Thursday, 25 September 2014

Port flow improvements

More examples of previous engine work.  This post is concerned with modifications to improve port flow.  Starting from a head off the street models which had an updraft inlet port of about 10deg. in comparison to the similar factory race heads with a small down draft angle.  I wanted to create a better inlet port shape without any welding on the head.  Any significant head welding requires the removal of valve guides and seats, re-heat-treating and re-machining the seat and guide holes.  That is if you want to do the job properly.

Cross section of a stock road head showing updraft inlet port.

The red area shows where I bore a circular recess to allow the fitting of a solid block from which to fashion a new port.

Making the insert block.

Machining the new port with the block fitted to the head.

Final block shape after machining the new port.

Comparison of flow between the standard port and that modified with the block.  This was achieved without any other detailed port work.  Further improvements were achieved by the removal of small amounts of material around the valve guide area.

There are more pix. in the Aermacchi engine folder on my picasa page and there is a PDF report with a lot more detail available for download from

I managed to get a race head casting from Dick Linton which was cast without ports or combustion chamber recesses.  I intend to machine this to use a narrower valve angle to allow for a more compact combustion chamber and steeper inlet port. It will also have significant squish areas which are absent from the original 1950/60s design.

Showing the basic casting.
More details are available for download in a report at

Monday, 22 September 2014

Some past engine modifications.

This is just a sample of some modifications that I have done to improve oil sealing by the application of "O" rings to eliminate various gaskets and the usual application of sealants.  In addition to ensuring good sealing the use of "O" rings simplifies assembly and disassembly.

O-ring around base of the barrel, replaces original paper gasket.
The mouth of the crankcase was counter-bored to accommodate the O-ring shown above.
The original pushrod tunnel relied on a precise gap to squeeze a flat polymer gasket by a given amount.  In general the gaskets could not be reused and in use they would harden and often leak.  This shows the barrel to head joint but the barrel to crankcase was sealed similarly. I have never liked this design, my improvements shown below.

Machining a groove in the crankcase around the base of the push-rod tunnel for an O-ring.  I hated the original sealing system which was prone to incorrect assembly.

O-ring fitted to the new groove.

A simple O-ring groove as I used at the bottom of the barrel was not feasible and so I adopted an insert sleeve sealed by O-rings in both the head and barrel.  This necessitated boring a circular recesses in both, shown here for the head.

Showing the recess bored in the barrel and the sleeve inserted into the head.  This system makes assembly fool-proof and results in a leak proof seal.  As a modification to existing components it requires precise setting up before machining the recesses to ensure accurate alignment.

Not exactly an obvious oil sealing modification, but this breather helps produce a slight negative pressure in the crankcase and so would rather let outside air in rather than let inside oil out.  One way valves in breather lines are not uncommon but in most cases they are too small.  I prefer to use reed valves from a 2 stroke engine which are designed for high flow.

Sunday, 21 September 2014

Back dated starting point

To add a logical starting point here are some pix. of the bike that I built in 2008 whilst in the USA.  There are more pix. in the Aermacchi folders on my Picasa page at and web site at

My race Aermacchi from the early 1970s. Most classic racing organizations require twin rear shocks so the replica will be of the previous incarnation of this bike.  Also without the mag wheels used originally.

 The starting point for a new replica in 2008.  A 1973 Aermacchi 350. I only needed the engine and maybe the rear hub.

New frame for the new replica.  I prefer bronze welding in place of fusion welding for a few reasons.  One is that the steel is not heated to such high temperatures and so it loses less of its strength.  Another is that the fillet size and shape creates less stress concentration at the joint improving fatigue strength.  The lower Young's modulus of the bronze also contributes toward less stress concentration at the joint. 

First trial assembly.  Everything fitted together.

 All together and ready to test.

Product testing.

It worked. 2009 AMA Grand National 500cc Champion.


I get quite a lot of people asking for details of some of the stuff that I do and make.  I am starting this blog in order to have a central repository for this information and also as a record to jog my own poor memory.

The main theme of this blog will be to document activities concerned with the design and construction of machinery for my own classic motorcycle racing activities.

In 1976 I ceased racing, not a declared retirement, I just never entered another race.  That was until 30 years later, 2006, I was invited by Mike Bungay of Sacramento, USA, to ride his Aermacchi at Daytona.  It was a crazy plan and I accepted immediately.  I also rode his bike at Barber and Mid Ohio in that same year.  I was hooked and regretted the long layoff.  We cannot change the past so I am now concentrating on getting as much racing as possible as well as building suitable bikes to ride.  In 2007 I raced in some endurance races in Spain on modern bikes. Between 2008 and 2010 I lived in the USA and purchased a couple of street Aermacchis for their engines as a base for constructing a replica of the race Aermacchi that I had back in the early 1970s.  Since returning to Spain to live I unexpectedly have had the opportunity to continue racing in the US.  For that I have Frank Camillieri to thank for providing both accommodation and a bike to ride with the USCRA.

I am currently building what I hope will be a very fast Aermacchi.

As this record is a long time coming I may initially post some past projects for the sake of greater completeness.