Forum

[hagis]

I think before I started pulling the engine apart I would look at the decompression valve and even replace it with a new one to be sure its operating correctly, the fact that the engine starts and runs ok would suggest its not mechanical?.

[stinkwheel]

Not necessarily. Starting and running isn't necessarily an indicator of it being mechanically well. They will keep running even with pretty extensive damage.



My 350 ran for over a year and completed the Round Britain Rally with the exhaust valve seat flapping in and out of the head. They'll run for a long time with a partially collapsed piston crown and/or a broken oil control ring too. Just get gradually slower and smokier.

[Alan R]

Hi guys----whilst I agree with all that's currently being said about tight de-compressor cables, tight tappets, sticky valves et al, there's one overiding factor that tells you to take the head off}--- ie...you don't know what piston is currently fitted !!!... My own bike is a case in point}--- There's nothing wrong with it but the head is still coming off this Winter as I've no idea what piston is in there...... "Better safe than sorry" seems to apply here...

[Felix]

A question about piston quality.
My stock 2000 500 runs well with 13000 miles covered, and I'm considering a top end decoke/refresh in the next year.
Is our host's European made std piston a good choice? I intend to reduce the piston's top land on the lathe.

[Alan R]

Hello Felix------ Whilst our hosts European-made standard piston should be of a better quality material it would appear that the basic design hasn't changed.....Those deep under-cuts and slots are still apparent just below the oil control rings......This means that only the material in the vicinity of the two gudgeon ( Wrist ) pin holes is actually connected to ( and thus supporting ) the piston crown...

You might want to consider the American-made High Compression piston and fit a 3mm compression plate to regain some lower Compression Ratio.....Problem here is that the barrel and everything attached to it, will also be raised by 3mm.....The rear rocker cover in particular becomes even MORE awkward to fit if not modified.....and the exhaust pipe run may need to be adjusted.....The top of your carb. is also double ***ng awkward to access ac well....

Then there is the Forged Piston option which ticks all the boxes for strength, good design etc. BUT, as you've probably noticed---it's almost TWICE the price of that European-made Std. piston................This is my 4th Classic Bullet and I know what I'm fitting if my bike turns out to have an OEM piston....

[Felix]

Thanks, Alan
I guess I'll just have to decide after disassembly.

[Bullet Whisperer]

I have used the American made 'High compression 9:1' pistons for numerous builds and I even suggested to Hitchcocks that they might want to consider renaming it the 7.75:1 piston, as that's all I seem to get unless I shorten a barrel to increase the CR further. Well made, reasonably priced no silly slot in the oil ring grooves and not excessively heavy. Also available up to +0.080" oversize for those in search of extra cubes. Might be worth considering.

[Tim NZ]

(Thanks for posting the pic correctly for me, Alan)


The 'European' 500 piston is the OEM item...


Initially RE used a locally made 500 piston, and piston failure in the early bikes was all but guaranteed prior to them swapping over to the better finished European item.




Unfortunately, piston failure is (was) nearly always outside the warranty period, and it is my belief that the factory chose to ignore the situation?!




The OEM piston is made to a std 'formula' that dictates 'X' taper, 'Y' ovality, etc etc, for close running clearance and quite operation. It is not too dissimilar in dims and weight to the original Redditch supplied item. It also fails in just the same way as the original Redditch item did: Skirt failure or Crown separation.


It is not just RE that this 'problem' applies to. Every Brit bike maker who fitted similar design (T-slot, split-skirt) pistons eventually suffered from skirt failure when the motors are reved too high too long. Unfortunately for the RE 500, part of the 'situation' stems from the deck-height of the piston in relation to the gudgeon pin; the crown is higher than in nearly all other similar bikes (with the exception of the 500 BSA).


Which makes it more susceptible to beating itself to death when ever piston slap is present...


Hence the advice to reduce the piston Top-land dia to no less than 0.028" smaller than the nominal bore size, to minimise the effects of the piston rocking at TDC & BDC (slap). But even that is no guaranteed cure, as the skirt is still going to flex.


Rev a 350 or 500/700/750 twin past 5500 rpm too often too long and the Oil ring will eventually fracture, and then the skirt fails too.




Split skirt T slot pistons' are not suitable for high revving motors. Experience dictates that if you want an older classic 500 motor to stay together with the OEM style piston, do not ever exceed 4000rpm. (55 mph in top)
If you insist of riding any faster, my experiences are that it is advised to replace the OEM piston by 6000m (10,000km). Even at that short a distance there will be evidence of skirt deformation; excessive oil consumption, piston slap, ring wear and compaction.




With no better than a 1-5 chance of eventual piston failure for the old 500; do you want to take a risk?


Keep the speed down, or fit a forged solid skirt piston...

[Alan R]

Hi Guys-----Those of you with little or no Automotive Engineering experiences would do well to re-read TIM's excellent dissertation on this subject, it says it just how it is......As a good example of a well-thought out engine design take a look at the humble Honda CG125.....Both the OHC and pushrod models will rev to max RPM and STAY THERE for mile after mile....The pushrod model uses a high level cam with short pushrods to help achieve stability at top RPM....As with the Bullets I've owned 4 of these 125's over the years and with a 30 mile round journey to work and back none of them ever failed !! ........ Forged pistons, forged steel con-rods, roller bearings, wet sump, self-priming oil pump, etc.....In their "after-life" they are now very popular with the Junior Grass-Trackers and Go-Karters I believe.....

REG, returning to TIM's listing-- his very last sentence captures your situation admirably ..."Do you want to take a risk ??....Keep the speed down or fit a forged, solid skirt piston"....Well, Human nature being what it is how well will you realistically be able to keep that speed down ??.....be honest now, we all get caught up in the moment, don't we ??------I know I do, and frequently at that !! ( At 68 and having built and raced bikes back in the day you'd think I'd know better, eh ?? ---LoL !! ).........Keep us up to date please ??

[Reg]

Thanks for all comments, (even the scarie ones). Reckon I'll start with the de-compressor and work my way down.Any ideas on how to check if it's sealing OK once removed from the head? Perhaps I could screw a spark plug in the hole temporarily, and see if I get compression?