I have three Sloper engines, a 1929 a 1930 and a1931, I am hoping to rebuild the 1930 engine as this was the original engine, however they all have some minor issues.
I have removed the barrels from all of them and have the following issues.
1929 is in generally good condition a couple of chipped fins and honed up OK it has a bore size of 3.284 (83.4mm), it was fitted with a flat top piston (only observation I have made in comparison to the others is that it has 9 cooling fins where the others have 10)
1930 again has honed up OK, a few broken fins nothing to bad and has a bore size of 3.242 (82.34mm) it was fitted with a flat top piston (Wisconsin DB190A) Only problem is the bottom of the cylinder where it sits in the crankcase has two chips broken out of it.
1931 honed up OK a couple of dark stains but no grooving, a couple of broken fins nothing to bad and has a bore size of 3.141 (80mm) again fitted with a flat top piston, this one has a fairly badly damaged bottom flange area...
My question is really, would the cylinders with damaged flanges be serviceable or would these broken areas cause problems? Attached pictures
Secondly, I could clean up and use the pistons again but was hoping to fit a three-ring piston with oil scraper, has anyone found a piston that could suitably replace the standard piston....
Thanks in advance and sorry if the enquiry seems like a dumb question but as we know parts are a bit scarce for these old girls so at times, we have to make good with what we have. Cheers
Hi, if you had one of the barrels with a broken flange sleeved the broken flange would not matter. AS you say finding a pristine barrel is almost impossible.
It seems to me that there is something rather strange here.
The 1929, 1930 and 1931 Parts book show the same Part No. 24-6 for the barrel.
The 4.93 OHV Sloper barrels I'm aware of have 10 fins, so I don't know what your 9 fin barrel is off. Surely, If BSA had altered the number of fins, the Part No. would be different? Who knows?
My view is that the damage to the 1930 barrel will not cause a significant problem, because I don't think the rings will reach the damage. I think you could clean up the damaged edges and put a chamfer on the inside. However, if you fit a piston with an oil control ring at the bottom of the piston, it may well catch. (But see below, re fitting a liner).
I have been using a 2 compression ring piston for the last 30 years, and haven't had any problems. Yes, I sometimes have to adjust the oiling if it smokes too much. I find the adjuster is quite sensitive. Can't really see the damage to the 1931 flange. There seems to be a brass shim covering it?
Now, I failed my 11+ maths exam, but as far as I can see, 80mm is 3.149", so it is 0.008" larger than the figure you give of 3.141". In terms of bore measurement, this is quite a lot.
I wonder if you have been using a pair of callipers to measure the bore, because with a bore of nominal 80mm diameter, plus a clearance of say 0.005", the actual bore size will be larger than the 80mm you have measured. (unless BSA made some undersized barrels?)
In theory, 80mm + 0.020" oversize would mean a rebore to 3.169" plus clearance. Plus 0.040 would give 3.189" and plus 0.060" would give 3.209" + clearance. Are my calculations correct, or have I missed something?? However, a rebore should always be done to the actual piston size.
I think you will need to sleeve either of the 2 larger bores. I used a Hepworth liner, and it didn't break through, but other liners may have larger OD's and break through.
I hope I've got the above correct. If not perhaps some kind gent could put me right.
Your best bet is to take the piston and barrel to an Engineering shop and get their advice, but they won't like the damage.
Thanks Steve, I have rechecked my measurement and due to the anvil in my internal micrometer being loose (slack me needs a boot up the R's )I have reset and tested it so my reviewed measurement is 3.159.
So time to find a machine shop and see what they can help me with....
I live a fair way from any shops that may be able to help though .......
The nine fin Barrel has got me intrigued now though, would be nice , reckon it will be about 535 cc I will go searching
Cheers .... have a good one
Aha. That makes a difference. So in theory the barrel is 0.010" over, which is a little on the high side. It might smoke a bit. However, you need to measure your piston carefully at right angles to the gudgeon (wrist?) pin to get the actual clearance, but if you might want to give it a run or two at Bonneville, 0.010" might be OK to prevent it from seizing! LOL.
I get the impression you are in the good 'ole USA with your Wisconsin piston?
There is a company in the UK, F.W Thornton 01952 252892. Email ENQUIRIES@FWTHORNTON.CO.UK
I understand they have the 493cc OHV Sloper piston in stock. Hepolite Part No. 2687. Apparently these are newly made in Australia, and have 2 compression rings and 1 oil control ring. However I don't know if the control ring is at the top or bottom. If it's at the top, you're laughing, because all you'll need is a rebore.
They stock Std, +0.020 and + 0.060. The compression height is 1 1/2" and the gudgeon pin is 3/4" dia.
They also stock liners. The piston will be quite expensive, but what price Fun?
Hope this helps. Let us know how you get on.
Ah Ah the smoke from the oversize bore has thrown you on to the wrong path!!! LOL I am actually in Australia..
I am led to believe that the ole Wisconsin piston was a bit of a common thing back in the good ole days ???? Measuring it, it is quite oval in shape and has very little contact surface with the bore , I don't think i will refit it.
I have been on FW Thorntons site ... I will have another look and flick them an email, I reckon I seen said piston and the third ring is actually at the top so I might give that a go..... That will give me some time to find a workshop that can sleeve one of the other cylinders for me, I have three engines so I might as well build a spare $$$$$ cant take it with ya!
I would still love to find out what that 9 fin big cylinder was off of ???? it was fitted to the 29 cases and was obviously functioning as it came in the bike, ah curiosity
Appreciate your input and by the way if I was your teacher I would have passed you in your Year 11+ maths exam LOL .. I will keep in touch Cheers
I'm not sure that the F.W Thornton pistons are still available – I think they sold out some time ago.
However I've used BSA B40 pistons on both my Slopers. The B40 has a standard bore of 79mm which means that +40 and +60 correspond to STD and +20 bore sizes on the Sloper. The base of the crown is 1mm higher than that of the Sloper which means that in theory a 1mm compression plate is required. However in practice this is not necessary as the Sloper piston seems to sit well down the barrel and doesn't come anywhere near to reaching the top of the barrel. The Sloper small end bush is much wider than that of the B40 and the bush will need to be reduced so that it is flush with the conrod. Also you will need to remove a small amount of material from the piston's gudgeon pin boss to accommodate the small end – I used a finger file sander. You need to ensure that you remove the same amount from each boss so that the piston sits centrally in the barrel. The piston skirt is slightly shorter than that of the Sloper and weighs about 1 gram less. The piston also uses circlips to retain the gudgeon pin which I feel is an improvement over the aluminium buttons which can wear.
Most machine shops have little trouble fitting a liner and will normally source something for you. In fact practically every Sloper barrel I've ever seen has had a liner fitted – the middle barrel in your photo seems to have one and the one on the left looks as though the liner has been removed.
I've attached a few photos which show a fitted B40 piston at the top of the bore and pictures that compare the two. The photo of the gudgeon pin bosses imply that at least 1mm will need to be removed from each; however this is not the case because the small end bush is proud of the big end eye and hence some material can be removed from the bush itself.
Cheers for that, very helpful , I have been looking at B40 pistons on line and the ones I found have a 5/8 Gudgeon were my rod has a 3/4 , so did you replace the small end bearing ?
In saying that it appears the SV sloper have a 5/8 pin so they would ok in that situation.
Handy to know about the compression height, looks like there is a bit of allowance there to play with,
I will keep hunting, thanks again
You can't have been looking at a B40 piston because they all have a gudgeon pin size of 3/4. I've attached a photo of a B40 piston compared to that of the Sloper - the gudgeon pin has been inserted into both pistons so that you can see the difference in the crowns. As can be seen the base of the crown is about 1mm higher than that of the Sloper, however even with no compression plate it comes nowhere the top of the barrel on either of my machines. The actual crown is somewhat higher, however the valves would need to descend 11mm before hitting the piston. A slightly higher compression ratio would certainly benefit the Sloper as in standard trim it is bit low really. I've also attached a photo of a Sloper 9:1 compression ratio piston, which has a considerably higher crown than that of the B40.
The other photo shows the Sloper small end. As can be seen the bush protrudes either side of the conrod eye and this material can be removed to help fit the piston.
The B40 piston I've fitted is low compression ( around 7:1) there are some B40 SS90 ones made by JP which have a much higher compression ratio. Kidderminster motorcycles have a +40 piston on Ebay at the moment and British Only in Austria also have some ( along with the SS90 pistons).
Pre-war machines are a pain because you simply won't find any standard parts. Consequently you will need hunt around for an alternative that's a reasonably close match and modify it if necessary. Parts catalogues are good source for finding a possible alternative, the JP piston catalogue is excellent in this respect, here's a link to it:
https://www.norndaautomotive.com.au/images/pdf/PISTONCATALOGUE2016v2.pdf
Generally speaking it's best to start with the same manufacturer and then widen the search if you can't find anything. With respect to pistons then there are surprisingly few 80mm ones available ( Ducati, Ford, Landrover). I searched the entire catalogue ( and other sources) and the B40 was by far the best alternative.
OK now I am on the right track, I have tracked several down here in Australia think I will head for the low compression piston, order submitted so just sit and wait now !
Piston arrived today, compared to the one that came out it looks to have the same compression height, just need to widen the inside gudgeon pin bosses and it can head off with the cylinder for a rebore.
Knocked up a facing / boring tool today to reface the inside gudgeon bosses to give me some clearance on the conrod, faced the conrod bearing back as well to tidy thing up, clean everything up, rebore the barrel and head back to the head / valve rebuild