Small block home tune

[shane BBoys]

Dickie.
The guys I ride with who run Casa & Muggy 186 have delly 25,s the Casa has a AF clubman and runs at 65 and the Muggy will do 70+ although this has had some work done down under before coming back to England; so should be good on your small block.
Shane.

[coaster]

I stand to be corrected but I thought the rings needed to be 'eased' back into the piston as the piston rises past the exhaust port. That means that the widest part should be at the bottom surely I had a 225 barrel ported a few years ago and even with the correct shape, the 1.5mm cast rings snagged in the exhaust port and broke the piston within 700 miles so be careful.

Great work so far by the way and I agree, doing it yourself and the feeling the results of your calculations and fettling is very rewarding. I did my own homebrew reed conversion to one of those ST225 ally/steel barrels, hacking away with my dremel, grinding out a finger port, widening the transfers like yours, very, very rewarding

Good luck

[dickie]

I stand to be corrected but I thought the rings needed to be 'eased' back into the piston as the piston rises past the exhaust port. That means that the widest part should be at the bottom surely I had a 225 barrel ported a few years ago and even with the correct shape, the 1.5mm cast rings snagged in the exhaust port and broke the piston within 700 miles so be careful.

Great work so far by the way and I agree, doing it yourself and the feeling the results of your calculations and fettling is very rewarding. I did my own homebrew reed conversion to one of those ST225 ally/steel barrels, hacking away with my dremel, grinding out a finger port, widening the transfers like yours, very, very rewarding

Good luck

Yes thanks, I think caution is needed here with regard to width and following advice from a couple of members away from here, I not going above 40mm wide, mayne only 38. To put that into perspective, the insideof the lines is about 39mm! So still a significant increase.

The rings do need easing back in, but in both directions. As the piston reaches the bottom of the exhaust porr it's slowing to a stop, so offers more opportunity for the rings to pop out than it does at mid stroke when the piston is at maximum velocity. So I don't think this is why your ring snagged. Was your port wide at the bottom?

[Adam_Winstone]

Ring breakage due to excessive port width is a massive problem with kits, especially since dynos have become popular. You'll note that most kits have needed to go with ever higher specification rings to cope with the exhaust port widths promoted by some tuners and kits, however, many of the older texts used to suggest 60-65% of bore port width for fast road use and max 70% for race use... even when using good Jap rings.

Over the last 10 years we've had an ever increasing occurrence of ring breakage on road bikes fitted with tuned cylinders and or modern kits 9even some using top quality rings) and in all cases the exhaust ports have been closer to 70% than 60%, all with good port shape. To combat this I've done a number of replacement tunes that have used similar durations to the tunes/kits that have let riders down but have kept the exhaust port width down closer to 60% and they've remained 100% reliable, with power being only marginally down (road use and tour, with no interest in peak BHP figures).

If port width really wasn't an issue, there would be no recent generation of kits with bridged exhaust ports.

Good luck.

Adam

[dickie]

Thanks Adam, that's interesting and falls in line with what others are telling me. In other words that I should stay below 40mm (62mm bore), which equates to about 64%. I'm aiming for 38mm with a small radius on the edges. I also wonder about confusion between chord length and arc length which makes a big difference.

Going to 38mm is still something like a 25% increase over the width of the existing port.

[Adam_Winstone]

Widening exhaust ports can really assist power gains/spread, however, there is little worse than being on the side of the road because of an element that you intentionally and unnecessarily introduced. I am aware of some TOP kits and tunes dropping rings after less than 1000 miles. Jap piston conversions became popular in the 80s/90s as the rings were some much more forgiving than Lambretta (or period aftermarket) pistons but tuners then pushed to find the limits (and found them!) of exhaust widths for these too, which is basically where we are today, with many kits being on the edge of reliability.

Thankfully, with a decent piston/rings and keeping the exhaust port reasonably narrow, you can end up with an extremely reliable motor that will wear the bore out before failing (many, many thousands of miles).

Broken rings will soon put you off going anywhere near the edge of reliability.

Adam

[dickie]

Having listened to the advice of those with more experience than me, I'm keeping the port below 40mm, so I've marked it out at 38mm so that by the time I've added a nice soft radiused edge to the exhaust port it'll be around 39, maximum 40mm. The outside edge of the black lines is where i'll be stopping, with a tiny amount further for the radius.
20180919_184901 by richardhenderson1968, on Flickr

As I've also decided to be a little more conservative with the carb, I can use the inlet manifold I already have for the PHBL25. This had already been opened out for an ill-fated SR185 kit that someone did for me a few years back. Surprisingly to me, the GP150 port was only very slightly smaller on the lower edge than the manifold. Anyway, it's matched all round now. I only touched the outside of the port; inside is not getting touched until it's properly marked up with the crank and piston in place.
20180919_183202 by richardhenderson1968, on Flickr

[dscscotty]

Nice work! Well thought out!

[dickie]

I've run into a problem; the GP 150 barrel which I picked up to use for this project doesn't want to go onto the studs. The spigot fits fine into the case and the original barrel (gp125) will go on but is a little tight.
20180922_181540 by richardhenderson1968, on Flickr
It's hard to show on a photo, but as the spigot enters the case, it hits the carb side of the casing, then as the thicker part of the spigot enters, it jams up against the thread of the barrel stud.

The gap is about 4mm and I anticipate using a 3.5mm packer, so the problem will be reduced, but I want it gone completely as it may cause a leak and if the barrel is tight on the studs it may be prone to four-point seizure.

Options I can see:

1. Drill the holes in the barrel out to 10mm
2. Open out the case a little
3. Fit waisted studs and cut the thread part down so that the thread never touches the barrel

Personally I reckon option 3 is the best, but a quick search on eBay etc isn't showing up any studs that are thinner than mine. Just cutting down the threaded portion of mine might be enough as the studs are about 7mm diameter once you get away from the threaded portion which is obviously 8mm.

[Adam_Winstone]

The cylinder studs are causing some of the problem. The originally rolled cylinder studs have a long thread section on one end and a shorter thread section on the other, with the shaft of the stud being a smaller diameter. If the threads are exposed, as yours are, then the studs try to align the barrel, rather than being free-floating to align and locate by the spigot and casing mouth.

Revisit your studs and see why you have so much thread sticking out beyond the casing mouth. You can even mark the threads that extend beyond the casing and then grind these off to allow more float.

Also try to determine whether all studs are causing the alignment issue or whether some fit OK. To do this you can remove the studs one by one to see if the barrel then travels all the way to the gasket face with no pressure. It is important that there is no pressure applied to the spigot as it can cause the spigot to crack over time, which is more of an issue when the cylinder is overbored and thin. You know that the barrel sits flush with ZERO studs fitted so you can start with ZERO and then try the studs one by one in the casing to see which gives issue.

This is actually a fairly common issue and it is important that you resolve it so that no pressure is applied to the spigot by the studs trying to pull the cylinder off line.

Good luck.

Adam

[dickie]

Thanks Adam, that makes perfect sense. I hadn't thought about the stress being applied to tbe spigot as a problem in itself, but once pointed out, it's obviously an issue.

You may have noticed that the casing was absolutely filthy, so I've created a lot of mess while cleaning it up; some of this probably got into the holes and is stopping the studs from seating fully home. I've tapped the holes as much as I can with a no2 tap, but I'll try to pick up a blind hole m8 today. Any remaining exposed thread will be ground off.

After that I'll try removing one at a time until I find the offending one.

[corrado]

I've created a lot of mess while cleaning it up; some of this probably got into the holes and is stopping the studs from seating fully home. I've tapped the holes as much as I can with a no2 tap, but I'll try to pick up a blind hole m8 today.

I use a can of carb cleaner spray with the thin spray tube, that usually cleanes out all stud holes.

[dickie]

So, I cleaned the threads with a plug tap and squirted them clean with brake cleaner as suggested by corrado, but these measures made almost no difference. So I took Adam Winstone's advice and removed a little thread from the studs where it was protruding; I may have been slightly over-zealous with this, but I'm not really worried about it.

Once the threads were removed, I could push the barrel all the way home, but I could feel just the tiniest amount of pressure on one side. when I looked closer, I could see that there was about 1mm of thread still remaining on one stud. This was enough to stop the barrel from seating perfectly. I would definitely have been able to tighten it sufficiently to get a leak-free cylinder, but there would have been stress on the spigot. Adam advised me off line about cracked spigots, so I removed it and Bob's-your-Uncle. Goes on easy now.

I also found that one exhaust stud was just slightly catching the stud. i don't think it was making a difference as it's so far from the spigot, but I wound it out one turn to be sure.

20180926_191702 by richardhenderson1968, on Flickr

If I get a chance tomorrow night, it'll be crank in and start some proper measuring.

[dickie]

Got the bottom end built up without any drama. Last one I did was riddled with leaks and I find looking for them particularly boring so I'm not taking any chances. Threebond all over the place.
20180927_184055 by richardhenderson1968, on Flickr

I used an arrangement of washers at 4 points to simulate a 4mm base packer and see what the results were. It gave 122 transfer and 152 exhaust which I'm pretty happy with, for the transfers and I'll raise the exhaust easily enough. The piston was only about 0.5mm below the exhaust, which I'll obviously tidy up.
20180927_194725 by richardhenderson1968, on Flickr
20180927_194737 by richardhenderson1968, on Flickr

Then I fitted the head I picked up for a tenner. It's got 175 written on it, so it must be good.
20180927_195445 by richardhenderson1968, on Flickr
first problem is that the studs are too short with the 4mm packer, even without a head gasket. I'll get some MB waisted ones. squish was 1.8mm.

So I'm left needing to decide whether to drop the packer to 3.5mm which will bring the squish down to 1.3mm, but reduce the transfer timing to 118, or get the head skimmed by 0.5ish mm. I'll almost certainly get the head skimmed, otherwise it defeats the object of trying to do a really good tune.

Still need to measure head volume, so that may play a part in my decision. I'll need to buy a burette first though, or just do it approximately by filling the bowl with the head off.

[dscscotty]

Concentrate more on port timings than squish, the timings will have more of an affect on performance and the characteristics of the motor than the squish, you can look at that at a later date if necessary.

[dickie]

Concentrate more on port timings than squish, the timings will have more of an affect on performance and the characteristics of the motor than the squish, you can look at that at a later date if necessary.

Cheers scotty. I was erring that way. Do you think I should go for 4.5mm packing which will raise my transfers to about 125 degrees? I only say this because MB's tech pages talk about mid 120 transfers being ideal. I was thinking about maybe 125 transfer, 178ish exhaust and consequent 26.5 blowdown. Problem is that while I don't mind getting the head skimmed I'm slightly reluctant to shorten the barrel; although I'm not sure why! If I just stick with 4mm, I'll go 122 transfer, 175 exhaust, 26.5 blowdown, but I won't need to get the barrel shortened.

[Warkton Tornado No.1]

Five modifications that you might like to take into consideration:

The PX studs are inexpensive & an excellent rolled, marginally oversize thread & the longer threaded end can often be used to further increase the strength of their fit within the crankcase by increasing the depth of the M8 holes if you have no access to Helicoils. I torque the head to 22.5 ft/lbs & don't want to be worrying about the studs pulling out.

Stud length can easily be compensated for simply by counter-boring the cylinder head holes deeper. Hard, stainless, very thick M8 washers are available via eBay that can take advantage of a modified counterbore size. 19 mm diameter if memory serves.....

Compression ratio, & therefore volume needs to be safe, but ballpark figures are good enough & will hardly be affected by squish variation.

The squish is very important as the piston sheds it's heat by getting to within the closest distance practicable. I always aim for 1.0 mm & whilst that may seem a faff, it is worth it for the optimum results. But then again, I always centralise the head & barrel with either hollow dowels or pin dowels.

Head gaskets are prone to fail. Personally, I refuse to use them in any engine build as there is always a way in which you can achieve the desired geometry without having to employ them.

I hope that helps.....

[dickie]

The PX studs are inexpensive & an excellent rolled, marginally oversize thread & the longer threaded end can often be used to further increase the strength of their fit within the crankcase by increasing the depth of the M8 holes if you have no access to Helicoils. I torque the head to 22.5 ft/lbs & don't want to be worrying about the studs pulling out.

I've just ordered some MB ones as they're waisted and around 11mm longer than the ones I have. I had considered adding helicoils, but dismissed it as unnecessary. Maybe I should reconsider?

Stud length can easily be compensated for simply by counter-boring the cylinder head holes deeper. Hard, stainless, very thick M8 washers are available via eBay that can take advantage of a modified counterbore size. 19 mm diameter if memory serves.....

Ok, I don't think I'll need to with the longer studs, but just in case, how deep can I go? typically of course.

Compression ratio, & therefore volume needs to be safe, but ballpark figures are good enough & will hardly be affected by squish variation.

I'm trying to optimise here, so I;m going for a corrected ratio of about 6 or 6.2:!

The squish is very important as the piston sheds it's heat by getting to within the closest distance practicable. I always aim for 1.0 mm & whilst that may seem a faff, it is worth it for the optimum results. But then again, I always centralise the head & barrel with either hollow dowels or pin dowels.

Yes, I'll be aiming for a similar figure.

Head gaskets are prone to fail. Personally, I refuse to use them in any engine build as there is always a way in which you can achieve the desired geometry without having to employ them.

I need to get the head and possibly the barrel skimmed to achieve a decent squish, so I've no need for head gaskets.

thanks

[Warkton Tornado No.1]

The PX studs are inexpensive & an excellent rolled, marginally oversize thread & the longer threaded end can often be used to further increase the strength of their fit within the crankcase by increasing the depth of the M8 holes if you have no access to Helicoils. I torque the head to 22.5 ft/lbs & don't want to be worrying about the studs pulling out.

I've just ordered some MB ones as they're waisted and around 11mm longer than the ones I have. I had considered adding helicoils, but dismissed it as unnecessary. Maybe I should reconsider?

The theoretical maximum strength for thread depth in alloy is only about 2 x 'D' which is 16 mm, but we're talking about a decades old die casting that not only will have hardened through work & age, but will also have a large zinc content because Innocenti knew how to produce aesthetically pleasing products with minimal scrappage. On occasion, I have carefully drilled down 35 mm or more, particuarly when the proximity of the stud is so close to the transfer port that the only way to maintain strength is to go extra deep. I also dowel the cylinder to the crankcase because I am a fussy b@st@rd.....

As for the comparison of Piaggio versus A.N.Other's products, I think that the likelihood is that a manufacturer of Helicopters is likely to have far better Quality Control in place, & having witnessed problems with over length front sprocket bolts & rear hub bearings with plastic ball cages, I know where my faith lies.

Stud length can easily be compensated for simply by counter-boring the cylinder head holes deeper. Hard, stainless, very thick M8 washers are available via eBay that can take advantage of a modified counterbore size. 19 mm diameter if memory serves.....

Ok, I don't think I'll need to with the longer studs, but just in case, how deep can I go? typically of course.

(See above)

Compression ratio, & therefore volume needs to be safe, but ballpark figures are good enough & will hardly be affected by squish variation.

I'm trying to optimise here, so I;m going for a corrected ratio of about 6 or 6.2:!

As per Bell's tuning guide, I only see the logic in uncorrected (or geometric) as it makes perfect sense to me. If that is of any use though, I aim for 10:1 maximum.

The squish is very important as the piston sheds it's heat by getting to within the closest distance practicable. I always aim for 1.0 mm & whilst that may seem a faff, it is worth it for the optimum results. But then again, I always centralise the head & barrel with either hollow dowels or pin dowels.

Yes, I'll be aiming for a similar figure.

Squish is often tricky to get spot on, & my 'cheat' is to VERY CAREFULLY achieve the best match of piston to head concentric ally after dowelling as close to it as possible by 'lapping' the two together. What I do is stick some abrasive wet & dry to the piston in the form of a ring (polo mint shaped) & lower it into the tightly assembled barrel & head. Then, with a lot of elbow grease accompanied with questions such as "Are we there yet?" the eventual result is a perfect match of profile.

Obviously, you cannot afford any abrasive to come into contact with the bore, so this is best performed prior to the final hone. Which reminds me, your bore could be better than it appears, IMHO. Sorry! It has took me years to eventually achieve a really good hone & the secret is to have a drill that goes REALLY slowly with loads of torque, & I have the Milwaukee Combi to thank for that.

Head gaskets are prone to fail. Personally, I refuse to use them in any engine build as there is always a way in which you can achieve the desired geometry without having to employ them.

I need to get the head and possibly the barrel skimmed to achieve a decent squish, so I've no need for head gaskets.

Top Man!

[dickie]

I drilled the holes a little further and tapped them so that they accept the full 17mm of thread on the MB studs. I'll probably use piaggio m8 next time, but I'd ordered these before you told me. C'est la vie.

Regarding geometric versus corrected, I suspect I'm about to open a can of worms, but corrected makes more aense to me, although the reflection from an expansion causes me to doubt my own opinion. I'm going to be running this with a clubman which I think makes that less of a concern and corrected ratio therefore more relevant?