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I haven’t built a new engine for a while and I was thinking of getting an RT 225 and fitting a 62mm or 64mm crank.

I have a had a few replies when I posted this enquiry on Facebook. Some where quite sensible but I thought I would start a debate so as to get a reaction from the non-Facebook people here.

What are your thoughts? Shoot!

(I like the engines that Adam Winstone (70x70 Mugello) and Mark Bennett (72x64 RB) as they have both tried hard to push the boundaries of torque-monsters).

Lol, funny that you mention me in this thread as one of the last configurations I started looking at was a 62mm stroke RT. The longer barrel makes this very viable, and port durations remain suitable.

I did make some notes but that's as far as I got with it, my attention going elsewhere.

I will watch this with interest.

Adam

I used a 60 stroke in my RT198, that was easy enough to port time & squish up bud.
As I understand, I'm sure a 62 stroke on a rt225 would be easy aswell to set up, but,
as on my TS1 when I used a RB25 crank I had to trim the piston base.

I used a DT 62mm crank in my RT195. I'm presuming port timings are similar/identical.

I ended up with 175exhaust, about 125transfer and mid 140s inlet.

The compression is ridiculous and I need to stand all of my 93kg on the lever to turn it over, but a thicker head gasket will solve that over the winter.

It rides very nicely with its s2 casa clubman and 25phbl. Just like a much better standard 200. It hasn't been on a dyno, but it feels like mid teens bhp, which will come as no surprise.

Knowledge wrote: I like the engines that Adam Winstone (70x70 Mugello) and Mark Bennett (72x64 RB) as they have both tried hard to push the boundaries of torque-monsters).

Mine's 70x70 Martin, nice and square.

Solid Air wrote:

Knowledge wrote: I like the engines that Adam Winstone (70x70 Mugello) and Mark Bennett (72x64 RB) as they have both tried hard to push the boundaries of torque-monsters).

Mine's 70x70 Martin, nice and square.

Thanks Mark Bennett.

dickie wrote:The compression is ridiculous and I need to stand all of my 93kg on the lever to turn it over, but a thicker head gasket will solve that over the winter.

I am not surprised by the high compression. I built a box-standard Casa 185 and the compression was so high I had to drill a decompression port above the exhaust port, but we are getting off-message here.

dickie wrote:I used a DT 62mm crank in my RT195. I'm presuming port timings are similar/identical.

I ended up with 175exhaust, about 125transfer and mid 140s inlet.

The compression is ridiculous and I need to stand all of my 93kg on the lever to turn it over, but a thicker head gasket will solve that over the winter.

It rides very nicely with its s2 casa clubman and 25phbl. Just like a much better standard 200. It hasn't been on a dyno, but it feels like mid teens bhp, which will come as no surprise.

Did you need to trench the case to fit that crank ?

Compression on any long stroke conversions needs careful consideration as many assume that they can simply use their existing head as the more remains the same, leading to lots of people running into problems. Indeed, a pal built an RT 225 with 60mm stroke and ended up selling the bike as he couldn't stand the difficulty kicking it over. I wish he'd spoken to me earlier as I would have happily reworked the head for him.

For my own 62mm stroke, 70 bore consideration I looked into converting a Mugello head (with extra fixings), rather than reworking an RT head, but it also occurred to me that the extra stroke could be configured with rod to use any flush face head, providing it was reworked for suitable comp ratio and squish band... returning to a 4 stud fixing.

You don't need to worry about the big cc being a problem for starting, providing you get the geo/corrected ratios appropriate. My 293 (72 × 72) Mugello kicks over fine.

Thanks Adam. I will take this on board if I go with a plan, though my point about the Casa kit was that this is supposed to be a bolt-on kit and you really shouldn’t have to re-machine the head, but we have said similar things before.

A seven or eight fixings head would be good as I think this is a weak-point on my current 76.5mm bore 283cc.

I might like to explore the idea of a steel liner down an ali barrel. The plan would be to fit a 71mm+ piston and have the option of oversized.

I have a Spanish 150 casing (the one made from a 200 casting) which has to have the crankcase mouth machined out to take a 200cc spigot. While doing this I could machine out the mouth an extra millimetre to make room for that bigger piston.

I’ve fitted steel liners before and they offer good tuning options as you can lower ports and re-angle transfers. You can also take the opportunity to use the top of the liner to lengthen the barrel to suit the longer stroke and this can also be used to centralise the head over the bore.

Yes, I used the lengthening of my Mugello as an opportunity to go slightly longer than needed so that I could spigot centralised my head to bore too. I think this is very worthwhile if you're having to machine a head anyway.

TrackTen wrote:

dickie wrote:I used a DT 62mm crank in my RT195. I'm presuming port timings are similar/identical.

I ended up with 175exhaust, about 125transfer and mid 140s inlet.

The compression is ridiculous and I need to stand all of my 93kg on the lever to turn it over, but a thicker head gasket will solve that over the winter.

It rides very nicely with its s2 casa clubman and 25phbl. Just like a much better standard 200. It hasn't been on a dyno, but it feels like mid teens bhp, which will come as no surprise.

Did you need to trench the case to fit that crank ?

No, well sort of ish. It turned fine, but it didn't have the clearance Darrell Taylor recommended. He probably knows a little more than I do, so I used a flap wheel to achieve it. I was quite surprised at how much work it took. I went through 3 wheels. It wasn't exactly difficult but harder than I expected.

Adam_Winstone wrote:Compression on any long stroke conversions needs careful consideration as many assume that they can simply use their existing head as the more remains the same, leading to lots of people running into problems. Indeed, a pal built an RT 225 with 60mm stroke and ended up selling the bike as he couldn't stand the difficulty kicking it over. I wish he'd spoken to me earlier as I would have happily reworked the head for him.

For my own 62mm stroke, 70 bore consideration I looked into converting a Mugello head (with extra fixings), rather than reworking an RT head, but it also occurred to me that the extra stroke could be configured with rod to use any flush face head, providing it was reworked for suitable comp ratio and squish band... returning to a 4 stud fixing.

You don't need to worry about the big cc being a problem for starting, providing you get the geo/corrected ratios appropriate. My 293 (72 × 72) Mugello kicks over fine.

Adam, I'm a bit surprised by this. I've assumed that I ballsed up my squish clearance and have made it too tight.

Some very rough maths tells me that a 62mm crank will increase geometric compression by around 7%, which doesn't seem like a huge amount to me.

I didn't measure the head volume for some reason (probably laziness). But when I rebuild over the winter I definitely will.

It's always worth working out both the geometric and corrected ratios (you can use the calculator on the Lambretta Images website, rather than do the maths) as many kits come with heads that are very high or surprisingly low. If you take a kit that is already quite high, it just gets higher.

Whilst the RTs are not crazy high, they need to be high enough so that they are not too low on 58mm stroke motors. However, they also need to be low enough so not too high when used with 60mm stroke cranks, which they're also promoted for use with. However, if you bump this up again with a 62mn crank, you're doubling down on the cc increase and raise in compression ratio.

The RT is well designed for 58mm stroke, designed to cope with an increase to 60mm (subject to what the owner feels is appropriate), but really should be carefully considered if intending to go to 62 stroke.

Always remember that a key selling point of the RT is the sensible porting, including a moderate exhaust duration. However, lower exhaust durations have the effect of being relatively high for corrected compression ratios as compression starts as soon as the exhaust port closes. Low port duration and a double increase in stroke does bump the ratio.