RB cylinder grading

[Warkton Tornado No.1]

Some time in the future I plan to build myself an engine using a used RB22 cylinder that I've acquired. As far as the bore is concerned, it is good to go though I will be inevitably porting for use with either of the 60, 61 or 62 mm stroke crankshafts that I already have. As the 62 mm has the little end to suit a 16 mm wrist pin, that looks the obvious choice. FYI a LH manifold to use air filtration is a must & there's 3D printing in an alloy composite available to me, so I'm keen to add that to any components available - besides the rear brake shoe pivot kits WIP

As this engine is likely a 'keeper' I should consider piston options other than the excellent DT/KDX/TS motorcycle type.

Now you all know about graded pistons, but what about the cylinders themselves? The cylinder has a stamped number, but am I hoping too much that somewhere there will be a record of what grade of bore it will be? Or do I have to measure the bore? As we are talking in only microns variations, the best standard to which I could DIY evaluate the size would be with a new 70 mm nominal piston (I have several) with feeler gauges. I might be answering my own question here, but WT No.2 could probably get the bore measured.....eventually.

I've been 'window' shopping for pistons but have a bit of an aversion to Asso, though they are cast & in grades A - D, which doesn't seem the case with the Wossner forged pistons....as far as I know. Lambretta specific pistons in grades A - C are available from Mitaka & that may be my best choice. Any relevant comments would be welcomed....

[Fast n Furious]

What compression height are you wanting Mark?

[Warkton Tornado No.1]

Hey Pete.

As I've long since done what I can to lower primary compression, the higher the compression height the better. I'm happy to repeg as well....

[Fast n Furious]

Forget the grade number stamped on the top of the cylinder. This is only relavent to a new cylinder. If the cylinder has seen reasonable service in the past then the aluminium will have a certain amount of thermal relaxation. So, the bore will have streched a smidgeon. (this has nothing to do with bore wear)
It's important to measure the bore all over using a bore micrometer to determine what is going to be a suitable piston grade.
Unfortunately, as far as I know, Mitaka don't do a suitable piston to fit the RB22 cylinder without some extensive mods like repegging.
Decades ago you would never consider using a Meteor piston, cos they had a dreadful coefficient of expansion and were complete pants.
However, times have moved on and investments have been made. I've had very good reliable results from their lastest lightweight 70mm incarnations which are available in 30 and 39mm comp heights. No repegging required and are available in 3 or more grade sizes.
There is also a 32mm comp height one in single or dual ring format as used on Scuderia engines. I dont know the ring peg positions for these.

Hope this helps.

[Warkton Tornado No.1]

Enormous help Pete

For one thing you've confirmed my belief in Mitaka which I already had on my shortlist.

However, I'd overlooked Meteor despite their prolific use in many of the current range of kits. A quick look through Sticky's Kit Book with my blinkers off prove that Meteor have indeed appear to have raised their game to OEM quality.

As for bore measurement, I've messaged WT No.2 & await his response....

[Warkton Tornado No.1]

Much quicker results than I thought

70.056 to 70.064 mm variation in the bore probably means that a 69.99 mm cast piston would be the only option. That's 0.0026" in real money....

[Fast n Furious]

Looks like a grade "D" then.

[Warkton Tornado No.1]

Or 'C' in Mitaka.....I think

I've emailed Ron concerning availability.

[dscscotty]

Hi, with regards to piston availability for RB 22 cylinders, having tried several alternatives
Including, suzuki, yamaha I'd advise using a Wossner as provided by AF, having used these in many road and race motors I can vouch for their performance and reliability over jap or other cast type pistons, they have greatly reduced ring pegs issues, and not prone to cracking etc, our current (Team DSC) 40+ bhp rb250 Group 6 motors rev to 10k, using 72mm wossner pistons, great reliability and no issues in 6 yrs racing.

[Warkton Tornado No.1]

I have considered the Wossner piston but the only size that AF seem to do is 69.95 mm:

https://www.afrayspeed.co.uk/pages/part ... tid=119585

[Norrie Bodge]

I’ve had Wossner 70mm+ pistons for the RB22 for a mates bike !
AF Defo stock oversizes

[Fast n Furious]

Different oversizes yes. Different grade sizes?????????

[Warkton Tornado No.1]

I've just seen that KHC pistons are available from 70.00 mm & up in 0.02 increments up to 70.08 mm.

I personally have no experience of the brand but recall them being discussed on this Forum....

[Fast n Furious]

I'm wondering if this unamed supplier could be sporting a typo Mark????? Very unusual for anyone to make pistons this size. Maybe it should read 70.20mm- to-70.80mm making them oversize rather than gradesize.

[phil23fair]

I'm wondering if this unamed supplier could be sporting a typo Mark????? Very unusual for anyone to make pistons this size. Maybe it should read 70.20mm- to-70.80mm making them oversize rather than gradesize.

WT1 is correct in that KHC pistons go up in 0.02mm increments. I was after a 70.8mm piston for an iron lined TS1. I too thought it was a typo and contacted the vendor who checked the stock and said their website was incorrect and that the pistons actually went up in 0.2mm increments. When my 70.8 mm KHC piston arrived, the box was marked 70.8mm but the piston crown was marked 70.08.
With regard to KHC pistons, I have a 71 mm one in another 25BHP iron lined TS1 and its done about 8000 miles without any problems except the rings wore out after about a 1000 miles. I replaced them (rightly or wrongly) with I think a pair of 71 x 1mm Wiseco rings and have not had any problems since. I have read on here though that the KHC ring pegs are prone to coming loose.

[Warkton Tornado No.1]

I'm wondering if this unamed supplier could be sporting a typo Mark????? Very unusual for anyone to make pistons this size. Maybe it should read 70.20mm- to-70.80mm making them oversize rather than gradesize.

WT1 is correct in that KHC pistons go up in 0.02mm increments. I was after a 70.8mm piston for an iron lined TS1. I too thought it was a typo and contacted the vendor who checked the stock and said their website was incorrect and that the pistons actually went up in 0.2mm increments. When my 70.8 mm KHC piston arrived, the box was marked 70.8mm but the piston crown was marked 70.08.
With regard to KHC pistons, I have a 71 mm one in another 25BHP iron lined TS1 and its done about 8000 miles without any problems except the rings wore out after about a 1000 miles. I replaced them (rightly or wrongly) with I think a pair of 71 x 1mm Wiseco rings and have not had any problems since. I have read on here though that the KHC ring pegs are prone to coming loose.

I'm fairly certain that the listing is correct:

https://beedspeed.com/collections/lambr ... ve-engines

I've not heard many positive things about the current vendor & the fact that ring quality is questionable makes me dubious about the total risk

From all responses gratefully received & my own experiences in addition to wanting to optimise clearance, the cast Mitaka may be the best bet

[phil23fair]

The listing is correct but the boxes were marked incorrectly. Beedspeed is where I got the piston from

[Fast n Furious]

Then it would appear that these pistons have been specifically made to serve "hone to size" sleeved cylinders.

[dscscotty]

Just to confirm AF do supply wossner in graded increments to match specific Rb/Ts1 bores,
eg: 69.95= A 69.96=B etc,
With regards to KHC Piston, I stopped using them years ago due to reliability/poor quality, crap rings, peg failures and cracking, breakages, put a khc or similar next to a wossner there simply is no comparison.

[Fast n Furious]

Aye. Ya only get what you pay for.