Dominator gudgeon pins - tight!

Are you sure that your replacement pistons are actually for a Norton 88? What is the fit in the conrod small ends like? My NoS pistons had tight pins but a good warming allowed them to slide into position in both the pistons and small ends.

Hi ,I fitted my old orriginal 99 pins to some new 650 old stock pistons, In otherwords they were all from the same period and in theory should have fitted and been able to float when hot,as per the Makers intentions.They were far too tight .I had to work on them to get a compromise .They are probably still not floating as they should and are a possible cause of the piston slap I get and may result in accelerated wear on the small end. The difference can only be a matter of half a thou or so. In hindsight I should have had the piston pin bore relieved (or found some 650 pins that were correctly toleranced.The circlip grooves should not be affected.My problem came from using unmatched parts,Your problem comes from poor manufacturing tolerances.A good engineer should be able to fix it. Its gone together but Its not a good job and may come back to bite you.

Previously roger_wilson wrote:

Having obtained some new old stock Hepolite pistons at +20 I had it rebored to the spec. However when it came to fit pistons the pins had to be drifted out despite heating beyond ânormalâ temps â i.e. rings removed and gas torch to high temp. Recognising this was extreme I relieved the piston boss by half a thou. to assist fitting as there was a one thou difference between the old pins and the new but the new were within recommended manufactured tolerances so thought this was ok however I had to âhammerâ the pins in!Advice please as to whether the 1955 dominator 88 pins should be fully free to "float" at working temperature? I note the old standard pistons had no oilway on the boss but these Heplex do.I have never experienced tight pins before on any bike and think I may need to ream out the boss another half thou. but what about circlip fit?

OWCH You never Hammer any inturnal parts these pistons need pre-warming before fitting and when dis-assembling There is a gudoin pin tool for fitting and dis-assembling pistons for the conecting rods yours anna j

Previously robert_tuck wrote:

Hi ,I fitted my old orriginal 99 pins to some new 650 old stock pistons, In otherwords they were all from the same period and in theory should have fitted and been able to float when hot,as per the Makers intentions.They were far too tight .I had to work on them to get a compromise .They are probably still not floating as they should and are a possible cause of the piston slap I get and may result in accelerated wear on the small end. The difference can only be a matter of half a thou or so. In hindsight I should have had the piston pin bore relieved (or found some 650 pins that were correctly toleranced.The circlip grooves should not be affected.My problem came from using unmatched parts,Your problem comes from poor manufacturing tolerances.A good engineer should be able to fix it. Its gone together but Its not a good job and may come back to bite you.

My thoughts are the same. They are coming back out and I will ream out to sliding fit when warm. Thanks to all replies I see we are all agreed as to how they should be.

Hello.

Continuing on the theme above regarding tight Gudgeon pins could I ask for some advice please.

I have a 1957 Dominator 600.

I have had a rebor to plus 40 thou and i am in the process of fitting new JE Pistons. My problem is the new gudgeon pins seem to be bigger than the ones I am replacing.They are obviously a good fit in the piston but do not slide in cold to the small end.

Could some one advise the Haynes manual suggests the pins should be a good slide fit into the small end as where the old ones when cold. The new pins seem to be 0.02 mm bigger sorry for the metric conversion but I only have metric measuring equipment and will slide fit by hand with a good push if the small end is heated with a hot air gun.

My worry is the pin will not turn and starve of oil.

Any advice would be appreciated

. Vinny_ Tray

The pins should to be free to turn in the piston when warm and in the small end cold and as you say oil should get in there too.you should get the little ends reamed to fit.They are probably slightly worn to the old pin size anyway,and could be a fraction oval too. In reality I'm embarassed to say I would tape up the pin /piston areas and tape polish the pin in the little end area!!. Would you buy a bike from me???!!!.PIstons need to be able to move sideways on the rod to accomodate bores being out of line a little, Don't want pins knocking against circlips.How do we sleep at nights?.

Dominator 99 (1957)

I recall after what I think was a first rebore on this engine about ten or more years ago that the gudgeon pin was too large a diameter to pass through the little end and I used an adjustable reamer to accommodate it without any problem. The gudgeon pins slid through the pistons with gentle heat though.

I have a note that the little ends of the connecting rods of that era are supposed to be aluminium and not bushed. They are aluminium but they were bushed.

I guess you cannot anticipate what modifications have been done on these old bikes.

I think the Dommy's were all bushed.Good that they are tight and will allow reaming to clear any ovality, loose would be no good.

I tried new GPM and JP Pistons in my ES2 both were bigger than the original heptolite. Like Richard I had to use an adjustable reamer to ease out the little end bush to allow a sliding fit. Fit in the pistons was fine.

Previously robert_tuck wrote:

I think the Dommy's were all bushed.Good that they are tight and will allow reaming to clear any ovality, loose would be no good.

I picked up the reference to the small ends not being bushed from the Haynes manual, Norton Twins (1974) Page 39. Para 18 where it says ' dural connecting rods are not bushed and must be renewed if excessive small end wear occurs ', so if this is the case it is obvious why they may be found to be bushed.

The Bacon Norton Twin Restoration (1988) on the other hand (page 48) states that forged light alloy rods are fitted and when referring to the little ends uses the words 'when fitted' as regards the bushes.

So who knows!.

This small attachment is from my 1949 Model 7 Parts List. It clearly shows that the Con Rods had bushed small ends from the start. Also worth noting is the Con Rods had Alloy Big End Caps. Something carrried through until the 650 & 750 engines came on the scene.

Haynes Manuals are usefull, but contain errors,some of which are serious, ie wrong assembly of gearbox etc.

Thank you

The nearest Norton parts list I have to the 1957 Dominator is for 1961/62 and that does list the small end bush as a part and the Connecting rod was supplied complete with small end bush.

I'll accept that Haynes was incorrect. Bacon is rather ambiguous on the subject.

Just why has a post, from July 2015, suddenly been resurrected?

Previously John Shorter wrote:

Just why has a post, from July 2015, suddenly been resurrected?

If you read Vinny's post you'll see why!

Thanks to all above for the valuable advice.

In my case the con rods are alloy and are fitted with bronze small end bushes.

From the advice above it would seem a hand reamer is in order to get the desired slide fit when cold.

Thanks again for the fast response to my problem.

Regards Vinny

IF IT'S OF ANY INTEREST i USED A 21/32 - 23/32 INCH ADJUSTABLE REAMER FOR THE LITTLE ENDS.

Previously John Shorter wrote:

Just why has a post, from July 2015, suddenly been resurrected?

Does it matter if it's of any interest?

Previously richard_evans wrote:

IF IT'S OF ANY INTEREST i USED A 21/32 - 23/32 INCH ADJUSTABLE REAMER FOR THE LITTLE ENDS.

Thank you, Richard - just what I needed to know!

Kevin