a piston would crack like that...nor if the replacements are OK or for how long.

Picture attached shows both.

I'll have a look in the shed and measure the old and new crown heights.

It would be nice to have a database of such measurements somewhere.

Attachments
dommie-88-pic-2-23-may-05-jpg

Previously David Cooper wrote:

All very confusing.

Originals were slotted type: the left one is numbered

LH 16338 STD MADE IN ENGLAND

The ones (Australian) in the picture are new May 2005.

One of these subsequently developed a hairline crack from the gudgeon pin up one wall to the crown. Found only by accident when I changed big end bearings. Scary... no idea why a piston would crack like that...nor if the replacements are OK or for how long.

Picture attached shows both.

I'll have a look in the shed and measure the old and new crown heights.

It would be nice to have a database of such measurements somewhere.

Hello The ones you may think are the Original ones are after market Hepoltie piston made in Bradford the original ones were BHB brico heavy industry and they made the cylinder heads too if you need some good pistons then get in touch with our Heavy twins expert Dale Middlehurst

you will find his contact detail in the roadholder

yous anna j

Thanks, Anna. No pistons for me at present I hope. I know the bike was rebuilt before I took it on. Out of curiosity - re: the slotted pair that came out - I've heard about tops of such pistons breaking off so I imagine I'm better off without them? Or is that only a Commando issue?

Sorry, Alan for changing subject.

I have a friend who removed the compresson plate which arrived under the barrels of his 650SS and then found that expanding hot con rods causes the pistons to clatter against the head when the engine warmed up ... apparently it was not a catastrophe but I understand he used plasticene to identify the problem.

Your newer pistons are of considerably higher compression ratio than the original slotted ones. Why do pistons fail?All sorts of reasons. If you subject slotted pistons to lots of high revs, the tops can come off. Best keep these ones below 5,000 rpm. Cracks in any piston (like your new one) will develop because of localised high stress. Again, lots of possible causes. There are many opinions as to the merits of the various makes of pistons. All I can say is that my box of knackered pistons gathered over the years are all Hepolites.

I used to run an 88 with the slotted pistons, Back in the 60's, it had a SS cam and 650 carbs.It was fast. I ran with others who had 99ss, 650ss and Atlas. I was kind to them, I waited for them to catch up.It went everywhere at 7000 rpm, I had a girlfriend in Lancashire,I lived in S London, I used to take her out on friday nights after work and riding up there. I did the Dragon Rally several times,I was not overtaken. The slotted pistons did not fall to bits, They did sieze at 95 mph on the last few miles of the M1 after a non stop run around England .They were not all bad. (the 88 was not as fast as a 650,I just rode faster!, and no i was not the fastest man ,that was Ron Chandler who left me for dead!).

Previously David Cooper wrote:

Thanks, Anna. No pistons for me at present I hope. I know the bike was rebuilt before I took it on. Out of curiosity - re: the slotted pair that came out - I've heard about tops of such pistons breaking off so I imagine I'm better off without them? Or is that only a Commando issue?

Hello well you have been told wrongly the Hepoilte spit shirt pistons are OK they have a extra strengthen boss the idea behind them is so when the piston gets hot they can flex better in there bore. providing they have been honed correctly cross hatch honing is very important. this allows the oil rings and compression rings to do there job of keeping the oil from getting on top of the piston and lubricating the gudgeon pin and little end and the sides of the piston skirt . if you look on our spares shop you may find some original pistons for a model 88 . yours anna j

The only thing now is I have a spare 99 crank, in perfect condition, clean and reground, plus 10 thou, if you know anyone who may be interested.

Alan

Yes,yes, yes....me please..........My 99 crankshaft is already at plus 45 thou and likely to die very soon.

The pistons are also plus 40 thou size running in a very worn bore with plus 60 rings.

If you still havea crankshaftfor sale and it will fit an alternator engine then please let me know asap.

Funnily enough I have a spare alternator 99 crank available as well if anyone's in need.

Hi Phil, mine is an alternator crank. I'll go and stick a mike over the big ends if you want.

Re-88 pistons,

One of my Dommi`s Hepolite pistons lost it`s top one time ,after 82.000 miles .I didn`t hang around in them days,yeh the thing broke around the oil ring,no damage was inflicted that time.I still did an engine rebuild though ,usingGMP pistons as supplied by that man in Cannock.I think they where made by an Italian outfit called Gandini,good and strong they are ,and no problems so far.

Cheers, Alan H.

Alan

Thanks for the offer but I already have two dynamo crankshafts sitting around that nobody else seems to want.

I made the mistake of thinking thatI couldresurrect (cheaply) an old Dunstall 99 engine that had sat in a barn for 40 years or more. Bad mistake!!! Not only were the innards all badly corroded but the crankcases were like collanders and the cylinder head cracked. I had to bin the special camshaft, followers and lightened rockers due to serious 'TinWorm' issues. I bought 2 dynamo 99 engines to cannabilize for replacement parts but apart from the rockers, timing cover bits, pistons, rods and barrel nothing else was of use.

My plan is to start again and put together another alternator engine, from scratch.

Phil,

My spare 99 alternator crank is still sitting on the shelf if you still need one.

Gordon.