Morning - I am rebuilding a 51 B31 which had been hotted up at one time. the piston in the cylinder was a triumph 650 that would give 9:1 compression i believe- in the triumph. I have no idea what it would give in the BSA but on a compression gauge it showed 60psion one kick. The piston has a weird scratch and I am replacing it. it was also .40 over and undersized at that. There is not enough material left to resleeve or rebore. As this is an iron head for putting around I want to put in a 7.5:1 piston in its place and run slightly oversized rings. What I would like to know is what triumph 650 piston would give a comparable compression ratio. Any Ideas?
Hi David, It sounds as though you could have a 750 Triumph piston fitted from a T140. This is a common conversion to get more power from a B31. These pistons fit with no problems except for the high compression ratio. The usual trick to reduce the compression is to use a cylinder base gasket as a template and make a 2mm. thick spacer which is then fitted between the cylinder base and the crankcases. Good luck. Beezageezauk
Hi, sorry to butt in, but would that be a 76mm T140 piston, fitted by boring the B31 barrel 5mm oversize? Any idea what compression you'd end up with with a 2mm comp plate? I just took the somewhat tired B31 I just got off eBay to bits for resucitation... :-)
Re: b31 350 piston replacement#3600 08/23/058:00 am08/23/058:00 am
beesa350. No problem with anybody butting in. This is what the forum is all about. Passing on information and helping fellow motorcyclists. Yes, the piston I have fitted is from a T140 Triumph but I don't know what compression ratio it gives with a 2mm comp plate. I guess it will be between 7 & 8:1. I suppose you can get it lower by using a slightly thicker comp plate. There is enough meat on the barrel to bore out even further and use a B44 piston. This takes the engine up to 425cc. Don't try to bore out for a B33 piston or you will end up with no barrel and a selection of fins. Hope this helps. Beezageezauk.
Hi Dave, I've just gone through the same sort of piston research process you're into, if I'd known about this site I could have saved a bit of time, but doing the investigation is a great way to learn about a machines innerds. For my self, if I was just going to plonk around on the bike I would stick with the Tri. 750cc piston coz firstly you get a little more grunt, and secondly their gudgeon pins are the same diameter as standard BSA, the 650cc piston gudgeons are thinner and require the small end bearing of the conrod to be reduced in diameter.
I'd appreciate hearing from someone with more than a months experience (as I have) on the following.... Re. compression ratios...just measure the "compression height" of a known piston then if you want to try different types of pistons, just obtain one of similar height and the compression should be the same. The "comp.height" is the the measurement between the centre of the gudgeon boss and the top of the piston crown. The hepolite piston list quotes the following for a 71mm bore...
std. flattop comp ratio 6.5:1, comp Ht is 1 5/32" nxt oversize '' '' 7.75:1 '' '' 1 5/8"
I think there's a miss print with the specs for their 9:1 piston as the stated height is 35.5mm I feel this should be around 42mm as the 7.75:1 item is 1 5/8" (or 41.14mm when converted) highest '' '' 9:1 '' ''
I'm laying myself wide open to be shot down here 'cos as I say I'm just a newby to this engine modification thing, so if I'm wrong in the above take it easy on me...ta.
Re: b31 350 piston replacement#3602 09/06/056:00 am09/06/056:00 am
Interesting discussion above re cr ratio and the distant from the pin to the top. The flat top BB31 piston (maybe 6.5:1 cr)has about 1 1/8 inches above the center of the pin. I would expect that nothing sticks up above the top of the barrel given there is no dome. The BB32 GS 8.0:1 piston is 1 1/4 inch to the top of the side then another 1/8 inch to the top of the dome.
The B44 piston (9.4:1 cr)has the same 1 1/8 inches above the pin as the BB31 then another 1/2 inch for the dome above that.
By comparison the early B34 GS pistons are 1 3/16 inches to top of the piston side from the center of the pin then another 5/16 for 9.0 cr, 1/8 for 7.5 cr and no dome for 6.8 cr.
With these B34 pistons a plate of 1/16 inch thickness knocks the 9.0 down to 8.4 and a 1/32 inch plate reduces the 7.5 to 7.2.
I would think that besides the bore and pin size, the pin location in relation to the top of the piston side would be the major factor in selecting a substitute piston. The dome would be the next consideration for compression and valve interference. The total height above the centre of the would only be approx for cr given the different shapes.
By comparison the little T100 piston is 1 1/8 above the centre of the pin then another full 1/2 inch of dome.
Without frequent roadside repairs there is no fun in riding!
Re: b31 350 piston replacement#3603 09/06/059:17 am09/06/059:17 am
...pistons are my pet subject at the moment Gordo, and thanks for not crucifying me for my efforts in the previous post. re the 6.5:1 piston,you said " nothing sticks up above the barrel"....I don't know having never seen this piston in the flesh. Hepolite does describe it as being "flat with valve pockets"
I've just ordered a piston,for my B31 telling the machinist I want a dome height of 13mm, thats close to 1/2", above what he calls "deck height" or what you call " the top of the piston side" I estimated this would give me a comp. ratio of around 8.5:1 Do you or anyone out there have in your possession, a B31 piston with a known comp. ratio of 8 or 9:1 that you could measure the dome height for me. I intend to check the comp. ratio once the engine is assembled using the liquid down the plughole trick. I can always reduce the dome height if need be, if it's too low....see me if someone needs a quality road bike piston...
BTW. it appears the T100 piston you mention has the same overall comp.height as the B31, 7.75:1 piston, 1 5/8", according to the Hepolite figures.
The 650 and 750cc Triumph items must all be similar hence their use in beezers.
Re: b31 350 piston replacement#3604 09/06/059:55 am09/06/059:55 am
.... hear ya Dig, when I'm up on the podium recieving the flowers and the kisses I don't want the threat of a scrutineers check hanging over me....
But seriously, I see no point in breaking the rules, I'm no speed demon who has to beat everyone. My reason for having a go is just that, to resurect another old darling thats been under a blanket in someones shed for over 10 years.
BTW, I've been given number 138 .
In my last post I asked if some one could tell me the dome height of a B31 piston when I suppose any of the 350cc B group would do...cheers.
Re: b31 350 piston replacement#3606 09/09/056:09 am09/09/056:09 am
Thanks all. According to the numbers inside the piston I have its from a T120 (#17844) and as the pin fit tight the conrod must have already been reduced.It is domed with an overall of 1 5/8ths. also has a pair of eyelashes cut into the valve flats - which surprised the engine guys I showed it to. I think to be safe i'll order a new one and throw in the spacer. so new question - what is suitable material for the spacer? Dave
David B 01 T-Bird 69 tr25w 51 B31(in many boxes)
Re: b31 350 piston replacement#3607 09/09/058:39 am09/09/058:39 am
Hi again David, I am sure I made my compression plate from some 2mm thick alluminium (Or is it "alluminum" in Sunny CA?). This material is easy to work with, compresses nicely when fitted and blends in with the crank cases because it doesn't go rusty. Put a thin gasket each side of it with a light smearing of gasket cement or just a bit of "Instant Gasket". Beezageezauk.
Hi Bazza, No, I didn't have to modify anything when using the T140 piston. The piston fitted as the original one did and was the same height at the top of the barrel. The pushrods were not touched either and the adjustment was taken up with no problems. The ignition timing was also left alone but I have been told that I could get a little more power out of the engine if I was to play about with this. However, I have covered about 15,000 miles with this set up now and I am more than happy with it so I don't really want to play about with timing. Maybe someone else can advise on this!! Beezageezauk
Re: b31 350 piston replacement#3610 09/10/0511:55 am09/10/0511:55 am
If the adjustment is at the valve stem, your pushrod length (if correct prior to the mod) is now off by 2mm + the extra gasket thickness. Taking up the diff at the adjuster puts the entire sweep of the rocker out of range. Result: any or all of the following: 1. premature wear 2. power loss 3. lower maximum RPM
I am sure that there is no need to panic over this modification as it has been done so many times before. The valve adjustment on this model is just above the camshaft at the opposite end of the pushrod to the valve stem and the pushrod to rocker operates via a cup system. 2mm difference over the length of the pushrods is not going to cause any significant problems with the running of the engine. If there were any problems with the three items previously mentioned they would have been noticed long before now. Beezageezauk.