LCGB Forums

What causes a 4 point seize in a cylinder. Scootering did a report about the cylinder studs causing the problem?

Yes if the cylinder studs are too fat or the hole that they pass through are too small that can cause it and so can (if my memory serves me right) over tightening the cylinder head nuts. The recommendation seems to be use vespa or original Italian studs

Usually this only happens to overbored cylinders. There is simply insufficient metal to structurally support the cylinder wall when it gets hot. The cylinder and cylinder head are being compressed by the 4 fixings. When the cylinder gets hot enough, it trys to expand but can't because it is being retained by the lateral fixings. So, it begins to twist instead causing the 4 point siezure.
Overtightening of the cylinder head nuts is a classical cause also but with some cylinders it can be a trying experience to work out what is the minimum torque required to effect a reliable seal between the head and the cylinder top. It's not only lambrettas that suffer with this issue. Most ally big bore, head retained kits are plagued here.
A crappy cylinder head with unsufficient mass can cause this problem as well, as this means the cylinder has to deal with more of the generated heat. Innocenti cylinder heads for large block motors should weigh in around 900grams. Spanish heads can weight as little as 700 grams and should only be used as workshop ashtrays.
High compression will also aggrivate the situation.
Ineffecient air cooling around the cylinder and head adds fuel to this fire.

There are ways and means around these problems, albeit extensive.

Whilst I agree with the previous post in most part, the simple answer is the lack of area from the cylinder bore to stud holes and the lack of cooling in this area due to obstruction in heat dissipation. I'm not a firm believer in the stud theory although in saying that I do tend to use the thinner studs in my builds. There is nearly always an underlying reason as to why 4 points seizures occurs, AFR issues, air leaks, insufficient piston to bore clearearance, lubrication etc, etc. Anything that creates excessive heat and the lack of cooling/ fin area down the channels become hot very quickly.

A friend of mine bought a nice standard original paint SX200 from a dealer. He claims to not be be told it needed running in and rode it hard from the get go
This has a 4 point seize

We checked the piston to bore clearance and it’s within tolerance, although it did have a large leak at the exhaust gasket after he swapped the exhaust

Combination of lack of running in and a leaky exhaust gasket caused the seize
I’ve yet to check the timing, but will do this later after I picked up the honed barrel yesterday. The scoot is so standard, the timing maybe still set to original specification, and another contributor to over heating

checked the timing and it is set to 23 degrees and will need adjusting
unfortunately the flywheel is seized to the crank and the puller thread in the flywheel is shagged..

What has been achieved with the generic Lambretta cylinder design, utilising the existing stud centres & casing is quite amazing. It's unlikely that Charlie Edmonds 'very high 30's BHP' Group Four could be surpassed, but 'never say never.' Getting on toward 200 BHP per litre with the restrictions imposed by the bottom end is some achievement.

What is a fact was reliable engines with sufficient power to spare, could lug his very tall, not lightweight body around & comfortably beat all other comers, whilst duelling with the likes of Guy Topper on his Group Six machine, all in weighing little more than two bags of sugar

However, although his engines would complete races, power would drop off toward the end. Simply through cylinder expansion creating too great a clearance to the piston was the cause of power fade. So, obviously, Charlie ensured sufficient stud to cylinder bolt down holes to eliminate seizures, because the alloy used expands far more than the cast iron the engine was originally designed for.

In other words, there really is no reason that four point seizures cannot be eliminated. Selected fits of the fasteners, sufficient cooling, material choices, ignition management, lubrication & fuel all come into play. However, in most instances of four point seizures, ignorance surrounding the selection of the cylinder studs will be the fundamental issue. To this day, there is a trend in thinking that new, replacement items are better than the old OEM components that were manufactured within strict guidelines & standards.....

Just to put things into perspective, our grp 6 race engines (Team DSC) RB250's, running 72mm bores, revving to 10'500rpm, with over 40bhp, fan cooled, have never had an issue, with 4 point seizures. So yes they can be avoided. Like I suggested in my previous posts the stud channels have the least area from cylinder wall to the stud bore hole, its gonna get hot, but generally its another cause. Also of note is that original cast cylinders had the stud exposed, so benefiting from cooling from the fan.

I took this a stage further with my Jet 200 TS1
(I will post pics next time i have it stripped.. promise )
The cylinder is secured to the casing with 4 M8x40 High tensile Socket caphead screws that have had their heads machined down to a diameter of 10.8mm.
The cylinder stud holes were then drilled and reamed open to 11.00mm from the top down to a depth 15mm from the bottom.
The top of the cylinder stud holes were then tapped M12x1.25 to a depth of 35mm ish.
The head stud holes were then drilled to M12 clear.
The head is secured to the cylinder with 4 M12x1.25x55 high tensile socket caphead bolts. ( 1 of which is drilled and tapped to M8 to take the head cowling bolt)
A small 2mm hole is drilled into the side of each stud cavity, about half way up, to relieve any trapped air in the void which would get compressed when the head was fitted and expand detrimentally when hot.
So, the cylinder is secured to the case with the allen bolts, eliminating cylinder compression, just like you get on a jap 2 stroke bike engine.
Admittedly, there isn't much of a shoulder to bear the forces on the bolt heads here, but experience has proven it to be sufficient.
The difference in material expansion coefficients between the new fixings and the ally cylinder under thrash conditions has shown no negative effects.
If the cylinder needs to expand laterally, it now can, without twisting.

Fast n Furious wrote:(I will post pics next time i have it stripped.. promise )

These must be the long awaited cowling modifications I was up @ Northallerton @ the weekend for my son's wedding & if I'd have known where you lived would have tracked you down to take my own photos

As for the rest of the engineering modifications you have made (& therefore require no images ) I have thought of doing so, if it were to be necessary, but, thus far, that has not been the case. However, being intrigued:

How are the cylinder base fixing screws torqued up? Is an extremely long extension used that fits the cylinder stud clearance holes?

Talking of which, as they are now redundant, for their most part, why were they opened up to 11.00 mm, or was that just to suit the cylinder base securing screws?

For the record, there are often inserts available that can be adapted, though they would need unscrewing for complete removal of the special cylinder base securing screws you use. I mention it as it could mean the cylinder head could remain with M8 fixings, desirable if larger bores such as 70+ mm are to be used because of head to cylinder sealing/spigotting.

I understand the desire to avoid entrapped air but we often get that with other stud fixings. Endplate studs if removed often come away with a bit of what I refer to as a 'Vegetarian Stonk' which is a smell that takes some getting used to.....

As you mentioned the cowling fixing, I have, on occasion & if there's room, fitted an M8 AV mount to reduce the vibration in conjunction with M5 studs/screws fixed from the fan side of the magneto housing in conjunction with M5 nylocs & washers. It saves the soft alloy threads & the M5's can be just tightened enough not to strain the head cowling to fracture. Worth doing if the cowling has been extensively re-engineered......

You can't use a long Allen key or T bar key cos there is always a risk of it snapping.
Unfortunately, a 6.3mm 1/4 drive socket measures around 12mm dia. I could have machined one down in the lathe but opted instead to use an M6.35/M6 hex adaptor that fits into my 10mm dia extension driver to reach down the fixing holes. This allows the correct torque to be applied without any undue risk. You just have to be a bit careful when withdrawing the tool that the adaptor doesn't come away from the extension. A small magnet on the extension gets around this should it happen.
The nominal diameter of the stock cylinder fixing holes is 9mm, So, a 12mm fixing is only encroaching toward the cylinder bore by another 1.5mm which leaves cira 4.5mm from the edge of the fixing hole to the cylinder wall on a 70mm cylinder. So, M12 superfine headbolts are acceptable with 70mm cylinders.
I don't use 72mm cylinders, so I've never been inclined to contemplate it.
I have in the past used steel inserts with an internal dia of 8.2mm to strenghen the cyliner base. This offers a bit more shoulder for the bolt head to sit on but I have since found them to be superfluous.

Thanks for the detailed response.

There is never a 'blank canvas' as far as cylinder modification/design, but if the cylinder studs are less of an obstacle, there is more scope for porting cylinders that do comply with existing OEM stud layouts.

Personally, I feel that to be quite relevant in the name of competitive engines. Track racing has rules about stud repositioning (for the moment, but in recent years the BSSO has allowed a lot of changes) but as far as sprinting/high speed records are concerned, moving studs outside of the two generic patterns is hardly within the spirit of the game IMHO.

ooooh...... "The spirit of the game"..... Now there's a contentious issue.

WT.Current BSSO regs still stipulate original stud spacing for all classes. However there is an open class which was set up to allow for development, which does allow for external stud fitting and other engine designs that do not comply with the more traditional engines. The class is non championship scoring but does offer track time for R & D and other entrants who wish to use the class as an opportunity for more track time on race wknds.

dscscotty wrote:WT.Current BSSO regs still stipulate original stud spacing for all classes. However there is an open class which was set up to allow for development, which does allow for external stud fitting and other engine designs that do not comply with the more traditional engines. The class is non championship scoring but does offer track time for R & D and other entrants who wish to use the class as an opportunity for more track time on race wknds.

Yes, thank you, I am aware.

However, apologies in advance to the OP for a he hijack.....

Having attended AGM's where there was conflict between those advocating that regulations should be 'locked' as far as practicable & those fans of major changes, I think most people would admit that there now exists an amount of confusion as far as the casual spectator is concerned. FFS, there is a Group Four 'E' now, which somebody thought meant electrically powered!

There are myths concerning scooter racing, one of them being that it should be cheap. That philosophy went flying out of the window when the Rapido cylinder was allowed in to Group Four "due to lack of availability of OEM type" The consequences were the Mugello etc followed by the decline in numbers of that class when it became evident that only a professionally built & maintained engine would stand a chance of being competitive. It would fair to estimate the cost in achieving the best Group Four engines since the allowance of kit cylinders to run into tens of thousands per tuning house.

Put another way, I am aware of there derinately being four unused Group Four Lambrettas within about a two mile radius of my home. Extend that radius to 35 miles & there may be ten or more.

Please don't say that Production Class caters for that unused machinery. Riders won't convert their machines because, fundamentally, they feel the ethos that got them into scooter racing has largely been ignored.

Not sure what you mean by ignored? I know of several grp 4's also within my locality, but nothing other than thier own
personal reasons not to race age being one, lol. It's my intention when time allows to have one of our team campaign an old cast iron cylindered engine, incidently an engine that was the last to to beat the legendary Dave Webster in grp 4.
With regards to grp 4e just a modern version of grp 4 really but with stringent regs, aimed at curtailing costs and trying to get back to that old concept parts being available to all and not just the financial clout of well sponsored teams or individuals.
Again apologies for going off topic.

Getting back on subject, have any of you watched Steve Richards (Steelweasle) recent video on YouTube about four point seizures?

He points out that cylinder studs cannot expand enough to fill the gap between the stud and the hole through the barrel: even fat studs would need to grow by a huge percentage to even start being a problem.



The stud talk starts about 5mins into the video

I don't think that anybody commenting on this Forum has implied that the stud clearance to the cylinder holes are the direct cause of four point seizures.

Four point seizures are most likely due to the cylinder not being consistent in operating temperature throughout. That may occur simply due to the engine being subjected to high load conditions prior to a proper warm up, in which case the piston will expand far more - & quicker - than the cylinder has had opportunity.

Things that will excaperate the scenario include bore cylindricity, finish, compression, piston design/material & - of course - the fuel & oil used.

As far as studs are concerned, best practice remains in using OEM type with their superior design in terms of strength as well as their quality. Their smaller diameter just happens to be a product of the manufacturing process involving thread rolling. Studs are designed to stretch & the Innocenti engineers will have specified the appropriate torque as the result. That value would be applicable now were the OEM gasket materials to be utilised. Fat chance of that As I never use head gaskets on any Lambretta engine, I apply a different torque in conjunction with Copaslip on the 'head nuts.

Warkton Tornado No.1 wrote:I don't think that anybody commenting on this Forum has implied that the stud clearance to the cylinder holes are the direct cause of four point seizures.

Until I saw this video, I'd never heard anyone suggest that the studs expanding was the cause. My understanding of the conjecture was that the barrel and piston were expanding approximately uniformly until they encountered the studs which then stopped them expanding further and this was a cause. Personally, I feel this makes sense but that other factors come into play: an obvious one being uniformity of machining on engine cases and barrels.

I'll bet my left one that barrels twisting due to heat profile differences is another important factor.