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61 Venom
#768041 03/13/19 10:04 pm
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Hi guys -- We have a 61 Venom in for repairs. Just ordered a +0.020" "Hepolite" piston kit and valves for it from Grove Classics UK. Seems we've heard several opinions n a few questions we have. Any Velo officionado's here to give us firm answers? Supplier says to put 0.004" piston to bore clearance on it? What ring end gap? What final hone grit?
It had been set up at 0.002" with an Omega KRT squish style piston - and had partially seized. Exhaust valve had also been clouting the piston - as valve timing incorrect. Does the cam have any I/D markings on it (not down that far yet) so we can degree it?
Cheers,
Brid.

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Re: 61 Venom
Brid Caveney #768043 03/13/19 11:03 pm
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2 thou clearance? no wonder it seized.

4 thou piston clearance is probably right, BUT it does depend on various factors; Alloy or Iron barrel? piston itself etc... (is it cam ground? split skirt? Comp ratio? etc).
Is it a 8.75:1 VM Clubman piston, or 8:1 VM split skirt piston?
Cam should have M17/8 engraved on it if it's a VM cam.
Hone: 400-525 grit at 30-60 degree angle.

Do yourself a favour... get a copy of 'Norm's Technicalities' by Norm Trigg, from the Velocette Owners Club of Australia. It'll tell you all you need to know.
It's worth much more than it's weight in gold for Velo guru info.


Re: 61 Venom
Joolstacho #768069 03/14/19 4:46 am
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Iron barrel --- piston on its way from UK - and just sold as a Hepolite, from factory part numbers - so just assuming it's a replica of the original, and same compression? We'll get to the cam tomorrow - and hopefully find out what it is. 400 - 525 grit sounds a bit fine for (what we're assuming is) cast iron rings? We are just rebuilding a 69 Rocket 3 engine - and that barrel was done with 280 grit - using cast rings. Not arguing - but last 'old timer Brit' we talked to said to do the Velo barrel with 150 grit! Compression wise -- It had an 0.048" compression plate - plus 2 paper gaskets on the cylinder base when we stripped it. Assuming that was there from the factory -- though most talk I've seen is of 0.031" and 0.010" plates. Be nice having anything to add to the factory manuals we have - but time is not on our side with getting the job done. The owner (79yrs old) has a couple of rallies he wants to ride it in - first week in April -- so, we have to push on.
Thanks for the reply.

Re: 61 Venom
Brid Caveney #768084 03/14/19 10:07 am
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The grit is determined by the rings fitted so Grove would be the best one to ask as they should know which are fitted. You would expect the rings to be cast iron and so a rough grit being needed with 150 being rough but you need that confirming. The Wassells Hepolites for the T120 have different manufacturers rings fitted both by Wassells and the dealers as an example. Omega's are made from a low expansion alloy so need tighter clearances that cast pistons but 2 thou sounds risky.

Re: 61 Venom
Brid Caveney #768099 03/14/19 1:15 pm
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Hepolite piston , solid skirt in an iron barrel = .0057/.006" clearance at base of piston.
omega( low expansion alloy )with an iron barrel = .0045"/.005 clearance.
regards
Rob

Re: 61 Venom
Brid Caveney #768100 03/14/19 1:36 pm
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Sorry for the off topic, but average cast iron piston rings in Rocket 3 cylinders will not last for long, you need to use Goetze rings ( cast iron with a spring in oil ring ).

Re: 61 Venom
kommando #768104 03/14/19 3:23 pm
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Kommando - Grove owner is on holiday (and not available it seems) and lady I spoke to couldn't answer the questions about the pistons/rings - so we are a bit stymied there. Need to put it to bed though - as need to get the bore done as soon as we get the new piston - to have a chance of completing the job on time.
Adam M -- the A75 has Grant compression rings and Emgo 2 piece oil rings (for some reason - the Grant 3 piece oil rings weren't correct for the JCC pistons)

Re: 61 Venom
Brid Caveney #768142 03/14/19 11:18 pm
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Camshaft has no markings on it at all - but according the the marks on the gears - the last perps had the timing 3 teeth out . Only question is cam positioning according to the picture in the factory manual we have?[img][IMG]http://i68.tinypic.com/260zoyh.jpg[/img][/img] [Linked Image] Any thoughts folks?
Cheers,
Brid

Last edited by Brid Caveney; 03/14/19 11:20 pm.
Re: 61 Venom
Brid Caveney #768143 03/14/19 11:19 pm
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Pic with cam -- is it correct? [Linked Image] The con rod is on the downstroke - just before BDC

Last edited by Brid Caveney; 03/14/19 11:25 pm.
Re: 61 Venom
Brid Caveney #768188 03/15/19 7:10 pm
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Brid, keep in mind that Velocette cams are pressed into the gear wheel that drives them, that is they are pressed in with no hard locating pin or keyway. So, seeing a gear wheel such as yours, that is off on the timing marks could be a factor of the pressed in job being off a little bit. Checking valve opening and closing positions with a degree wheel is the only sure way to make it correct.
Fullminator

Re: 61 Venom
Fullminator #768275 03/16/19 5:14 pm
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Fullminator - thanks for that thought. Thanks to the help of a well known Velo guru - we've ascertained it is an M17/8 cam, and it appears the centre line through the exhaust lobe/cam spindle corresponds with the timing mark on the gear. Once we get the barrel and head back on, we'll degree the cam, and check the exact figures.

Re: 61 Venom
Brid Caveney #768665 03/20/19 1:09 am
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That looks to be a Venom cam (M17/8). The MSS cam (M17/7) looks very different. The Venom lobes are much wider.


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Re: 61 Venom
Brid Caveney #768786 03/21/19 7:29 am
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Hi Mates
it the valve timing is right in the last photo, the cams look 90 degrees out, also I think the Velos have hunting teeth, they take a few turns to line up again after rotation also I rebored an 500cc Alloy cylinder/ Omega piston w/- 4 thou clearance that was spot on
Barry

Re: 61 Venom
Brid Caveney #768787 03/21/19 7:31 am
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and that bush on the idler shaft should be firmly in the timing cover ( i would think)
Barry

Re: 61 Venom
barry2 #771413 04/18/19 5:56 pm
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Barry, we put 0.005" on the piston, and it is fine. Owner has just done 200+ miles on it, and other than it being a bit tough to fire up -- is running nicely. It's coming back in tomorrow, for a check over.
Cheers all,
Brid


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