Compression/height Issues - Rapido Kit

[LI225Canada]

Was having issue with my Rapido 225 Classic kit and I got it figured to not enough compression. (I could not keep up with a P200! So humiliating.)

Without any cylinder base gasket or a head gasket I get a squish measurement of 1.5mm. (It was >3.0mm!)
I am going to have to run it without a base gasket to get the piston up all the way up a TDC so will use Wellseal or similar on the base. This is due to me having a Spanish case with the tall spigot height.

I was going to get the base on the cylinder head machined down.
My question is how much?
Should I go for 0.5mm? More? Less?

I am assuming the Wellseal will take up some of the height but I do not know how much as I have never used it.
If I use Wellseal on the base and head maybe I will not need to get the head machined at all?

Any input is greatly appreciated.

[shocky]

I try and run all my top ends without a gasket if possible less to go wrongi use yambond normally I am for 1.2 mm squish but normally upen the bowl up a tad

[Norrie Bodge]

I was in same sittu
When fitting my reed tuned 200 cylinder done by Mick Abbey in Harrogate
I couldn't get it any lower than 1.9 with thinnest base gasket & no head gasket...
Rang Mick & he said 1.9mm will be fine & it has been (touch wood)

[B-Race Tuning]

Do you know the thickness of the two gaskets you removed? If your base gasket was 1mm (average), you are effectively lowering your cylinder by that. In doing so you're reducing your transfer and exhaust timing and increasing your inlet timing. From saying that you're disappointed with the current performance, I think you'll regret going down the route you're suggesting. I also prefer not to use gaskets on the top end, but sometimes it can help you out. If you keep using a base gasket (get at least three, so that you have one the same to do your final build, bearing in mind that during measurement you'll be compressing the "test" gaskets), build up, correctly torque down and measure your squish and compression ratio. If squish is now, say, 2.5mm, get the head machined so it spigots INTO the cylinder by 1.1mm. use loctite as a head sealant, JD. correct your compression ratio once the squish has been achieved. Wellseal, yamabond, loctite etc. add nothing in real terms regarding their thickness when used as a base/ head gasket.

[LI225Canada]

Do you know the thickness of the two gaskets you removed? If your base gasket was 1mm (average), you are effectively lowering your cylinder by that. In doing so you're reducing your transfer and exhaust timing and increasing your inlet timing. From saying that you're disappointed with the current performance, I think you'll regret going down the route you're suggesting. I also prefer not to use gaskets on the top end, but sometimes it can help you out. If you keep using a base gasket (get at least three, so that you have one the same to do your final build, bearing in mind that during measurement you'll be compressing the "test" gaskets), build up, correctly torque down and measure your squish and compression ratio. If squish is now, say, 2.5mm, get the head machined so it spigots INTO the cylinder by 1.1mm. use loctite as a head sealant, JD. correct your compression ratio once the squish has been achieved. Wellseal, yamabond, loctite etc. add nothing in real terms regarding their thickness when used as a base/ head gasket.

Original gaskets were 0.5mm each.
With the base gasket 'in' the top edge of the piston (Not the crown) was below the top edge of the cylinder by about 0.8mm so Im thinking that I do want to lower the cylinder and the timing.
Machining the head to get it to sit INTO the cylinder seems like a good idea.

[Solerunner1]

First thing I would do is plug your numbers into a port timing calculator. Then you can see how the timings change with and without the base gasket (which you'll see by altering the deck height measurement). You can also check how the bottom of the exhaust and transfer ports line up with the piston at BDC.

After that you can see how much and where you have to remove metal. Personally, I would take it off of the barrel, rather than the head. More than likely the top of the barrel, but definitely check. Assuming you're in Canada, if you can't find anyone you trust to get the job done right, send me a message. I can sort you out. I'm below your southern border though.

[LI225Canada]

Being that putting together a Lambretta top end is not that much work, relatively speaking, decided to try putting it all together with the recommended Yamabond type stuff. Used Hondabond4.
With just the cylinder on with the base gasket removed it just looked right. Piston at bottom centre was just below the ports and the skirt just cleared the intake nicely at the top pf the stroke. Not very technical I know.
Once all together and torqued down got an squish measurement of 1.1mm at the very tip of the solder with it going to 1.5.
Will get the motor in the frame in the next couple of days and do another compression test before I start it.
Will let yah-all know how it goes.

Cheers

[LI225Canada]

Got the motor back in and did a compression test and got 100psi, not as much as I had hoped. Was shooting for 130 but there is still 100 miles remaining on break-in period.
Starts good and did get out for a spin around the block.
First thing I noticed was how smooth it is. Nice pull up to 40-45mph, and that was as fast as I could go legally.
The vibration going through to the handle bar is noticeably less. Also a lot nicer ride when putting around in first & second.
So-far-so-go. Will get it out on a faster road in the coming days.

[LI225Canada]

To follow up. Got the Lammy up to 55mph today! yay! This is a first. Motor is running nice and smooth and is also noticeable more quiet. And was able to keep up with a P200.
Thanks for all your help.


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