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Hi All

I bought from a friend a 1964 BSA A65 Rocket. Lovely looking bike, did quite a bit of restoration including, gearbox, tinware and respray, but a few knocks at low tick-over prompted the complete strip down of the engine. On investigation, the timing side bush had definatley gone and right hand journal was out of tolerance - a bit of rock on the conrod but no up or down yet. However, whilst the engine is completely stripped, taking the opportunity to have the crank reground, new conrod shells, sludge trap cleaned and timing side bush replaced.

The previous owner replaced the spec'ed 9:1 pistons with 7.5:1 to "save the bottom end". However, with a refurbished bottom end, I'm planning to restore back to 9:1.

The bore is still in tolerance (+40 rebore done 7K miles ago by the previous owner), so intend to replace with either Hepolite or JP - thus the big question. Is there a major improvement in performance from going from 7.5:1 to 9:1 (i.e is it worth the bother), and which is better - Hepolite or JP? The difference in price between the two makes is nearly double, but can there be such a difference in quality to warrant the price differential.

For interest, the bike is on Youtube

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If I were you I would go with the new Hepolites. They are made in Taiwan by a large piston maker & are very good quality. I've fitted them in a couple of engines, one of which, a Triumph triple, has been running for ten thousand miles with no problems. I have measured each set & found them to be accurate to within a couple of tenths & to all weigh the same. They are also a similar weight to original English made Hepolite & BHB types. The only thing to watch out for is the rings. The latest type are fitted with decent rings, but earlier examples had terrible quality oil rings.
As for JP, if my experience is anything to go by, they are best used as boat anchors. The 650 AMC JPs I bought were much heavier than the BHBs, so I sold them on & found some old stock pistons.

Martyn.

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Compression ratio makes little difference to wear on the timing or rod bearings, more how often you use wide open throttle from low RPM or sustained high RPM. Doubtful you can feel 2 thousandths vertical play, especially if there is any oil in the bearing. Are you really seeing rock of the rod or side clearance?
If you have 0.002" rod clearance and 0.9" wide bearing the angle would be 89.873 degrees. On a 6" rod the end would tilt 0.013".
Point is, measure first, then decide.

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Allegorically at least I have heard many people say that the JP are very much heavier than the originals . Not such a good thing if you don't want to rebalance it.


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JCC/Hepolite are good pistons.
At +0.040 i'd use standard t140 rings, i like the 2 part oil rings myself.
Or even better Riken with 3 part ones.
Just my opinion.

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Definitely Hepolite/JCC or other branded versions of the made in Taiwan JCC.

I'm up to about 30k on my current set and compression remains good. Careful assembly using everything I've learnt from this site has helped.

NickL knows his stuff so I'd certainly listen to him regarding ring choice. I have usually gone for Hastings rings as a simple of the shelf purchase as it has been said they are better than what JCC (who make the Hepolites) used to supply in the past. I dont know whose rings Hepolite are now supplying. I havent needed rings for several years so havent kept up with it.

Ray


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JP are a comparatively small company, and are great for small runs of weird and wonderful pistons, particularly for Veteran, Vintage and pre-War.

As the others have said, the JCC pistons (modern Hepolite, etc) are excellent. The current 7.5:1 pistons will be more tolerant of lower octane fuel, so they might be worth retaining if they're in good condition.

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E & V engineering, a site sponsor, carries beautiful A65 pistons

https://shop.shopevengineering.com/...182&shopBy=-8415&catalogId=-2087



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Originally Posted by Gordo in Comox
E & V engineering, a site sponsor, carries beautiful A65 pistons

https://shop.shopevengineering.com/...182&shopBy=-8415&catalogId=-2087



Gordo

Yes, definitely.
A little over the top for normal road use though................

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Difference between the two is about 5mph! at top end. With 7.5:1 the bike will still go really well.

If you do want 9:1, consider buying from MAP in the US, they sell good quality pistons and sell the rings seperately so you can get decent rings for your good pistons if you choose (and thats what I think is most important)

Map Cycle BSA Pistons + Rings

I use the JE pistons in my bikes (as sold by Ed V), been very happy with them and had more Pinking with the 7.5:1 pistons that I ever have with Ed's Pistons....

...But apreciate that some people are on a tighter budget.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

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

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Reading the original post: bore is in spec. My response to the question is… Not.
Sounds like you could decide to continue with it. If you were careful with the parts as removed, My 2c is hone and rings and leave it as is and you will not notice or regret it. If your buddies Spitfire is a bit faster, how often does that show up? Free advice is worth the price!
IMHO, The early A65 could likely benefit from any help NOT geared to speed tuning and the way the U.S. riders messed up a decent product.

Last edited by KC in S.B.; 01/06/23 4:44 pm.

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Just went back to view your profile. Not set up. You did post a video so you know how.
Probably not politically correct, but was thinking I could see if your age might indicate the mind set. Oh well….
ME? 7.5:1 would be just fine, last a long time!

Last edited by KC in S.B.; 01/06/23 9:29 pm.

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I’m sure this will raise a few hairs.

A while back I did an expansion test comparing Hepolite original 9-1 piston and Emgo 9-1 pistons.
For the test I used boiling water and a micrometer.
I found no significant change as they both expanded the same well within bore clearances tolerances. .
Looking at the Emgo pistons the castings looked as good as the Hepolite. I’ve used Emgo pistons with grant rings ever since.
Now I understand Hepolite is manufactured in Twain as is Emgo,
Do your own test make your own choice.
I use 9-1 they just seem to have a stronger pull and 5mph difference may or may not be a true feature. It’s that solid pull up a hill that gives me confidence I’ll get to the other side without using second gear. For me I would never consider any lower than 9-1.

I know this isn’t relevant but my 67 WC Hornet I run 10.5 - 1 pistons with 25% race gas and 75% 91 octane, ET Ignition. Now that sucker pulls like a train. Granted those pistons are standard BSA.

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Originally Posted by KC in S.B.
Just when back to view your profile. Not set up. You did post a video so you know how.
Probably not politically correct, but was thinking I could see if your age might indicate the mind set. Oh well….
ME? 7.5:1 would be just fine, last a long time!


Thanks all - reading through the post, will hone, fit new rings and keep the existing pistons. Might change my mind next rebore. Re mind-set KC, I'm an ex fast jet engineer so its not about how fast I can set off from traffic lights, I'm just OCD about getting the best performance, reliability and longevity from the motor.

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Originally Posted by chappers1962
Originally Posted by KC in S.B.
Just when back to view your profile. Not set up. You did post a video so you know how.
Probably not politically correct, but was thinking I could see if your age might indicate the mind set. Oh well….
ME? 7.5:1 would be just fine, last a long time!


Thanks all - reading through the post, will hone, fit new rings and keep the existing pistons. Might change my mind next rebore. Re mind-set KC, I'm an ex fast jet engineer so its not about how fast I can set off from traffic lights, I'm just OCD about getting the best performance, reliability and longevity from the motor.

I don’t disagree with Richard’s comments, my own engine started off with 7.5:1 pistons. I upgraded it and it’s now about 10:1. With the 7.5:1 it had good acceleration. As it stands now it’s still a quick bike but there are other things which will get good performance without blowing the head gasket.

One motor I built for a friend has 7.5:1 pistons fitted, a standard cam but I altered the tappet radius, new carbs etc. that thing goes really well and better than a stock bike with 9:1’s

High compression is a good thing but without improving everything else too, it won’t gain you much, and 5mph is the difference it gave me with no other changes made.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
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Hah! “…I'm an ex fast jet engineer so its not about how fast I can set off from traffic lights, I'm just OCD about getting the best performance, reliability and longevity from the motor...”

Check mine,.. Me too. 47yrs 91 jets.


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AS we all get older these old dungas get harder to kick over .... not the same i know but i have 7:1 in my A10 and it is easy to kick over , starts well and doesn't seem to object too much to the crap that comes out of the fuel pumps

I am very pleased with the performance and it has the feel of a bike "that just wants to go" . Nice and smooth ya know . In reality its not much slower than the A65 with 9:1


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
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The bike in the vid sounds fine , if its got good working parts stay with them, 7.5 :1 will still give decent performance, smoother running and a slightly better combustion space so its win so
me loose a wee bit.
On the other hand , a fresh bottom end would be the best time to go for 9:1.

Given that the petrol age is slowly changing , stay with 7.5:1. The bike will last longer,run smoother,
and you can use poor fuel.

But its a rocket , meant to be sporty, your choice.
Short sporty thrashes, go HC,
Long runs touring, go LC.
If it was me and the bits were there and working I would stay with LC.


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Not sure what company I'm in here yet, but thought I'd chime in.

I have built a couple maximum effort motors, but mostly stock and mild tunes. I still regularly ride a majorly modified motor. The others are just well-tuned stock builds.

With age, trials, and a declining tolerance for finicky-ness I've come to the conclusion that I like low compression machines for street use. If I need more power, I'll chase it (to the extent I reasonably can) with displacement and tuning.

I dislike breakdowns and blowups a whole lot more than losing races about town.

The heavily modified bike is stroked and bored, but still runs 9:1 for reliability. It has treated me well over many miles. Earlier builds at high compression took me down a rabbit hole of case breathing mods, ignition machinations to quell pinging, hunts for high octane fuel, strengthened starting mechanisms, etc. It was fun and I learned a lot.

Jason


Recovering perfectionist and absolutist learning to go easier in all ways

2009 Buell Ulysses (a lovely, capable, ugly machine)
1971 Harley Sportster (no end to the troubles. built in college)
1966 BSA Lightning (lightly modified for reliable use)
1964 Harley Duo-Glide (stock and unrestored)
1958 Harley Frankenpanshovel (hotrod!)
1949 Harley Hyrda-Glide (all the parts, none of the assembly)
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Welcome aboard! I think you will find this is a friendly place, and it’s fun to see what kind of trouble the other guy is having !! For being fairly simple machines, there seems to be no end of issues to deal with and keep you interested.


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Originally Posted by gavin eisler
The bike in the vid sounds fine , if its got good working parts stay with them, 7.5 :1 will still give decent performance, smoother running and a slightly better combustion space so its win so
me loose a wee bit.
On the other hand , a fresh bottom end would be the best time to go for 9:1.

Given that the petrol age is slowly changing , stay with 7.5:1. The bike will last longer,run smoother,
and you can use poor fuel.

LC.

That’s a good point since governments around the world are slowly increasing ethanol content in fuel. Still able to get 5% in UK at a premium price, but expect they will increase to 25% ethanol soon as part of the so called green agenda which will force another large proportion of older vehicles off the road - which I’m sure is the intention.

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his is getting off topic a bit........but i have a few comments,,,, as per usual lol.

9-1 is not and never was considered 'high compression' Many bikes and cars
are and were around 9-1 static compression and ran/run ok on pump fuel.
In the days of 5 star or 100ron fuel plenty of blokes ran 10.5-1+ that would
be considered 'high compression' for a hemi style single plug head.
My old crate (a65) runs fine on 'super unleaded 98ron' and is ok on premium
unleaded 95ron if that's all i can get, the regular fuel here is 91ron and requires
the timing to be knocked back to about 30degs and the target for fully advanced
set to around 4k rpm. This of course can only be achieved easily with electronic
ignition. E10 fuel here is 94ron and the bike will run ok on it with the timing set
to 32 degs.
The compression ratio on my bike is 9.1-1 when i measured it last. It's important
to radius and blend all piston sharp angles to prevent hot spots and assist flame
travel.
If you are able to get a squish working on the head. running on regular fuel is
no problem, it's largely about getting turbulence in the combustion chamber.
Most cars these days with that capability run very high ratios and run on regular
fuel which 'back in the day' we would have considered kerosene/paraffin lol.

Once again only my opinion but i was always taught that with these old heaps
'compression is free horsepower' so i hate to see it wasted.

Nick

Last edited by NickL; 01/08/23 12:51 am.
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i run 9.5 to 1 in a 72 T120, which has a lousy head compared to the A65, i think.

its been twin plugged and i pour in 87 r+m/2 gasoline with no difficulty.

twin plugging is harder to do on an A65 but there are people on this forum who have done it.

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Originally Posted by NickL
his is getting off topic a bit........but i have a few comments,,,, as per usual lol.

9-1 is not and never was considered 'high compression'

Nick

I disagree, I had a 1952 Ariel KH twin , 7.5:1 CR was the High option, std was around 6.5 :1 , at the time petrol was only around 75 octane.


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the whole compression V fuel thing ... part of the reason the Army liked the M20 at it massive 4.5 : 1 was because they would run on the shittiest fuel you could put in them .... war time fuel was not so great

my A10 runs well on the 91 and is 7: 1 ....

i made the mistake of putting some of that bio fuel in the a65 at 9:1 OMG it was not happy .


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