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Engine advice sought

I would be grateful of some advice from the collective intelligence and experience of the forum: I have a RE WD/CO/B which I am restoring. When I got it it ran, but the engine vibrated very badly compared to my WD/C and WD/CO, but it was a noticeably more spirited with it. When I stripped the cylinder off to get it rebored I found it had a high compression piston. I rebored the cylinder to plus 20 thou and refitted a plus 20 thou 'high' compression (7:1) piston. I haven't run it since, but decided to try and get the crank balanced in an attempt to eliminate the vibration, but keep the relatively
sporty feel.

Not withstanding the balancing discussion on another thread, I have struggled to find anyone who would balance a single cylinder engine, so am looking at refitting a standard 6:1 compression piston. What do people think? Should I pursue the balancing route or refit a standard piston? Will a standard piston resolve the balance issue, or could I still end up with a rough running engine? My primary aim is to get an engine with minimal vibration to make it more pleasant to ride and reduce the fatigue on the engine. I am also going to be in lockdown for some months yet which complicates the issue as I can't take the engine anywhere. My aim was to get the bike finished whilst in lockdown.

Many thanks,

Tom

email (option): Tomillward@hotmail.com

Re: Engine advice sought

Just a few thoughts on your concerns: How much lighter/heavier is an original piston over what you have now? If within a few grams difference, piston weight should not be a problem.

(Also what is the source for a “high compression” piston for an WM20 as I’m not familiar with such)

I’d suggest checking all the engine and transmission mounting bolts for proper tightening, and even if tight now, if the machine has run with loose fasteners, the mounting points can wear oversize from vibration as well as damage to the bolts. You may need to address these issues first if present.

Also, check to see if the engine head steady is properly mounted and tight.

email (option): teladelujo@msn.com

Re: Engine advice sought

Just a few thoughts on your concerns: How much lighter/heavier is an original piston over what you have now? If within a few grams difference, piston weight should not be a problem.

I’d suggest checking all the engine and transmission mounting bolts for proper tightening, and even if tight now, if the machine has run with loose fasteners, the mounting points can wear oversize from vibration as well as damage to the bolts. You may need to address these issues first if present.

Also, check to see if the engine head steady (assuming it has one) is properly mounted and tight.

email (option): teladelujo@msn.com

Re: Engine advice sought

Just a few thoughts on your concerns: How much lighter/heavier is an original piston over what you have now? If within a few grams difference, piston weight should not be a problem.

I’d suggest checking all the engine and transmission mounting bolts for proper tightening, and even if tight now, if the machine has run with loose fasteners, the mounting points can wear oversize from vibration as well as damage to the bolts. You may need to address these issues first if present.

Also, check to see if the engine head steady is properly mounted and tight.

email (option): teladelujo@msn.com

Re: Engine advice sought

.

email (option): teladelujo@msn.com

Re: Engine advice sought

Trying to edit to correct my original error has been impossible...! Just making things worse.

Sorry, I see you have an RE. I didn’t get past the WD.... assumed BSA. Still, same suggestions apply.

JDE

email (option): teladelujo@msn.com

Re: Engine advice sought

Tom I went all through this last year. My new high top 'Bullet' piston was over 3 ounces heavier than a standard flat top piston. I had the crank assembly statically balanced and it definitely improved, but it always did vibrate before. On the other hand, I fitted exactly the same type piston to my WD/G and that bike is still as smooth as it ever was. I had initially put this down to the G's heavier mainshafts, rubber foot rests and rubber mounted handlebar.

Here is the thread. http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1419649&cmd=show

Ron

email (option): ronpier@talk21.com

Re: Engine advice sought

Assuming you are in the UK I would suggest you contact Bassett Down Balancing. Nearly 10 years ago I fully rebuilt my 350cc AJS rigid trials bike. At the time I could not find a new,old stock piston for it,tried everywhere. I ended up using an Australian JP piston which everyone frowned upon.The problem was that it was far heavier than the Hepolite one that it had been running.We machined quite a bit of material out of the inside of the crown,but still couldn't get the weight down to the Hepolite.(Can't remember the figures)
Because of this I thought it best to get the crank balanced to suit the piston. Bassett Down did it,what I found interesting was that they drilled different amounts of metal from different places on either flywheel. From memory the balance factor we set was about 52% because of its use for trials at mainly low rpm. It has worked a treat,that engine is so smooth and is still running beautifully even though its done alot of hard work. I think for a road running single a balance factor of 65-70% is more normal,although there will always be a compromise,moving the sweet spot up or down the rev range depending on the figure used. My main point is that the dynamic balancing,whatever the factor chosen will always help to get the flywheels to roll more evenly as a pair if that makes sense. Apparently BD do plenty of Harley cranks,without fail the owners always comment on how their bike never ran so smooth before.

Re: Engine advice sought

There is an inherent problem with dynamic balancing of a roller bearing crank and I gather some companies won't do them as it involves stripping them down to balance each individual item. Static balancing is a reasonable compromise for our old singles. Ron

PS my guy is in Ringwood, so not a million miles from you. Let me know if you want his phone number.

email (option): ronpier@talk21.com

Re: Engine advice sought

Ron Pier
There is an inherent problem with dynamic balancing of a roller bearing crank and I gather some companies won't do them as it involves stripping them down to balance each individual item. Static balancing is a reasonable compromise for our old singles. Ron

PS my guy is in Ringwood, so not a million miles from you. Let me know if you want his phone number.
You give the balancing company the crank assembled without the rod. They dynamically balance it the same as a shell bearing bigend. You'd then take it away and reassemble with the rod. I don't know a balancing company that'll assemble the crank.

They only have to balance each end of a conrod and piston etc if it's more they 1 cylinder. Obviously with a single there's nothing to balance it to.

email (option): horror@blueyonder.co.uk

Re: Engine advice sought

I'm not quite with you Dave? Surely the piston and conrod all come into the balance factor, even on a single? Ron

email (option): ronpier@talk21.com

Re: Engine advice sought

Ron Pier
I'm not quite with you Dave? Surely the piston and conrod all come into the balance factor, even on a single? Ron
Absolutely right Ron, otherwise they will never know the balance factor they have set it to. Horror is also right about sending in a trued up crank with no rod,along with the separate piston and rod etc for them to weigh.That is exactly what I did with my AJS. Now that engine has been done I wouldn't bother having it done again as the flywheels now rotate as a pair,not trying to rock from side to side.I do have some pictures somewhere of that crank. When the time comes for a different piston all it will mean is a slightly different balance factor if the replacement is heavier or lighter. It was worth doing though,and if an engine does ride like a jackhammer I'd definitely have it done

Re: Engine advice sought

The crank should be supplied to the balancer trued up and without the rod fitted....They need to know the weight of the piston, rings, pin and clips....The weight of the remainder of the big end assembly...rollers, roller cage if applicable, nuts, locking plates and screws if applicable...They also have the rod....I believe they calculate a percentage of the rod weight and factor that into the calculation...

The parts listed above are the ones I have always had to send up (or supply the weights of each item)...I believe they then clamp a weight representing a percentage of the out of balance weight (the balance factor) to the crankpin and then adjust the flywheel weight to get the whole thing in balance....

If you don't know the actual balance factor for a single cylinder crank it seems 60% is the favoured figure...BSA cranks for the heavyweights we mainly deal with were balanced by the factory to 58%....Basset Down do them to 60% and my local British bike shop (now gone) always used that figure as well for the same cranks...

I would note again here as I have previously, I'm not an expert on the subject at all and have only picked up some information here and there whilst discussing the job... I've always 'farmed the job out' to someone with the knowledge and the right equipment to balance the cranks dynamically... Ian

email (option): ian@wright52.plus.com

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