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Discussion Starter #1 (Edited)
Hi All,

Following @Andytheflyer's rebuild in 73 TR7RV in a 71 T120R frame restoration, I'm curious to know if everyone else rebuilding 750 twins also simply bolts engine components back together, or if anyone checks parallels and perpendiculars, or if anyone has checked 'em during a rebuild and found 'em accurate on 750 twins?

Reason I ask is, rebuilding Japanese engines in the past, I'd always just bolted 'em back together, never knew there might be inaccuracies but never experienced a problem. Otoh, long-time reading of particularly John Healy's, @Truckedup's and @speedrattle's posts on BritBike, and some experience of manufacturing cock-ups in triple engine components has me interested in what others do.

The discovered problems I've read of, and I wonder if anyone checks for:-

. Crank, camshafts and/or gearbox mainshaft axes not parallel.

. Crankcase cylinder barrel mating surface different heights on different crankcase sections.

. Crankcase barrel mating surface not parallel with crank axis.

. Conrod big-end and/or small-end oval.

. Conrod big-end and small-end not parallel.

. Conrod over-length (0.1 mm. over-length indicates failing conrod?).

. Conrod not in centre of bore.

. Cylinder bores' axes not perpendicular to barrel base mating surface.

. Cylinder barrel crankcase and head mating surfaces not parallel.

. Skimmed cylinder head.

. Cylinder head barrel and rocker-box mating surfaces not parallel.

. Rocker-box cylinder head mating surface not parallel with rocker spindle axis.

Regards,
 

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It's very important to look at the wear pattern on the parts when the engine is disassembled. You may be able to spot some issues and correct them...If the engine was a smoothie before rebuilding, change nothing other than worn parts and make sure the piston weight is close as possible to the worn ones.

I have rebuilt two 750's. and perhap a dozen 650's, many from mistmatched parts.And my race bikes....I did make sure the machine shop referenced the cylinder bores from the bottom flange. I checked the rod big end to make sure it was round, .001 out of round the rod is scrap and it should never be resized..All rotating parts should be mocked up in the bare case to make sure they rotate smoothly and to check end play... In the long run it'll be very expensive to have all the stuff checked if you can't do it and brutal expensive to make maching corrections..You can certainly go anal on this stuff.... I have money to wager it'll make little difference one way or another for a street bike...
 

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i check for some of this stuff when i have time. i confess i don't always have time and have to depend on long-dead british apprentice fitters. these are easy:

Crankcase cylinder barrel mating surface different heights on different crankcase sections.

. Crankcase barrel mating surface not parallel with crank axis.

. Conrod big-end and/or small-end oval.

. Conrod big-end and small-end not parallel.

when i put the crank and cams in, it has to turn absolutely freely. sometimes with the cams, getting them to turn freely means tightening the crankcase bolts in a certain order. it varies with different cases.

it doesn't take a big investment in tools to do the rudimentary checks, but when you get into measuring parallelism on shafts then we're getting beyond my abilities. for the rods you can check on straightness by slipping a 0.6875-inch bar through the little ends when it's together. if the bar won't go in, the rod is twisted or bent.
 

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My engine rebuild is always on the same engine and i replace the failed part. If the main bearings are old, i check for any play, if in the gearbox, i check the 2 caged roller bearings as the rollers sometimes fall out. I do pay attention to the surface the barrel bolts to and bolt it up with the case bolts slightly loose to ensure it is as flush as it can be. The rebuilds do hold together for many years until a major part fails. The 2 rebuilds i have done were brought on by, first one,main bearing failure. Second one, Crank snapping decelerating from 80 mph. Odd repairs every few years on pushrod tube leaks which, if i change the piston rings, stay leak free for quite a few years. I rarely measure anything at all. Since fitting a 1969 TRC6 crank from a running engine, it is a fairly smooth running engine. My bikes are ridden very hard on shortish rides for fun. Just this week, i went out on a low temperature day.30 mile ride and the engine was still very cool and you could keep hands on the cases without getting burned. It is a very cool runner which i put down to better oils that reduce the friction a lot better than oils i used 40 years ago.
 

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Hi Stuart, I look closely at piston wear pattern. I have a real depth gauge that will reach crank installed in case with rods removed. Very hard to get repeatable results, so I don't trust the measurements. I have bar to put though installed rods & use it. That's as good as I can do. I've not seen all that many motors apart, but I've not seen the very crooked wear on pistons.
I've not seen the huge steps between case halves. However it seems the cases are not perfect in real life. Tend to be lower in center on the last few motors. I never checked them back in the day. It was decided in each case to reassemble as is, since piston wear pattern was even. I attempt to line case halves best I can, but I find they really don't have a lot of wiggle room to align them. I try to get the best average I can. I find sealant on cly base gasket really helps seepage.

Every head I've ever seen or heard of is not parallel top to bottom. I've neve checked rocker shaft to box face for parallel. With push rods it doesn't seem to matter. The push rods don't push straight anyway. I have flattened top of head & rocker boxes to reduce leaks. I always use sealant on covseal box gaskets & they don't seep. I use sealant or blue Loctite 243 on any stud or bolt hole that goes to oil space.

I measure clearance & out of round at rod small end. I look at big end bearing wear. If perfect pattern I just reuse rod & always plastigage clearance. Big end out of round, I've only measured a few now with bore gauge/dial indicator type. Was about .00025". New LF Harris was pretty much spot on round. I can't measure rod length accurately.

I can't accurately measure cyl base/top parallel, but like cyl head I don't think it's too critical. I can't measure bore perpendicular to base.

Skim of head I find very puzzling. Triumph gives no spec. My head was skimmed .009". After is still thicker than some report new head is. Certainly would be nice to know.

It is imperative the cyl is mounted to the boring table with the base flange. I don't know of anybody not doing that around my area. Many use quickway bore set up. Basically the table has oval hole for cyl. Cly put in from bottom upside down. Then a "clamp" plate pushes cly up onto bottom of table tightly. Clamp plate has clearance for boring bar. If you go to Franz/Grubb website you can a photo of one in their shop. Other brand boring tables can mount differently. Torque plate can reduce distortion of bore. I don't think Triumph used them though. Other styles of boring tables in use, but so far as I know they align from bottom flange.

I find piston weight of modern pistons to be quite close in weight the same box set. Also from set to set. Modern pistons can be different weight from originals by 20-30g or more. LF Harris rods are much heavier on big ends. Visually big end alloy looks thicker going into rod shaft. I don't have much experience of this though as I only recently got rod scale. Regarding feeling of motor, I don't feel piston total weight makes such a difference as left to right difference. Seems many "best" balance factors are used & owners report good results. But all these are dynamic balanced crank assemblies. From 62-85%. So it seems??? Left to right is more important than factor?? Problem is the balancer didn't do before measurements. That's about twice the charge so owners declined. That's what I want to know.
I examine every part closely & fit them best I can. Has worked out well. I never ever take a chance on rods. Any defect at all they are replaced.

Don
 

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I tend to adopt the axiom...if it ain't broke don't fix it.

That is, if a motor comes to me and has a problem, I go looking for the root cause of that problem and make corrections.
If the motor is just tired or old and needs a freshen up then I don't go looking unless I see something that makes me. TBH your eye is a very critical examiner and can be trusted to pick up problems - but not all problems.
I often roll shafts and pushrods etc and look for wobble, I also sometimes use a straight edge and check 'stuff' as I go, but that's generally because I am interested, not because I have to.
I always measure big and little ends...just because of the age of these machines and their often chequered history [not everyone was as loving or caring].
 

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Hi Stu.

I'm still in the process of getting ready to pull the crankcases apart.
My focus has been to collect all the replacement parts, tools, and seals involved.
Once I commit I don't want any road blocks causing long periods of delay.

I'll report any issues when the engine comes together.

The 750 Twin Block has already been prepared re bored to take the new pistons and rings and honed .

The machine shop reported: "Surface Block: Prepare bottom of block, set up on mill, and machine minimum amount."

I made it clear any machining was to be very minimal.
I didn't want oil leak issues resulting from altered pushrod tube seal crumple distances.

The engineer said that it was important that the block had parallel surfaces to ensure perpendicular re boring, but he would keep the skim to a minimum.
The block surface which mates with the head looked skimmed, but I could tell it was very minimal judging by the surface.


Thanks

R R
 

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I have rebuilt or restored 20+ Triumph big twins (although only two 750s) and never found one so abused or incorrectly manufactured that it was out of spec apart from requiring typical oversize/undersize machining. Only significant note is that the 500s seem to wallow out valve guide bores WAY more often than 650s.

...and that excludes the two 650s with grenaded cases)
 
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