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Re bore questions
#747875 09/04/18 11:54 am
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Hi All,

Sorry if this topic has been done to death but its my first time attempting a re bore!
Have just got my self a new set of JCC pistons and Hastings rings +040 ready for a rebore, on my 65 thunderbolt rocket.
the old pistons were +020 and blowing a good amount of smoke from both cylinders under load, esp.. the rhs, this was due to corrosion marks low down on both bores, as this bike sat for a long time in the states before it was resurrected, also some scoring on both pistons that looks like it might have gotten a bit warm at some stage,valves & guides are good, no play evident no evidence on back of valves of oil! and only 1000 miles since I o/hauled the head.
My question is what would be a safe piston/ bore clearance for the JCC +040 set up also ring gaps, new pistons measure 2.988 at bottom of skirt 90 deg from pin and 2.983 under the oil ring groove
I ride the bike mostly on short trips and the odd long haul at no more than 65mph so its never under any major duress so fingers crossed this re bore will give me many happy miles smile

Cheers, Mick


Mick M, in OZ
1965 Thunderbolt Rocket.
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Re: Re bore questions
NickL #748004 09/05/18 4:52 pm
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Originally Posted by NickL
4.5 thou piston clearance is the norm with a 10 thou ring gap.


plus 1


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Re bore questions
Allan G #748174 09/06/18 9:52 pm
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Thanks Guys,
Once again this sites info has made a daunting task easy grin
Stuck the barrel in the mill and bored it out and finished of with a 180 grit bore hone .0045 clearance no dramas!
Dry assembled the pistons and rings as published in other threads reset valve clearances and am stoked to say that after the first 50 kms all is good, running just as good before strip down, but with no smoke no rattles and plugs are looking pretty good, maybe a tad on the rich side but will leave for the moment.
Will take it out for another blast after work today then do a re torque on the bolts drop the filter and get some fresh oil in it for a good ride on the weekend thumbsup.
Once again thanks to all for the info.

Cheers, Mick


Mick M, in OZ
1965 Thunderbolt Rocket.
Re: Re bore questions
mickjmag #748265 09/07/18 6:45 pm
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Sounds great Mick!!

the barrel I'm running with now was bored to the later spec of .004", unlike the .0045 it would start getting fussy and wanting to cool down when running in, once run in it was fine but never had an issue with the .0045 and always try and keep to that value.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Re bore questions
Allan G #760028 12/21/18 10:22 pm
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So i've clocked up nearly 400kms on the T/bolt after rebore and its going great,but I've a query regarding oil on top of the pistons???
The bike is not smoking at any stage during riding or idle( had a mate follow me) or using any measurable amount of oil(oil changed twice) since first start.
Whilst looking at the plugs the other day I stuck the bore scope down the plug holes "curiosity" and notice what looked to be oil on top of the pistons(very thin coating not pooling) the bores still have very visible cross hatch and the pistons are shiny new on top, plugs looked great no oil burning evidence, I'm thinking the oil maybe from the valve guides after shut down but when the engine was apart the guides and valves measured well in spec (have only done 1500kms since head rebuild) so its got me curious as to where and how this occurs, any thoughts folks?


Mick M, in OZ
1965 Thunderbolt Rocket.
Re: Re bore questions
NickL #760135 12/22/18 10:37 pm
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Originally Posted by NickL
It'll bed in with use, don't worry.

Thats what I'm hoping for Nick


Mick M, in OZ
1965 Thunderbolt Rocket.
Re: Re bore questions
mickjmag #760142 12/22/18 11:42 pm
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If your really curious a colour tune will give you a little information. Or rather it will tell you if it’s excess oil or fuel by the state of its burn colour. But best bet is to look again when it’s all run in.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Re bore questions
mickjmag #760171 12/23/18 5:35 am
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DOPE
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what does burning oil look like through a colortune?


every day you do not take a chance is a day of your life that you will never get back.
Re: Re bore questions
mickjmag #760182 12/23/18 11:19 am
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Have you been going too slow and not making it pull hard?

If you think it’s coming down the valve stems or between the guides and the head, look into the ports.


Amateur Loctite enthusiast.
Re: Re bore questions
triton thrasher #760185 12/23/18 12:17 pm
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Originally Posted by triton thrasher
Have you been going too slow and not making it pull hard?

If you think it’s coming down the valve stems or between the guides and the head, look into the ports.

No certainly not babying the engine have been using the hills around my area to keep it loaded up.
Actually went for a ride today about 40 kms stopped and pulled plugs half way through ride and they looked great certainly not burning oil, checked bores when i got back and no oil was present so I'll see if anything appears after it sits overnight.


Mick M, in OZ
1965 Thunderbolt Rocket.
Re: Re bore questions
mickjmag #760738 12/28/18 10:05 pm
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Big ride yesterday with some friends 220kms to be exact, took the tools with me and checked the plugs at our lunch stop both plugs have a nice tan color right down into the base , so I'm pretty damn happy with the set up, no oil was visible on the pistons so as Nick L said I think it just needed more miles thumbsup


Mick M, in OZ
1965 Thunderbolt Rocket.

Moderated by  Allan G, Jon W. Whitley 

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