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MalcolmM

159 engine rebuild

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Hi All,

I am in the process of rebuilding the engine in my Bultaco Sherpa 350 (1976).

At the moment the bore is STD although badly worn so thinking of taking it out to one under the maximum size (any thoughts)

Also big end bearing, no noise and no up and down movement on the bearing, but there is a mile of sideways movement between the webs, is this normal?

It will get rebuilt with all new bearings seal and gaskets etc, 

Any help thoughts or recommendations would be gratefully received.

 

Edited by MalcolmM
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Hi Malcolm, yes, the big end does have a lot of lateral play as the con rod is kept central by the 2 alloy spacers each side of the little end bearing before the gudgeon pin is fitted. I usually make new ones and aim for about 5 thou clearance, fiddly and time consuming but I like things to be “ just so” ! Use C 3 clearance bearings on the crankshaft, I use a a NJ 205 ECP/ C3. By SKF on the ignition flywheel side, it makes re-assembly so much easier plus is much longer lasting on that side. Make sure you get the correct O rings set for your motor, they often get left out, not a good idea ! Oils, everyone you ask have their pet theories, it is your motor, your choice. Fully synthetic is fine for modern motors, not for our 50 year old ones. I think I have tried most, but have settled on Lucas semi sync. At around 32 to 1, that is 187 cc to 6 litres of fuel.  I may be right, or wrong, but my motors run just fine at that. Finally, get your nose into a good workshop manual, if you get stuck, put that hammer down and ask ! You will find lots of good info on here !  Over to you.......

 

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Johnjsy, thanks for the reply. You even answered my next question about what mix to use.

As it had not been run for 20 years I was using Castrol R at 50:1 (bit smokey in the man cave but smells great) but always knew it was going to need some major looking at. Will give the semi synthetic at 32:1 a go when rebuilt.

Cylinder all booked in, new piston, bearings, gasket and seal set on order.(should keep me quiet for the weekend)

Just got to figure out where all those gears go.

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Johnsy -  I was about to ask why you think synthetic oils are unsuitable for 50 year old engines, but b40rt beat me to it !

In the past,  my own experience with 2-stroke engines that have been stored in average conditions and  unused for a long time is that without exception, after being put back into use they consume their main bearings (and/or con. rod bearings) shortly after, even though those engines may have sounded surprisingly quiet and OK initially. This seemed to happen irrespective of the make of bike or brand of (mineral) oil used.

My last 2-stroke bike acquisition in May 2016 was a 1996 moped that had stood in a garage unused since 1999. With 17.5K miles on the clock, a 20 year old bike that had its first MOT when I put it back on the road !!.......This little bike that I still own and use for the occasional local errand is up to now the ONLY one I've owned that has broken the bearing failure pattern - and I'm quite convinced that the reason for this is that it has been lubed with Fully Synthetic oil from new.

Malcolm - I will tell you that Castrol R is definitely unsuitable, even if you find the smell irresistible  - don't let the stuff anywhere near your rebuilt engine PLEASE.......... 

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Lorenzo. Thanks, don’t worry i will definitely not be using R when it’s rebuilt.

out of interest what mix do you run your Bultó’s on.

Back in the day (1976) I used to run on 50:1 mineral oil, I know oil has come a long way in past 40 years.

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As for the size of the rebore, get the cylinder measured up by the person who is going to do the rebore and they will be able to tell you the smallest oversize that will work

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On 1/13/2019 at 10:18 PM, MalcolmM said:

Johnjsy, thanks for the reply. You even answered my next question about what mix to use.

As it had not been run for 20 years I was using Castrol R at 50:1 (bit smokey in the man cave but smells great) but always knew it was going to need some major looking at. Will give the semi synthetic at 32:1 a go when rebuilt.

Cylinder all booked in, new piston, bearings, gasket and seal set on order.(should keep me quiet for the weekend)

Just got to figure out where all those gears go.

Castrol R is not the best oil to use it may smell lovely but its a full on vegetable based oil and is intended for highly stressed engines at max revs most of the time, speedway engines, race engines and some scrsmbles-m/x engines.This oil produces a lot of hard carbon and gum and needs to be cleaned out really regularly, which usually meant a complete strip. 

Fully synthetic oil is fine for any older engine especially four strokes, with two strokes the petrol mix ratio is less oil and this can cock up the carb settings, jets etc. 40 years ago there was no fully synthetic oil for sale to the public, although it was formulated in the chemical industry around the Second World War. As with life there are certain provisos such as the old Jaguar straight six DOHC engines needed a 20/50 oil otherwise using (15/40) the lighter oils were dragged up into the combustion chamber weakening the fuel mix to the point where you holed pistons.

Edited by section swept

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