AJS 31 CSR Piston rings

robcurrie · Post by **robcurrie** » Fri Mar 16, 2012 4:38 pm

I have a 1962 model 31 CSR with the Speed kit, I have stripped down the engine because the casings need some welding repairs. While it is open, I would like to replace bearings and rings. My dilemma is that the pistons do not seem to have standard width ring grooves. The pistons are standard (bore 72mm) made by BHB part no. 028446, there is also E62 stamped on the crown. The compression rings measure 2mm thick and the oil ring groove is 4mm. Inside diameter of ring groove is 64.5mm. Could anyone verify whether this is correct, or suggest what rings I could use.

Don Madden · Post by **Don Madden** » Fri Mar 16, 2012 7:22 pm

The original pistons were made by BHB & stamped with AMC part no. I don't know what the E62 stamp is. I have uploaded a sketch of the genuine piston with dimensions to the photo section & it should be available soon. You will have to convert your metrics to the original Imperial dimensions yourself.

I have similar sketches of all the 72mm twin pistons. It can be confusing as all can be fitted in any of the 600cc/650cc but only the correct ones will not cause problems. Cheers, Don.

robcurrie · Post by **robcurrie** » Fri Mar 16, 2012 8:33 pm

Thanks for the reply, Don. Piston measures 3/32" (2.38mm) taller crown. As a comparison, the dimensions I have from top to bottom of your sketch are: 0.433" ; 0 ; 0.202" ; 0.355" ; 0.51" ; 1.71" ; 2.48" . Top and middle ring grooves are 0.080 wide x 0.141 deep ; oil ring groove 0.156 wide x 0.141 deep.

The 'bike was running fine and I have only done the stripdown because of an oil leak at the front of the crankcase. It has the twin monoblocs, very "square" cams - not "pointy" and it pulled like a train when it came onto the cam. It also has alloy mudguards and flanged alloy rims. It is possibly the only CSR Speed kit model 31 in South Africa.

I am a bit concerned that the ring grooves have been altered as they are not standard Imperial sizes, and I don't know where I will get replacement rings. I saw a photo of BMW 72mm rings and they looked a bit thicker than usual and they should also be Metric sizes, so I think I will pay the BMW agent a visit tomorrow. Edited by - robcurrie on 18 Mar 2012 6:54:46 PM

Don Madden · Post by **Don Madden** » Mon Mar 19, 2012 1:48 am

I also could not access the Jampot.com website for a couple of days, glad it is back. I got your email this morning but it is Sunday here & I ride somewhere most Sundays so could not answer until this evening. I've done some more research--The 028446 is the standard size for 1965 G12CSR/31CSR & is 9:1CR as opposed the the 8.5:1 normal for your 1962, which accounts for the higher crown. The rings & other dimensions should be the same so your piston has been altered, no doubt due to the world-wide shortage of the correct parts. I have a sketch of that piston, also.

The speed kit cams are indeed quite "square" & pumps up the power considerably. Mike, (now Michell), Duff, the Canadian racer & team partner Hugh Anderson of New Zealand had a factory supplied 31CSR that produced over 64hp for the 1962 Thruxton Marathon. They outran the Norton 650 ridden by Phil Read/Brian Stetchell easily until sidelined by mag problems. After several lost laps they took up the persuit again & fought back to the lead but overheated, (tiny), brakes caused them to lose lap times. They rode for two more hours with no brakes but finished well down in the pack but minus much shoe leather. "For want of a nail, the kingdom was lost."

Cheers, Don.

lightning7a · Post by **lightning7a** » Mon Mar 19, 2012 6:15 am

I could not get the standard rings for my 63 G12 csr and altered the pistons to take early Toyota Corolla rings, the oil ring is a 3 piece arrangement.They have been in for about a year, no problems or oil consumpstion. Cheers

robcurrie · Post by **robcurrie** » Mon Mar 19, 2012 7:51 pm

lightning7a, would it be possible to provide more information about the rings you used, such as the Toyota engine model, ring width (outside diameter to inside diameter) and thickness (height) for all the rings?

The pistons that I have have already been regrooved (dimensions as per my earlier post), so my options are limited. I am still trying to find dimensions for BMW R60 rings, as they might be the right size.

robcurrie · Post by **robcurrie** » Mon Mar 19, 2012 7:57 pm

I had a quick look at Stan Weiss' engine bore stroke website http://users.erols.com/srweiss/tablersn.htm#Toyota , but the closest Toyota he lists has a bore of 73mm

cf160 · Post by **cf160** » Sun Apr 08, 2012 3:31 am

73mm is just 72 +.040"
I guess this is an old posting by now...
Dave (cf160)

robcurrie · Post by **robcurrie** » Thu May 17, 2012 10:16 am

Follow-up: I purchased a set of BMW R60/2 - R69/S piston rings from Ulis Motorradladen in Germany. They fit perfectly! I will assemble the engine this coming weekend and report back in a few weeks how it runs. Cheers, Rob

Crafty · Post by **Crafty** » Mon May 21, 2012 9:00 pm

You could remachine the grooves to fit the rings you have available. This needs accurate machining.

Or you could use the 73 mm rings if they fit in all other respects. Just regap and put them in. 1 mm oversize will have little effect on the end result.