1935 Model 8 Piston rings, advice please.

CLAUDE BESSON · November 30, 2019, 12:34:15 AM

I'd say that the most important is to ease the engine, ie no klonking nor overloading. Some gentle accelerations, and then let it quieten. And change the oil oftenly during the run in..

Exstress · November 29, 2019, 09:41:12 PM

Thanks Claude,

I did manage to get an actual Sunbeam Model 8 piston from a guy who advertises in the VMCC monthly magazine. It is fitted but I have not had chance to run the bike yet.

What sort of mileage and speeds would you recommend for the running in period.

Regards, John

CLAUDE BESSON · November 29, 2019, 06:09:59 PM

Hi, an alternative is the Royal Enfoeld 350 OHV piston, the crown is 1mm higher than the BSA B31 one (I tried..). RE had 70mm bore, so you should have to go to a +40 to have 71mm.
Hope it will help

Exstress · September 12, 2019, 10:44:10 PM

Thanks for the replies.

The Piston is a BSA B31 made by JP and with a compression ratio of 6.5.1 which is obviously not suitable. I will try and get the correct one this time.

singleminded · September 12, 2019, 06:57:59 PM

If the B31 piston you got is a standard compression ratio the the hieght between pin and crown will be wrong.Hepolite number SW10962, described as Flat with Valve pockets.
The 7.75:1 has, according to my book got the same compression height..Hepolite number 9939, described as Dome with valve pockets.
Do you have a Hepolite number for the one you bought.

wessex_man · September 12, 2019, 02:45:46 PM

Looks like it's going to be a bit on the low side compression wise and requires heavy modification. I'd think again.

You definitly could do with one like you have taken out. Hepolite part number 3238 I had a +40 and +60 both gone now the last one at last Shepton.

The later ones have a larger gudgen pin than the earlier but the small end can be reamed to suit.
here's some pic's I took when selling them.

Exstress · September 12, 2019, 02:08:54 PM

Quote from: singleminded on August 30, 2019, 11:07:12 PM
If there is enough meat in the barrel then you might find a slightly bigger but more available piston..
A 1946/1958 B31,, 7.75:1 cr is 71mm, has the same compression height and gudgeon pin but may need to be shortened..There are also some 600/650 twin pistons that may work..My Hepolite book is not very comprehensive I'm afraid.
I have a 9A and pistons are going to be a problem for me in the future..I'm sure one of the Model * owners out there has had the same problem and can give better advice..
Post this to the facebook page as well, the more people that see your question the better..John

John, I have got myself a B31 piston which unfortunateley is nothing like the piston fitted to my model 8. Can you advise if the piston on the right is a B31 or perhaps the one fitted in my bike is wrong anyway. The distance from pin to crown is substantially different.
Regards, John

Exstress · September 05, 2019, 03:57:59 PM

Apologies..... Its a BSA B31 piston manufactured by JP pistons. Advertised at 71mm diameter. My fingers sprung into action before my brain!

singleminded · September 05, 2019, 07:37:56 AM

I thought that the B50 was a 500 with an 84mm piston? is there another B50.

Exstress · September 04, 2019, 06:55:42 PM

Thanks for the replies.

I have found a supplier has a Hepolite Piston showing in stock but he hasn't actually found it yet! He also has Model 8 valves and end caps. I will confirm if this is the case.......

If not and I go down the B50 piston route, will I be ok to get a standard piston at 71mm diameter for my cylinder that has never had a rebore but needs one now?

Thanks, John

singleminded · September 03, 2019, 08:44:35 PM

Hi Sean..In my hepolite book there a few 650 twin pistons that look like they could do the job.
50/58 6T is close in 8.5:1..
RE 692 Meteor in 6.5:1
A10 650 in 8.5:1.
these are close and have a 3/4" gudgeon pin..but the decision, as always, lies with the bikes owner..John

singleminded · September 03, 2019, 08:31:22 PM

I saw you post earlier in the year Tom and made a note of the details ,,Thank you.

Tom Snow · September 03, 2019, 03:46:51 PM

Hi John,

I needed a piston for my 1928 Model 8 earlier this year, the only options I could find were to modify a piston from another make or have a new one made. I chose to have one made and JP in Australia did the job on time (six weeks) and for the quoted price. The piston they made for me included a one piece oil control ring which works fine so far; I have ridden the bike three times and covered 210 miles with no problems. My only advice would be :- 1. Fit the piston according to the manufacturers instruction, ignore all the old wives tales, if your machine shop won't H.T. Howard in Slough will. 2. Use good quality oil like Castrol XXL SAE 40, 3. Run the engine in properly don't thrash it in !!

Good luck,

Tom.

wessex_man · September 03, 2019, 02:59:17 PM

My Hepolite piston sold some time ago. When selling it someone told me they were using triumph ones but I certainly can't confirm that not knowing the crown height wrt to pin and bore etc.

singleminded · August 31, 2019, 05:24:28 PM

possibly the 48/58 350 bullet, g2 or clipper in 7.25:1 but may need shortening.

Exstress · August 31, 2019, 10:15:02 AM

Thanks for the advice John.
I will do some research into the BSA piston. Someone has also mentioned the Royal Enfield 350cc piston will also be suitable.

Regards, John

singleminded · August 30, 2019, 11:11:07 PM

Have you typed '350 piston' into the search box yet?
Try it ..Wessexman had a new piston for sale last year.

singleminded · August 30, 2019, 11:07:12 PM

If there is enough meat in the barrel then you might find a slightly bigger but more available piston..
A 1946/1958 B31,, 7.75:1 cr is 71mm, has the same compression height and gudgeon pin but may need to be shortened..There are also some 600/650 twin pistons that may work..My Hepolite book is not very comprehensive I'm afraid.
I have a 9A and pistons are going to be a problem for me in the future..I'm sure one of the Model * owners out there has had the same problem and can give better advice..
Post this to the facebook page as well, the more people that see your question the better..John

Exstress · August 30, 2019, 04:36:44 PM

I have popped the head and barrel off and sadly the bike needs a new piston and rings plus a rebore.

I contacted FW Thorntons at Telford but they don't have any pistons suitable for a 350cc. Please could any member recommend anywhere else I could try?

Thanks in advance.

John

Rick Parkington · August 18, 2019, 03:29:11 PM

To add to Paul's sound advice, it is essential to make sure that the supports for the rocker box are level before beginning to tighten the bolts. Screw them in lightly and make sure the box is settled squarely on each, if necessary shimming with thin washers, as this is another cause of the box breaking.
Cheers Rick

Exstress · August 15, 2019, 05:35:32 PM

Thank you Paul, that is precisely the reason I asked the question. There is no substitute for experience.

If it's ok I will post some photos of the piston and barrel for your comments?

Regards, John

phutton · August 15, 2019, 02:33:16 PM

Hi John,

The beauty of the Sunbeam engine is that it is very simple, and the description of removing the head and barrel in the 1935 handbook covers most aspects. The only thing it doesn't mention, and this is very important, is that before you start to undo the bolts holding the rocker cover, you MUST ensure that the piston is on TDC. If you don't there is the real possibility that the load from the valve springs could cause the aluminium casting to crack. The same applies on re-assembly. As good engineering practice dictates, the bolts should be done up evenly.

There is no head gasket, as you say, but there is a gasket between the barrel and the crankcase.

Good luck with it!

P.

Exstress · August 15, 2019, 01:13:56 PM

Hello, my 1935 Model 8 is smoking rather badly and I think its time to change the piston rings.

I have carried out this work on sixties bikes but have never had the Sunbeam engine apart. Is it straightforward or is there anything I need to be aware of whilst dismantling please?

Also, once the head is off presumably there is no head gasket, just matching surface facings like the rest of the bike?

Any advice will be gratefully received.
Thanks, John