brt650 Posted August 26, 2009 Report Share Posted August 26, 2009 Hello, I'm fitting a new piston kit to my 159 350 and I have had to fit 3 base gaskets to get the piston height correct.Is this normal?It's a mahle piston. I've also had to machine 2mm from the exhaust side of the skirt as well.My bultaco dealer said I may have to modify the piston. So I just thought I'd ask if anyone else has had to carry out these mod's while fitting a new piston kit. Thankyou Quote Link to comment Share on other sites More sharing options...
hewson Posted August 26, 2009 Report Share Posted August 26, 2009 Hello, I'm fitting a new piston kit to my 159 350 and I have had to fit 3 base gaskets to get the piston height correct.Is this normal?It's a mahle piston. I've also had to machine 2mm from the exhaust side of the skirt as well.My bultaco dealer said I may have to modify the piston. So I just thought I'd ask if anyone else has had to carry out these mod's while fitting a new piston kit. Thankyou Hi there , i had the same problem with my 199b 340 , i had to get the mahle piston because it was the only 86mm one i could find. I just trimmed down both skirts to match my old piston .... Quote Link to comment Share on other sites More sharing options...
dschigoda Posted August 26, 2009 Report Share Posted August 26, 2009 Hello, I'm fitting a new piston kit to my 159 350 and I have had to fit 3 base gaskets to get the piston height correct.Is this normal?It's a mahle piston. I've also had to machine 2mm from the exhaust side of the skirt as well.My bultaco dealer said I may have to modify the piston. So I just thought I'd ask if anyone else has had to carry out these mod's while fitting a new piston kit. Thankyou In my M199 Sherpa T 350 I used a Wisco replacement piston. No mods needed. The required bore clearance was larger for the Wisco vs stock, because one is forged and the other is a cast aluminum piston. One expands much more than the other when at operating temperature. Quote Link to comment Share on other sites More sharing options...
steve Posted August 26, 2009 Report Share Posted August 26, 2009 Hello, I'm fitting a new piston kit to my 159 350 and I have had to fit 3 base gaskets to get the piston height correct.Is this normal?It's a mahle piston. I've also had to machine 2mm from the exhaust side of the skirt as well.My bultaco dealer said I may have to modify the piston. So I just thought I'd ask if anyone else has had to carry out these mod's while fitting a new piston kit. Thankyou Out of interest how do you determine the correct height? Which measurement was different to stock? Quote Link to comment Share on other sites More sharing options...
hewson Posted August 26, 2009 Report Share Posted August 26, 2009 You should'nt really have trouble finding the correct piston for a sherpa t ( bultaco uk or hughes if your in the USA ).In my case my 340 199B was on its last rebore and i needed a 86mm , the mahle piston i purchased may of been for a frontera or pursang . I think i took about 5-6mm off the skirts to match my original one . If your in the uk ring bultaco uk Dave renham is always helpful and knows his stuff ...... Quote Link to comment Share on other sites More sharing options...
brt650 Posted August 27, 2009 Author Report Share Posted August 27, 2009 Out of interest how do you determine the correct height? Which measurement was different to stock? Hello Steve, I don't have a manual. for spec's, But the piston ring was half it's width above the cylinder's surface.I have rebuilt a lot of other bikes and they don't normally protrude to that extent. It's a 84.2mm piston .I'm resizing the combustion chamber also to suit the new piston diameter. Thankyou for all your response's. Quote Link to comment Share on other sites More sharing options...
hewson Posted August 27, 2009 Report Share Posted August 27, 2009 (edited) Hello Steve, I don't have a manual. for spec's, But the piston ring was half it's width above the cylinder's surface.I have rebuilt a lot of other bikes and they don't normally protrude to that extent. It's a 84.2mm piston .I'm resizing the combustion chamber also to suit the new piston diameter. Thankyou for all your response's. Hi again ,are you sure your using a bultaco piston ?????? Edited August 27, 2009 by hewson Quote Link to comment Share on other sites More sharing options...
peterb Posted August 28, 2009 Report Share Posted August 28, 2009 Hi brt650, this is an interesting thread, The original piston for a 325 Bultaco was a Mahle, I think it was 83.2mm diameter. I have not seen replacement pistons from these times so cannot comment as to how they fit in. Seems a bit much to machine out the head to fit a piston, not quite the norm. Are you sure the skirt heights match for inlet and ex timing, are the transfer cut aways matched for height and width. From what you write, it appears the piston crown height is too tall for your bike, I guess it is too late to reconsider using another type of piston. I don't think the difference between a forged and cast piston is going to affect the thermal expansion properties of the piston material - does anyone know about this? I gather that for casting aluminium, a grade to LM4 is normally used, may be the same for forging. It is normal for a 2T piston height at TDC to have the piston crown flush with the top of the barrel, that is the shouldered edge of the piston. Good luck, PeterB. Quote Link to comment Share on other sites More sharing options...
bultacosteve Posted August 28, 2009 Report Share Posted August 28, 2009 The piston skirt length has no bearing on why the piston is protruding from the cylinder. The location of the wrist pin bore is what is causing the problem. Simply put its the wrong piston. Many of the Mahle pistons can be retrofitted to most bultaco's. Sounds to me like you have a piston that was for another 350 360 model witha different stroke. It is very common to have to file the intake skirt to obtain the correct skirt length. The other thing with using too many base gaskets is that it will change port timing. You would be far better to obtain the correct piston for the bike. You are permanently changing the cylinder head by removing material from it where as a piston change won't result in anything permanent. Part 2 is the subject of cast versus forged pistons. They make a huge difference. Wiseco pistons are forged and expand rapidly compared to the cast Mahle pistons. When running a wiseco the cold piston to wall clearance needs to be over 3 thousands of an inch to prevent cold seizures. I work in a modern motorcycle service department and every 2 stroke wiseco piston that I have ever removed from an engine has seized. It is usually due to the owners not properly warming up their bikes or running the wrong oil mix. They are very finicky and and need to be set up and used very carefully , but if they are all that is available then that is what people will be using. Stick with the Mahle pistons whenever possible. Steve Quote Link to comment Share on other sites More sharing options...
brt650 Posted August 28, 2009 Author Report Share Posted August 28, 2009 (edited) I agree bultaco steve that the piston I recieved is for a different model.However it had the same pin to crown height as what was left of the old piston. There was 2 base gaskets fitted when I dismantled the engine.I too believe the Mahle piston to be the better choice, but saying that. My riding buddy fitted a 3rd over mahle piston kit to his 199a and didn't check the ring locator pins.The top ring rotated and removed the top of the piston.And only after 15 mins.These piston kit's in Aus are $315 delivered.A pricey item for sure and come with no warranty. He checked the pins on the new one and I checked mine.They were fine. The combustion chamber size is 83.2 mm and now my piston is 84.2.I agree it's not much , but I'm skimming it anyway.I don't want the piston or ring coming into contact with it. I've checked my port timing. I have raised the cylinder 2mm.The transfer ports are 4.5mm above the piston at it's bdc. So 2mm closer will still be alot of difference.Don't see the point of changing them as a 350 has a mile of power anyway.I'll keep you posted when and if I ever get it going.I'll compare it to paul's 199a and see if there is a noticeable difference. The inlet side of the piston skirt was shorter than the exhaust side. I had to shorten the exhaust side 2mm so that I could rotate the engine.With a 1mm gap at the skirt to crankcase.There is still a overlap of 4mm to the exhaust port at top dead centre. Please feel free to coment as I haven't machined the head or finished bolting in the engine. But please note that piston is staying. If anyone has a piston sitting around and could let me know the skirt lengths I 'd greatly appreciate it.That measurement needs to be from a 159 350 as paul's 199a piston was different. Failing that does anyone have a 360 pursang barrell and chamber to suit.That'll make interesting riding in the tight stuff Thankyou Brian Edited August 28, 2009 by brt650 Quote Link to comment Share on other sites More sharing options...
peterb Posted August 28, 2009 Report Share Posted August 28, 2009 Hi Brian, I wonder if at some stage a previous owner has fitted a crankshaft, or complete engine from an Alpina. These had a 64mm stroke, and would account for you having the Mahle piston extend further out of your barrel than it should. My own Bultaco has a 64mm stroke Alpina crankshaft with Sherpa gear cluster and Alpina barrel. I have a few very old stock, but new Mahle pistons for the 325 Sherpa and will measure them up over the weekend, will also measure the old Alpina piston port heights. With the crank pin re-located to achieve the 64mm stroke, there would be a change of 2mm up and 2mm down in the stroke when compared to a 60mm stroke Sherpa. Bye, PeterB. Quote Link to comment Share on other sites More sharing options...
hewson Posted August 28, 2009 Report Share Posted August 28, 2009 Hi again , i've just took the head off a spare 199a 325cc engine i have , and this is as high as the piston goes with no base gaskets .The 199b 340 i have as a raised liner but its still the same with the piston top near flush with top of the liner ...Cheers Quote Link to comment Share on other sites More sharing options...
brt650 Posted August 28, 2009 Author Report Share Posted August 28, 2009 Wow this is the reason you join forums.Can't take for granted that I know everything. The photo's are great. The crankshaft may well have been changed.The barrell and cases have 159 numbers on them. Funny you mention different crank Hewson as the bike was fitted with a 71 sherpa tank and alpina seat. I fitted a new rod kit to the crank.I bought that from BultacoUK.It was the same as the one I removed.I'll check the stroke tomorrow . I'm not going to be happy if the crank has been changed. The chances of getting the right one will be hard for sure.Or maybe this engine was a special test engine? Quote Link to comment Share on other sites More sharing options...
brt650 Posted August 28, 2009 Author Report Share Posted August 28, 2009 Well, Peterb you are correct.It has a 64mm stroke crank. Bugger, bugger, bugger. Lucky I have my trusty italjet to keep me going. I'll have to try and chase a crank now. Thankyou everyone for your input. Brian Quote Link to comment Share on other sites More sharing options...
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