Oiling Problem

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Oiling Problem

Postby arsey30 » Mon Jun 22, 2009 8:47 am

Quite right, but this was a new G12CSR, with an oil drilling missing, I still find it remarkable after all these years.

Dave.

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Oiling Problem

Postby freddie 136 » Mon Jun 22, 2009 7:35 pm

quote:

As has been stated previously, a reputable engineering shop will want to bore the barrels to the pistons. Both should be taken to the shop, along with the piston specifications which should have been supplied.



Hi, thanks for your reply,the pistons were took with the barrels to the engineering shop and rebored to match,the pistons where tried in the new rebores and had a [3]thou clearence,there was no information in the box'swith the new pistons.

does any one have more info re clearence's, piston to bore please, thanks to all for your help.

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Oiling Problem

Postby itma » Mon Jun 22, 2009 7:44 pm

A bit too tight that, in my opinion.

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Oiling Problem

Postby SPRIDDLER » Mon Jun 22, 2009 10:06 pm

Probably worth repeating a post of mine of a year or so ago..
Following a rebore I obtained a new +60 GPM piston and rings from JSL and took them to the borer. He asked me for clearances. I asked JSL for clearances and was told "We don't give them. Any engineering shop worth its salt will know what clearace to allow". He didn't.
I've been told by a couple of engineering firms that there should a be a leaflet of rec clearances with each piston...
I researched the web etc and came to the conclusion that as a rule of thumb for an air-cooled engine etc. a clearance of 1.5 thou per inch of bore was about right. I went with this and all has been well.
One reads that clearance depends, among other things, upon coefficient of expansion of the piston material, which we have no way of determining in our sheds.
I read an article by 'Humbernut' in Real Classic mag a couple of years ago where he determined the clearance by taking measurements cold and after heating his home-made piston and the standard original barrel to working temp.
Bit too long-winded for me tho'.
Perhaps JSL will respond as this query has arisen several times? Or maybe Rick the pisston ring can enlighten us?
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Oiling Problem

Postby Eric » Mon Jun 22, 2009 10:16 pm

Before anyone could give you the information about clearance you would have to specify the make and if relevant the type of piston because as I and others have said in our earlier responses the clearance is dependant upon this, you cannot really use a a rule of thumb. Having said all of that 0.003" clearance really does seem a bit too small for me. But I have just looked it up in my red book just to confuse things even more, because I think there is an error for the 600cc engine and maybe this is something to do with your problem. The 500cc twin has a clearance of about 0.005" the 650cc around 0.006" whilst the 600cc is shown as 0.001" or less and this just cannot be right. You need to go back to whoever the pistons were purchased from and ask them for the re-bore dimensions if its JSL they should have it to hand. If the information was not in the carton the pistons were suoplied in and you did purchase them from JSL please let us all know so that we can complain upon mass because its essential information. Only the piston manufacturer who knows exactly what material has been used and how this expands at the temperatures in our engines can really know and this should be passed on to you the end customer via the seller.

The red book clearances would have been for original type pistons that had wire wound skirts to restrict expansion in that area so modern pistons without this may require more clearance but 0.001" cannot repeat cannot be correct so please dont base anything you may do upon this figure.

Edited by - Eric on 22 Jun 2009 10:22:28 PM

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Oiling Problem

Postby SPRIDDLER » Mon Jun 22, 2009 10:36 pm

quote:

You need to go back to whoever the pistons were purchased from and ask them for the re-bore dimensions if its JSL they should have it to hand.


Err, Eric, which bit of my post above: "JSL told me they don't supply that information as any borer worth his salt will know".
Freddie already posted above that he bought the +20 piston and rings from JSL.
Believe me, I had quite a, what shall I say?, a prolonged 'run in' with JSL in an endeavour to get the piston manufacturer's recommended clearances. Consequently I avoided them for a long time until I relented in my desire to support the club and bought a list of bits including an exhaust camshaft bush which, after 10 minutes of running screwed oil out of the cam box into the mag drive and onto my boots and inside the rebuilt mag due to the the helix inside the bush having been cut the wrong way. Apparently I was "the first person to complain", however another supplier identified the source of this, allegedly, long-standing faulty item before I'd finished explaining the symptoms.
I don't mention these experiences to knock JSL from whom I have received satisfactory parts, but nevertheless the bitterness of poor quality / support lasts far longer than the sweetness of a quick delivery.

Edited by - SPRIDDLER on 22 Jun 2009 10:39:22 PM

Edited by - SPRIDDLER on 22 Jun 2009 10:40:51 PM
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Oiling Problem

Postby Don Madden » Tue Jun 23, 2009 2:37 am

But 0.001in was correct for the original G11 pistons & the other AMC engines using these wire-wound, split skirt pistons. Like other AMC pistons, they were tapered from bottom to top & cam ground oval when cold, but expanded to straight & round when at operating temperature.

While most piston makers specify checking clearance near the bottom of the skirt, AMC often specified at the top but included the expected growth. This can be confusing if you don't know.

The way to be sure is to measure cold, then at 350F & add 0.001in. to the difference.

Cheers, Don.


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Postby Eric » Tue Jun 23, 2009 8:53 am

Neville you seem to be upset that I ignored your post but when I started to write mine yours was not there so my reply was for freddie 136 in support of Kens quickie as they were there.

Don if you think 0.001" is correct for the G11 then why are the other clearances I quoted so much larger? They are all based upon the "Piston skirt diameter TOP" figures related to the given bore size from my copy of the AMC red book I thought all of the twins had the wire wound pistons?

Looking in the book again the figures for the 600 twin would even suggest an interference fit when cold if an upper limit piston was fitted to a lower limit bore e.g. Cylinder bore size: - 2.835" - 2.834" with Piston skirt diameter TOP: - 2.8341" - 2.8334". Not sure where the TOP of the skirt maybe perhaps the waist?

Its also very clear in the same book that the singles have much larger clearances but did not use wire wound pistons e.g. 250cc 0.008", with the 350cc ranging from 0.007" to 0.010" dependant upon what engine type.

One thing this does show is that the 0.003" clearance quoted as used by freddie 136 was too small unless it was either side and therefore 0.006".

Seizure or partial seizure of new replacement pistons does seem to be a very common problem with many engine re-builds, perhaps we could ask JSL for comments and even an article in the Jampot about the subject.

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Oiling Problem

Postby SPRIDDLER » Tue Jun 23, 2009 9:43 am

Hi Eric. Sorry, didn't mean my prod to be rude - should've added a

I will email JSL asking for clarification with regard to their GPM pistons since this subject has occured quite often and as far as I know has never been properly resolved, which is a shame as it may inhibit their sales of these, hopefully profitable items in their range.
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Oiling Problem

Postby itma » Tue Jun 23, 2009 4:16 pm

whats the point of working on clearnace given for original AMC wire wound pistons, which incidentally were selectively matched to the bore as well, when you are not using one?

So many Pistons nip up because;
# they are bored to the wrong clearance
# they are rebored by a an operator that is n ot familiar with the needs of older air cooled engines[or who plain dont care]
#they are bored fromthe wrong end of the cylinder, they should be bored from the bottom not the top; most rebore firms do it the former way cos its easier for their semi skilled machinist.
# plain and simple and the one you dont want to hear, its your fault; too much heavy hand on the throttle,too many folk who wont believe that riding about at 30mph for 500 mile is NOT the way to run it in.
Any make of piston will nip up under such conditions.
Far too easy to blame the piston.

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