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Thread Like Summary
76degree-triumph, Hillbilly bike, leon bee
Total Likes: 5
Original Post (Thread Starter)
by Mercian
I’m thinking of rebuilding my 750 Bonny and I thought I’d take the opportunity to update the engine. It’s a road bike so reliability, flexibility and smoothness are more important than outright power. I’ve read that the low compression pistons tend to make it a bit smoother and that the Norman Hyde half race cams tend to liberate a bit more low down torque. This seems like a combination that would work for my purposes. Has anyone done this and if so, how did it perform?

If you wanted to build to ultimate money no object T140v road engine, what would it be?
Liked Replies
by JubeePrince
Seach here for cam related posts from user: Pete R - R.I.P.

He knew these bikes well and had some great suggestions for cam and cam timing combinations for the 750 engines. He was a great poster and was always generous with his knowledge of these old bikes.


1 member likes this
by Tigernuts
I set out with similar objectives when building my TR7 engine nearly 10 years ago. I'd built several T140 engines previously but not since the early '80s, so I was very out of touch with where to get parts from and what had changed since the 1976/'77'78 Bonnies I used to work on. One major change Triumph made was to fit a much stronger timing side crankshaft bearing (NUP306). This is a must, in my opinion. Otherwise, the importance of getting everything spot-on during the engine build can't be over-emphasised. Thorough sludge trap cleaning (done with care to avoid making any new punch marks in the crank web); avoiding getting big end journals reground unless genuinely necessary, and then making certain that whoever does it grinds the correct .080" radius each side; valves & guides - avoid the usual items which aren't usually very good (they often come with clearances that are way too large) and invest in Hidural 5 / Colsibro guides (Norman Hyde sells good ones) and G&S valves, with the guides honed to the optimum clearance after fitting. JCC (aka Wassell) pistons are good but unless things have changed, the rings they come with aren't the best. Goetze rings are, as far as I know, the best available in UK.

Compression ratio - there are three piston types, 8.6:1, 7.9:1 and 7.4:1. I have 8.6:1 in my engine and they're fine, though 7.9:1 would probably be good,and more tolerant of high ethanol content petrol. A good cam combination is the standard inlet cam with 70-9989 exhaust cam. These are easily available and good quality, and a fair bit cheaper than Hyde's.

Care should be taken to verify that main bearings are made in UK, Germany, Japan, not Far-Eastern items. RHP / SKF are good.

Proper attention to every detail, eg: checking and flattening mating surfaces etc. Copper head gaskets are better, in my opinion, than the composite type, which need repeated re-torquing. Good quality gaskets throughout are important - worth spending a bit more if you can find better quality than the usual offerings, which can be made of material which splits easily and / or with holes that don't align perfectly.

I'm sure others will give plenty more suggestions (and probably criticise mine), but I hope this is of some use.
1 member likes this
by DMadigan
You get around that by putting in a middle bearing and whilst you are at it make the crank 90 degree offset. Solves two problems at once, vibration and breakage.
1 member likes this
by TR7RVMan
Hi Hillbilly, I'd love to ride vintage Triumph with that crank! That set up costs a bundle. But if you have the spare $$ why not.

Funny how getting older changes your outlook on money. We had several younger folks die recently. (meaning 50-70 years). You can't take it with you. Money has zero value until you spend it. Of course we don't want to spend our complete future $$ now, but if you can afford it, why not spend on the 270 crank conversion.

I'd want to ride one first, but in the last year I've been rethinking my life & $$.

If I liked the way it runs I'd move forward with spending another $5-10K on my bike. It cost me more than bike was worth to overhaul motor. So you can't look at resale.

The cost of full overhaul in USA with best parts, doing all work yourself, subletting only cyl bore, valve guide replace, hone, seats cut. Meaning you reassemble head. Adding a few trans gears etc. Crank dynamic balance sublet. Your looking at $5500 or so. Staggering how the costs add up. This is San Francisco area labor for the little sublet work you can't really do at home. Cut that labor in half, you save only about $300. I have receipts to back this up...

It takes countless hours to fully sort the motor during overhaul. If you paid shop to do this add a good $4-5K in labor. You'd be hard pressed to find shop to even do this kind of quality work. Who will pay $11k for motor overhaul. I'm mean every last bit done to "perfection" as good as it can get. End of day it's still a Triumph. Still only worth the price of a nice clean well sorted T140, $7500 -8000 on a good day to a desperate buyer. So you have to write it off as good fun & satisfaction.
1 member likes this
by Hillbilly bike
Hillbilly bike
I know what you mean...And although a tuned 750 can run at 8000 rpm, it is inviting a broken crankshaft
1 member likes this
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